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: 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: Observation in Science

c) The puri puffed up completely
[Solution Description]
Qualitative observations describe characteristics or qualities that cannot be measured numerically. In the puri experiment, such observations include:
1) Whether the puri puffs up (yes/no).
2) Smell or smoke from the oil.
3) Splattering of oil.
Quantitative observations would involve measurable data like time or temperature.

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: Data Interpretation, Scientific Reasoning

b) Three consistent trials with controlled variables showing the same trend
[Solution Description]
Reliable scientific conclusions require consistent, repeatable results with proper controls. Multiple trials showing similar patterns with controlled variables provide the strongest evidence.
Step 1: Conduct minimum 3 trials per temperature to check consistency.
Step 2: Ensure identical dough composition and thickness for all tests.
Step 3: Verify trend holds across trials (e.g., always faster at higher temps).
Step 4: Check no other variables changed between tests.
Step 5: Statistical analysis showing significant difference strengthens conclusion.

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: 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 Wonder, Hypothesis Formation

d) The role of imaginative questioning in understanding natural phenomena.
[Solution Description]
This question highlights how scientific wonder involves comparing seemingly unrelated large-scale phenomena to understand magnitudes and scales in nature. It encourages abstract thinking and estimation, which are critical for developing hypotheses and models in science.

Key Concept: Nature of Scientific Inquiry

b) To show how science starts with simple observations
[Solution Description]
The text uses the puri example to show how everyday observations can lead to scientific questions. It explains this demonstrates systematic investigation and notes scientists still don't fully understand this phenomenon.

Key Concept: Curiosity, Observation, Scientific Inquiry

d) It motivates asking questions and exploring unknown aspects of observations.
[Solution Description]
Curiosity drives the student to question a common observation, prompting deeper investigation into the underlying principles of heat transfer and material expansion. This initial wonder leads to forming hypotheses and conducting experiments, exemplifying how curiosity fuels scientific inquiry.

Key Concept: Systematic Investigation

b) It ensures that the results are due to the specific variable being tested.
[Solution Description]
Changing only one variable at a time ensures that any observed effect can be attributed specifically to that variable. If multiple variables are changed simultaneously, it becomes difficult to determine which variable caused the observed outcome. This principle is fundamental to maintaining control and validity in experiments.

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 at a time in an experiment ensures accurate results, which is correct as per scientific methodology. The reason provided explains why this is necessary: keeping other variables constant helps isolate and identify the specific effect of the changed variable. This explanation correctly justifies the assertion. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Observation, Hypothesis Formation

b) Higher oil temperatures increase water vaporization in the dough, leading to larger bubble formation
[Solution Description]
A valid hypothesis should propose a testable explanation for the observation (larger bubbles at higher temperatures). It must connect the independent variable (oil temperature) to the dependent variable (bubble size) with a plausible cause-and-effect relationship. Option b) suggests that increased vaporization due to higher temperature leads to larger bubble formation, which aligns with basic principles of heat and phase changes.

Key Concept: Observation and Measurement

b) To compare results under different conditions
[Solution Description]
Measuring the time taken for a puri to puff up provides quantitative data that can help in understanding how different factors (like oil temperature or dough thickness) affect the puffing process. This measurement allows for objective analysis and comparison between different experimental conditions.

Key Concept: Variables in Experimentation

c) Whether the puri puffs up or not.
[Solