Key Concept: Scientific Investigation

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that observations lead to questions, which is true as scientific investigations begin with curiosity-driven questions based on initial observations. The reason explains how observations are made using senses, which is also true. However, the reason does not directly explain why observations lead to questioning; it merely describes the process of observation. Thus, both are true but the reason is not the correct explanation.

Key Concept: Scientific Investigation Basics

c) Asking questions
[Solution Description]
The first step in scientific investigation is asking a question or expressing curiosity about something observed in nature or everyday life, as mentioned in the syllabus.

Key Concept: Experimental Design

a) Size of the rolled dough
[Solution Description]
According to the syllabus, when investigating how oil temperature affects puri puffing, other factors like dough thickness and dropping method should remain constant for valid results.

Key Concept: Scientific Investigation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that changing only one variable while keeping others constant ensures reliable results, which is true because it minimizes confounding factors. The Reason correctly explains why this is important—it isolates the effect of the independent variable. Therefore, both statements are true, and the Reason justifies the Assertion.

Key Concept: Scientific Investigation, Controlled Experiments

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion states that changing multiple variables simultaneously helps identify the specific cause of an effect, which is incorrect because altering multiple variables at once makes it impossible to determine which variable caused the observed effect. The Reason correctly explains that controlling one variable at a time ensures clarity in attributing results to that variable, which is a fundamental principle of scientific investigation. Thus, the Assertion is false, but the Reason is true.

Key Concept: Scientific Investigation Process

d) To test hypotheses systematically and improve understanding.
[Solution Description]
The syllabus highlights that scientific investigations involve moving beyond simple observations to structured experiments that test hypotheses. Experiments help refine understanding through careful data collection and analysis, rather than just confirming biases or memorizing facts.

Key Concept: Scientific Curiosity

c) Curiosity about the world
[Solution Description]
The text emphasizes that curiosity is fundamental for scientific exploration, as shown by examples like observing puris puffing up and questioning natural phenomena. It states that all scientific experiments begin with curiosity.

Key Concept: Observation in Science

d) In everyday places like kitchens
[Solution Description]
The text specifically mentions that science is everywhere and gives examples of kitchen observations like puris puffing up. It states you don't need fancy equipment - even home kitchens are good places for observation.

Key Concept: Scientific curiosity, Observation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that observing a puri puffing up in hot oil raised questions about its uneven thickness. This is true as mentioned in the syllabus, where such observations initiate scientific curiosity. The reason states that scientific investigations start from everyday observations, which aligns with the syllabus emphasizing curiosity-driven questions from daily life. Moreover, the reason correctly explains why the assertion occurs—because science begins with simple observations that lead to deeper questions.

Key Concept: Role of Observation & Creativity

c) Balancing careful observation with creative thinking.
[Solution Description]
The syllabus emphasizes balancing **careful observation** (rigorous data gathering) with **creative thinking** (exploring new ideas). This combination ensures reliable yet innovative scientific progress, unlike approaches that prioritize only one aspect.

Key Concept: Scientific Questions

d) It should be focused, testable, and based on measurable observations.
[Solution Description]
A scientific question must be testable, measurable, and based on observable phenomena. For the puri example, it should focus on variables that can be controlled and measured, such as thickness, oil temperature, or dough composition. The question should lead to hypotheses that can be tested through experiments.

Key Concept: Scientific Investigation

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that a puri puffs up due to steam getting trapped inside the dough, which is accurate as the heat causes water in the dough to vaporize, forming steam that expands the dough. The reason explains that this expansion leads to one side becoming thinner, which is also correct. However, the reason does not fully explain why one side becomes thinner; it only describes the effect of steam expansion. Therefore, both the assertion and reason are true, but the reason is not the correct explanation of the assertion.

Key Concept: Focused Scientific Questions, Variables

d) How does increasing the oil temperature from 150°C to 200°C affect the time taken for the puri to fully puff?
[Solution Description]
To determine the most focused and testable question, we need one that clearly identifies both the independent variable (oil temperature) and the dependent variable (puffing of the puri). The question must also specify measurable outcomes. Among the options, "How does increasing the oil temperature from 150°C to 200°C affect the time taken for the puri to fully puff?" directly specifies the range of the independent variable and what is being measured (time taken).

Key Concept: Scientific Question Characteristics

c) Why does a puri puff up when fried in hot oil?
[Solution Description]
A focused scientific question is clear, specific, and can be investigated through experiments or observations. It should not be too broad or vague. Among the given options, "Why does a puri puff up when fried in hot oil?" is a focused scientific question as it targets a specific phenomenon and can be tested experimentally.

Key Concept: Scientific Question Formation, Systematic Investigation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that the given question about oil temperature and puri puffing is scientifically valid because it follows the principle of changing one variable (temperature) while controlling others. This aligns with the Reason, which explains that focused scientific questions enable systematic investigation through variable isolation. Both statements are true, and the Reason correctly justifies the Assertion since controlled experiments form the basis of scientific inquiry.

Key Concept: Systematic Investigation, Controlled Experiments

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that changing only the oil temperature while keeping other factors constant is essential in the puri puffing experiment. This aligns with the principle of controlled experiments where isolating one variable helps in accurately determining its effect. The reason explains that systematic investigation mandates altering one variable at a time to ensure valid results by eliminating confounding factors. Since the reason correctly justifies the assertion, both are true, and the reason is the correct explanation of the assertion.

Key Concept: Scientific Process

b) Asking a clear question
[Solution Description]
The scientific process always begins with asking a clear question about the phenomenon being studied before proceeding to design experiments or record observations.

Key Concept: Controlled Experiments, Variables

c) The time taken for sugar to dissolve completely
[Solution Description]
The dependent variable is what is measured in response to changes in the independent variable (stirring rate). Here, the time taken for sugar to dissolve depends on how fast the solution is stirred.

Key Concept: Scientific Investigation

b) Plants grown under white light
[Solution Description] The control group provides a baseline for comparison and should be exposed to natural white light (or no special treatment) to ensure that any observed effects are solely due to the experimental conditions (blue or red light).

Key Concept: Observation and Inference

c) Recording the color change in a chemical reaction
[Solution Description]
An observation is the act of noticing and describing events or characteristics using the five senses or instruments, without making interpretations. It is a factual record of what is seen, heard, or measured.

Key Concept: Scientific Inquiry Basics

c) Asking a question
[Solution Description]
The first step in the scientific method is to ask a question or identify a problem based on observations. This involves curiosity and wondering about how things work.

Key Concept: Scientific Inquiry

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that scientific investigations start with observations and questions, which is true as science relies on empirical evidence and inquiry. The Reason explains that curiosity motivates scientific exploration, which is also true and directly supports the Assertion because curiosity leads to the formulation of questions and hypotheses, initiating the scientific process.

Key Concept: Scientific Inquiry, Hypothesis Testing

d) Assertion is false, but Reason is true.
[Solution Description]
In scientific inquiry, a hypothesis is an educated guess that provides a testable explanation for an observed phenomenon. It does not need to be proven true; rather, it serves as a starting point for investigation. Experiments are designed to gather evidence that either supports or refutes the hypothesis. The Reason correctly explains that the purpose of a hypothesis is to offer a testable explanation, not necessarily one that must be proven true. Therefore, the Assertion is false because a hypothesis can still be valuable even if experiments refute it, while the Reason is true and correctly describes the role of a hypothesis in science.

Key Concept: Systematic Investigation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In systematic investigation, it is important to change only one variable at a time while keeping all other conditions the same. This approach helps in accurately determining the effect of the specific variable being tested. The assertion states that changing one variable ensures accurate results, which is true because isolating variables reduces ambiguity in observations. The reason provided explains why this practice is followed—by keeping other conditions constant, we can clearly attribute any observed changes to the altered variable. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Observation, Experimentation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that high oil temperature can cause puris to burn instead of puffing uniformly. This is true because excessive heat leads to rapid evaporation of water from the dough, which disrupts the even puffing process. The reason explains that rapid water evaporation causes localized overheating, which is scientifically accurate and directly supports the assertion. Thus, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Scientific Observation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that changing only one variable at a time ensures reliable results, which is correct because it aligns with the principle of controlled experiments mentioned in the syllabus. The Reason explains that this isolates the effect of the specific variable, which is also true and directly justifies the Assertion. Thus, both are true, and the Reason correctly explains the Assertion.

Key Concept: Systematic Investigation, Data Recording

b) Uncontrolled minor variations in dough preparation or oil heating
[Solution Description]
Inconsistent results in a controlled experiment often stem from unaccounted variables or measurement errors. Potential issues include slight variations in dough preparation (e.g., uneven kneading), inconsistent oil temperature maintenance, or subjective judgment of "puffing." Proper calibration of instruments and standardized protocols are critical to minimize such errors.

Key Concept: Scientific Observation

d) Moisture evaporates faster from one side, making it thinner
[Solution Description]
The thickness difference in a puri occurs due to the way steam gets trapped inside the dough during frying. When the dough is dropped into hot oil, moisture turns into steam and expands, causing the puri to puff up. One side becomes thinner because steam escapes more easily from that side, creating uneven expansion.

Key Concept: Scientific Inquiry

d) Assuming the reason without testing
[Solution Description]
Systematic investigation involves controlled experiments, careful observation, and recording data. Jumping to conclusions without experimentation or ignoring observations contradicts scientific methods.

Key Concept: Controlled Experiments

a) Thickness of the dough
[Solution Description]
The independent variable is the factor that is deliberately changed or manipulated in an experiment. In this case, the thickness of the dough is being altered to observe its effect on puri puffing. Therefore, the thickness of the dough is the independent variable.

Key Concept: Controlled Experiments

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that changing one variable at a time helps identify cause-and-effect relationships, which is true as it isolates the effect of that variable. The reason explains why this is done—by keeping other variables constant, we ensure that any change in the outcome is solely due to the manipulated variable. Thus, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Observation and Questioning

c) Observing the phenomenon
[Solution Description]
The scientific process starts with careful observation and questioning. Before conducting experiments or forming hypotheses, one must first observe the phenomenon and ask relevant questions about it. Observing that puris puff up when fried is the foundational step.

Key Concept: Systematic Investigation

b) Record them systematically
[Solution Description]
Observations made during an experiment should be recorded systematically to analyze the results accurately. This step is crucial for drawing valid conclusions.

Key Concept: Scientific Questions and Observations

a) Curiosity and careful observation
[Solution Description]
Science begins with curiosity and careful observation of everyday phenomena.

Key Concept: Scientific process, Iterative investigation

c) Investigate whether pricking holes in the dough affects puffing differently at various thicknesses
[Solution Description]
The scientific method is iterative. After drawing initial conclusions, good scientists develop new questions to deepen understanding. Testing alternative explanations helps validate findings, while simply repeating identical experiments has diminishing returns. Publishing immediately would be premature without further validation.

Key Concept: Key Principles of Scientific Investigation

b) To ensure results are consistent and not due to chance
[Solution Description]
Repeatability ensures that experiment results are consistent and reliable when reproduced under the same conditions by other scientists. It validates findings and eliminates bias or errors unique to a single trial.

Key Concept: Data analysis, Drawing conclusions

b) There is likely an inverse relationship between dough thickness and puffing time
[Solution Description]
Analyzing repeated experimental results shows correlation between thickness and puffing time. The consistent pattern across trials suggests causation, not just coincidence. Random variation is accounted for by the range given. The conclusion should reflect these careful observations without overgeneralizing.

Key Concept: Designing Experiments

d) All of the above
[Solution Description]
To ensure valid results in an experiment studying the effect of sunlight on plant growth, all variables except sunlight exposure should be kept constant (controlled). This includes factors like water amount, soil type, and temperature. Changing only one variable (sunlight) while keeping others constant helps isolate its effect.

Key Concept: Controlled Experimentation

b) Dough thickness
[Solution Description]
To ensure accuracy in experimentation, only one variable (here, oil temperature) should be changed at a time while other factors (dough thickness, dropping method) must remain constant. This is explicitly mentioned in the syllabus.

Key Concept: Variables in Experiment

c) Thickness of the rolled dough
[Solution Description]
A controllable variable is something that the experimenter can change intentionally. From the syllabus, examples include the thickness of the dough, type of flour, temperature of oil, or method of dropping the dough into the oil.

Key Concept: Variables, Scientific Investigation

d) Testing different oil temperatures while keeping all other preparation factors identical
[Solution Description]
The most reliable experiment should test one variable at a time while keeping all others constant. Testing different oil temperatures while using identical dough composition, thickness, and preparation method isolates the effect of temperature alone. This follows proper scientific methodology of controlling variables to determine causal relationships.
Step-by-step reasoning:
1) To study oil temperature effect - must keep flour type same
2) Must maintain same dough thickness and shape
3) Same frying technique needed
4) Only temperature should vary between trials
5) This provides clear cause-and-effect relationship
Therefore, the best option is testing different oil temperatures with all other factors constant.

Key Concept: Scientific Investigation Approach

b) Ask a scientific question about the phenomenon
[Solution Description]
The first step in any scientific investigation is to ask a clear and specific question. According to the syllabus, this involves identifying what you want to investigate, such as understanding why a puri puffs up when fried. The other steps (identifying variables, conducting experiments, etc.) come later.

Key Concept: Variables in investigation, Scientific approach

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that the thickness of the dough affects the puffing of a puri by determining the amount of trapped steam. This is true because thicker dough retains more moisture, which turns into steam during frying. The Reason claims that thicker dough allows more water to evaporate into steam, increasing pressure for puffing. Although both statements are true, the Reason does not correctly explain the Assertion. The puffing depends on the steam being trapped inside the dough, not just the amount of steam generated. Therefore, while both are correct, the Reason is not the correct explanation for the Assertion.

Key Concept: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that one side of a puri puffs up thinner than the other due to uneven rolling of the dough. This is true because if the dough is rolled unevenly, one side will be thinner. The Reason explains that the thinner side allows steam to escape more easily, causing it to puff up. This is also true and correctly explains why the Assertion occurs. Steam builds up inside the puri during frying, and the thinner side offers less resistance, leading to puffing on that side.

Key Concept: Dough Composition

d) Maida puris develop a distinctly thinner side compared to atta puris
[Solution Description]
Maida lacks bran and fiber, making the dough softer and less elastic than atta. This affects puffing because steam escapes differently—maida puris tend to be crispier but may not expand uniformly, often resulting in a thinner side due to weaker dough structure.

Key Concept: Dough thickness, Oil temperature

b) Heat distribution causes uneven expansion, making one side thinner
[Solution Description]
The uneven puffing of a puri is primarily due to the difference in heat distribution and dough thickness. When the puri is dropped into hot oil, the side touching the oil first starts heating up faster. The moisture inside the dough evaporates, creating steam which causes the puri to puff. The thicker side takes longer to cook and may not puff as much because the trapped steam escapes from the thinner side first. Additionally, the method of dropping the puri (angle, speed) affects how uniformly it cooks.

Key Concept: Phenomenon of Puri Puffing, Heat Transfer

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that the thinner side of the puri puffs up more due to lower thermal resistance and faster steam escape. This is true as thinner dough offers less resistance to heat transfer, allowing steam to form and expand faster. The Reason explains that the thinner side heats up quicker because of reduced thickness, leading to rapid vaporization of water and expansion of trapped air, which is scientifically accurate. Additionally, the Reason correctly explains the Assertion by detailing the mechanism behind the phenomenon—reduced thickness accelerates heating and steam formation, causing the puffing effect.

Key Concept: Identifying Variables

b) Dough thickness, oil temperature, and dropping method.
[Solution Description]
To isolate the effect of flour type, other variables like dough thickness, oil temperature, and method of dropping must remain unchanged. This ensures that any differences observed are due to flour type alone.

Key Concept: Controlling Variables

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that oil temperature affects puri puffing, which is true as higher temperatures cause faster water evaporation from the dough. The reason correctly explains this phenomenon by linking higher oil temperatures to quicker evaporation and puffing. Thus, both are true, and the reason accurately explains the assertion.

Key Concept: Controlling variables and measuring changes

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that excessive oil temperature prevents proper puffing of the puri, which is true because high heat causes rapid surface hardening. The Reason explains this phenomenon by stating that quick cooking traps steam, hindering uniform expansion, which is scientifically accurate and directly justifies the Assertion.
Experimentally, if oil is too hot, moisture inside the dough turns to steam but cannot escape due to the hardened crust, leading to uneven puffing or no puffing at all. Thus, both statements are true, and the Reason correctly explains the Assertion.

Key Concept: Controlling Variables

c) Temperature of the oil
[Solution Description]
Independent variables are factors that can be changed or controlled in an experiment to observe their effect on the dependent variable. In this case, the thickness and size of the dough, type of flour, temperature of the oil, and how the dough is dropped into the oil are all independent variables because they can be directly controlled by the experimenter.

Key Concept: Analyzing Results

d) Revise the hypothesis and retest with new experiments
[Solution Description]
If the experiment's results do not match the hypothesis, you should revise the hypothesis based on the new evidence and conduct further tests. Science involves refining ideas through experimentation.

Key Concept: Hypothesis Formation and Testing

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that uniform thinness leads to greater puffing due to even steam expansion. This is true as thinner dough allows uniform heat penetration, creating steam evenly throughout. The Reason explains that dough thickness impacts heat distribution, affecting steam formation—which is also scientifically valid. However, while both statements are correct, the Reason does not directly explain why uniform thinness causes more puffing; it only links thickness to heat distribution. Hence, the Reason does not correctly justify the Assertion.

Key Concept: Vapor pressure

c) Formation and expansion of steam bubbles inside the dough
[Solution Description]
As the puri is submerged in hot oil, the moisture inside turns into steam, creating vapor pressure. This trapped steam expands the dough, causing it to puff up due to bubble formation and expansion dynamics.

Key Concept: Hypothesis Formation

c) A possible explanation based on observations that can be tested
[Solution Description]
A hypothesis is a possible explanation based on observations, which can be tested through experiments. For example, "A puri puffs up because steam forms inside the dough." It is not just a guess but an educated assumption backed by initial observations.

Key Concept: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that changing the thickness of the rolled dough affects the puffing of the puri. This is true because experiments show that thinner dough tends to puff up more easily. The reason given is that thinner dough allows steam to expand more easily, which is scientifically valid as steam expansion ($V \propto T$) is a key factor in puffing. Additionally, the reason correctly explains why the assertion is true, as it links dough thickness to steam expansion. Hence, both are true, and the reason is the correct explanation of the assertion.

Key Concept: Dependent Variable

c) Size of the fried puri
[Solution Description]
The dependent variable is what you measure or observe as the outcome of the experiment. Here, the size (diameter) of the puri is measured after frying, so it is the dependent variable.

Key Concept: Control Variables

b) Oil temperature and frying method
[Solution Description]
To ensure accurate and fair results, all variables except the one being tested (here, dough thickness) must remain constant. These are called control variables. Examples include oil temperature, frying method, and type of flour.

Key Concept: Identifying Variables

d) Oil temperature
[Solution Description]
The independent variable is the factor that is deliberately changed in the experiment to observe its effect. Here, oil temperature is the factor being varied to study its impact on puri puffing.

Key Concept:  Identifying variables: what can be controlled and observed

(c) Dough thickness and oil temperature
[Solution Description]
In this experiment, the aim is to study how the type of flour affects the puffing of puris. Here, the type of flour is the independent variable because it is changed during the experiment. The observed variable is the puffing of the puri, such as puffing height, puffing time, or amount of puffing. Dough thickness and oil temperature should be kept the same for all puris so that the experiment remains fair. Therefore, they are controlled variables, not observed variables.

Key Concept: Dependent Variable Identification

b) Time taken for the puri to puff up
[Solution Description]
The dependent variable is the observable outcome that changes in response to the independent variable. Here, the dropping method (independent variable) is altered, while the puffing time (dependent variable) is measured as the result.

Key Concept: Identifying Variables

b) The conditions that can be deliberately changed
[Solution Description]
In an experiment, the researcher can deliberately change certain factors to observe their effect. These are called controlled variables or independent variables. Examples include thickness of rolled dough, type of flour used, temperature of oil, etc.

Key Concept: Controlled Experiment

c) To clearly identify the effect of the changed variable
[Solution Description]
Changing only one variable at a time ensures that any observed effect on the outcome can be attributed solely to that variable. If multiple variables are changed, it becomes unclear which factor caused the change in the result.

Key Concept: Systematic Experimentation

d) Dough thickness
[Solution Description]
When testing different dough thicknesses while keeping oil temperature constant, the independent variable is the dough thickness. The study aims to understand how this specific change impacts puffing, making it the primary factor under investigation.

Key Concept: Variables in Experiments

b) Oil temperature
[Solution Description]
To determine how dough thickness affects puffing, the independent variable is the dough thickness. All other factors that could influence the result, such as oil temperature $T_{\text{oil}}$, frying method, and dough composition, must remain constant (control variables). The dependent variable is the puffing outcome.

Key Concept: Measurement Tools and Techniques

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description] The Assertion states that a ruler is used to measure the length of an object accurately, which is true because rulers are standard tools for length measurement. The Reason provided is that rulers have standardized markings that allow for precise length measurements, which is also true and correctly explains why rulers are accurate for measuring lengths. Therefore, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Experimental Variables, Systematic Investigation

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion states that recording only whether the puri puffs up is sufficient for analyzing the effect of oil temperature. However, this is incorrect because scientific experiments require multiple measurements to establish a reliable relationship. For example, measuring the time taken to puff can indicate how quickly the reaction occurs at different temperatures, while thickness measurement can show variations in the degree of puffing with temperature changes.
The Reason claims that these additional measurements provide no further insight, which is false. In reality, they are crucial for comprehensive analysis. Hence, the Assertion is false, and the Reason is also false.

Key Concept: Controlled Variables

c) Time taken to puff up
[Solution Description]
The dependent variable is what you measure or observe in response to changes in the independent variable. Here, oil temperature is the independent variable, and the time taken for the puri to puff up is the dependent variable because it changes based on the oil temperature.

Key Concept: Controlling Variables and Drawing Conclusions

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that accurate recording of experimental data is essential for drawing valid scientific conclusions. This is true because without precise records, the results cannot be verified or replicated. The reason explains that detailed observations help identify patterns and relationships between variables, which is crucial for reliable conclusions. Since the reason correctly justifies the assertion, both are true and the reason is the correct explanation.

Key Concept: Experimental Design

b) To ensure the results are not influenced by external factors
[Solution Description]
Changing only one independent variable at a time ensures that any observed effect on the dependent variable can be attributed solely to that change. If multiple variables are altered simultaneously, it becomes difficult to determine which variable caused the observed effect.

Key Concept: Experimental Variables and Conclusions

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that controlling one variable at a time allows scientists to attribute effects to that variable, which is correct because isolating variables eliminates confounding factors. The reason explains why changing multiple variables complicates identifying causation, which directly supports the assertion. Thus, both are true, and the reason correctly explains the assertion.

Key Concept: Observation and Measurement, SI Units

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that changing the unit (km/h to m/s) affects only the numerical value, not the physical interpretation. This is correct because the physical quantity (speed) remains unchanged; only its representation changes.
The Reason explains that unit conversion involves a constant factor, preserving the physical meaning. This is also true since $1 \, \text{km/h} = \frac{1000 \, \text{m}}{3600 \, \text{s}} \approx 0.2778 \, \text{m/s}$.
Further, the Reason correctly justifies the Assertion because the constant factor ensures the underlying physical interpretation (speed) remains consistent regardless of the unit. Thus, both Assertion and Reason are true, and the Reason explains the Assertion.

Key Concept: Observation and Measurement

b) Asking focused questions
[Solution Description]
The first step in any scientific investigation is to ask focused questions based on observations. This helps in forming hypotheses and designing experiments.

Key Concept: Tools, Qualitative vs. Quantitative Observation

d) The puri puffed up quickly
[Solution Description]
Qualitative observations describe qualities or characteristics without using numbers. "The puri puffed up quickly" is descriptive and lacks numerical data, making it qualitative. The other options are quantitative as they include measurable quantities.

Key Concept: Observations in Everyday Settings

c) Noticing whether a puri puffs up when fried in hot oil
[Solution Description]
Observations can be made in simple environments like the kitchen by examining changes in sensory details or measurable outcomes such as time taken or yes/no results, as demonstrated by observing puri puffing.

Key Concept: Observing changes in experiments

b) Temperature of the oil
[Solution Description]
Controlled variables are factors that remain constant during an experiment to test the effect of changing one variable at a time. In the given context, the thickness and size of the rolled dough, type of flour used, temperature of the oil, and method of dropping the dough into the oil are controlled variables.
The option "Temperature of the oil" is one such controlled variable as it can be kept constant or changed systematically to observe its effect on puffing.

Key Concept: Multi-variable analysis, Measurement accuracy

d) Documenting time, puffing success, oil splatter, and smell for each trial
[Solution Description]
Reliable data collection requires systematic recording of all observable changes (such as sensory details), repeating trials, and noting environmental conditions that might affect the experiment.

Key Concept: Observing changes and responses in experiments

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that keeping the thickness of the dough constant while varying the oil temperature is an example of controlling variables. This is true because in scientific experiments, controlled variables are the factors that remain unchanged to ensure that any observed effects are due to the independent variable (oil temperature). The reason explains what controlled variables are, which aligns with the assertion and correctly justifies it. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Observation and Measurement

c) To understand additional aspects of the phenomenon
[Solution Description]
Sensory details like smell, sound, or visual changes provide additional qualitative data that can help explain the phenomenon being studied. They complement quantitative measurements (e.g., time taken to puff) by offering insights into underlying processes, such as oil degradation or moisture evaporation.

Key Concept: Scientific Measurement

c) Measuring the time it takes to puff up
[Solution Description]
A quantitative observation involves measurable numerical data. Measuring the time it takes for the puri to puff up is an example of such an observation.

Key Concept: Time Measurement, Observation

a) Higher oil temperature reduces puffing time
[Solution Description]
Higher oil temperature increases the rate of steam generation in the dough, reducing puffing time. Here, $180^\circ \mathrm{C}$ oil causes faster puffing (8 s) compared to $150^\circ \mathrm{C}$ This inverse relationship between temperature and time is correctly stated in option a).

Key Concept: Measurement, Variables, Systematic Investigation

c) Prepare puris with varying thickness but identical oil temperature and flour
[Solution Description]
A valid scientific approach involves changing only one variable (here, dough thickness) while keeping others (oil temperature, flour type, dropping method) constant. Observations must include measurable outcomes like puffing time or thickness of the thinner side. Option c) follows this systematic method.

Key Concept: Systematic investigation, Measurement control in experiments

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that controlling only one variable at a time is essential for determining causality, which is true as per the syllabus's emphasis on systematic investigation. The reason explains why this is necessary—if multiple variables change together, the cause-effect relationship cannot be isolated, making the reason both true and the correct explanation of the assertion.

Key Concept: Systematic Observation

c) To analyze results systematically and draw reliable conclusions
[Solution Description] Detailed notes help in understanding the experiment better, identifying patterns, refining experiments, and uncovering new insights. They ensure reliability and accuracy in scientific investigations.

Key Concept: Analyzing Results

b) Comparing different variables (e.g., thickness, temperature) to see how they affect puffing
[Solution Description]
Analyzing results involves comparing the recorded data to identify trends or correlations. For example, plotting graphs of puffing time ($t$) vs. dough thickness ($d$ mm) can reveal whether thicker dough takes longer to puff. This method helps draw meaningful conclusions.

Key Concept: Systematic Observation, Data Analysis

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that systematic recording of both qualitative and quantitative data ensures reproducibility in experiments. This is true because detailed documentation allows others to replicate the experiment under the same conditions. The reason explains why both types of data are essential: quantitative data provides numerical measurements (e.g., time taken, temperature), while qualitative data describes observations (e.g., color change, texture). Since the reason correctly explains why systematic recording of both types ensures reproducibility, the correct option is a).

Key Concept: Systematic Recording, Anomalies, Pattern Identification

b) The omission hides whether differences in puffing times are due to pricking or other factors.
[Solution Description]
Pricking holes can significantly alter puffing behavior. Without recording which puris were pricked, the scientist cannot distinguish if variations in puffing times are due to other experimental conditions or this unrecorded change. This leads to potential misidentification of patterns or anomalies in the data.

Key Concept: Heating Effect of Electric Current

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that the heating effect of electric current is used in electric heaters to keep us warm. This is true because electric heaters convert electrical energy into heat energy. The Reason explains that when electric current passes through a conductor, it produces heat due to the resistance of the conductor. This is also true and correctly explains why electric heaters work—the resistance in the heating element causes it to heat up when current flows through it. Therefore, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Experimental Design

c) To ensure clarity in identifying the cause of observed changes
[Solution Description]
Changing only one variable at a time allows us to clearly identify its direct impact on the result. If multiple variables are altered simultaneously, it becomes difficult to pinpoint which variable caused any observed changes, leading to inconclusive results.

Key Concept: Experimental Control, Data Collection

b) Use the same oil temperature and dough thickness but test multiple puris for each storage condition.
[Solution Description]
To isolate the effect of storage time, all other variables (oil temperature, dough thickness, etc.) must remain constant. Testing multiple samples reduces random errors, and consistent measurements ensure comparability.

Key Concept: Scientific Method

c) Ask a scientific question
[Solution Description]
The scientific method begins with asking a clear, testable question about the phenomenon being observed. For example, "What factors affect how a puri puffs up when fried?" This guides further investigation.

Key Concept: Electricity in Daily Life, Pressure and Its Effects

c) Heater performance remains unchanged
[Solution Description]
First, calculate the heater's power under normal conditions using $P = V^2/R$: $P = (220)^2 / 44 = 48400 / 44 = 1100 \, \text{W}$.
Atmospheric pressure changes do not directly alter electrical resistance or power dissipation in a closed circuit. The heater's performance depends solely on voltage and resistance, which are unaffected by external pressure in this scenario.

Key Concept: Observation and Curiosity in Everyday Life, Designing Simple Experiments

c) Assertion is true, but Reason is false.
[Solution Description]
The assertion correctly states that observing a puri puffing up is an example of applying scientific inquiry to everyday phenomena. However, the reason provided is false because the puffing of a puri depends on multiple factors such as dough thickness, oil temperature, and moisture content, not just the thickness of the dough alone. Therefore, while the assertion is true, the reason is false.

Key Concept: Pressure and Its Effects

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that a syringe works on the principle of pressure difference, which is true as it relies on creating a vacuum by pulling the plunger, causing fluid to enter due to atmospheric pressure. The reason correctly defines pressure as force per unit area, and this definition explains why pressure differences cause fluid movement in syringes. Therefore, both statements are true, and the reason correctly explains the assertion.

Key Concept: Ecosystems and Environmental Balance, Climate Change and Solutions

b) Wolves stabilize at 25 due to reduced prey
[Solution Description]
In a balanced ecosystem, predator-prey dynamics follow a direct relationship. With 1000 deer supporting 50 wolves, the ratio is 20:1 (deer:wolves). If deer reduce by 50% to 500, the wolf population will eventually decline proportionally to maintain balance.
Assuming the same ratio, 500 deer would sustain approximately 25 wolves ($500 / 20 = 25$) after several generations due to reduced prey availability. This demonstrates how disruptions in one part of an ecosystem cascade through trophic levels.

Key Concept: Reflection and Light

c) Light converges to a focal point after passing through a convex lens.
[Solution Description]
A convex lens is thicker in the middle and thinner at the edges. When parallel rays of light pass through it, they converge at a focal point on the other side due to refraction. This bending occurs because light slows down as it enters the denser lens material, causing the rays to change direction toward the normal and then diverge again after exiting.

Key Concept: Pressure, Mixtures and Solutions

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that a sharp knife cuts better than a blunt knife due to higher pressure. This is correct because a sharp knife has a smaller surface area at its edge compared to a blunt knife. According to the formula for pressure ($P = \frac{F}{A}$), when force ($F$) remains constant, decreasing the area ($A$) increases the pressure ($P$).
The reason correctly explains the assertion by stating the relationship between pressure and area, which is indeed inverse proportionality as per the formula. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Behavior of Materials

c) Steam forms inside the dough, making it expand
[Solution Description]
When a puri is fried, the moisture inside the dough turns into steam due to heating. The steam tries to escape, causing the dough to expand and puff up. This happens because steam occupies more volume than liquid water, creating pressure that inflates the puri.

Key Concept: Pressure and Weather Events

c) A small pressure difference causes slow air movement (breeze), while a larger difference causes faster movement (strong winds/cyclones).
[Solution Description]
Air moves from regions of high pressure to low pressure due to the pressure gradient force. A small pressure difference results in a slow movement of air, causing a gentle breeze. A larger pressure difference accelerates the air more, leading to stronger winds or cyclones. The greater the difference, the faster the air moves to balance the pressure.

Key Concept: Pressure and Area

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that a sharp knife cuts better due to higher pressure for the same force. This is true because pressure ($P$) is calculated using $P = \frac{F}{A}$, where $F$ is force and $A$ is area. A sharp knife has a smaller cutting edge area ($A$), so for the same force ($F$), pressure increases.
The reason correctly explains this phenomenon by stating that pressure is inversely proportional to area, which aligns with the formula. Since both the assertion and reason are true, and the reason correctly explains the assertion, the correct option is: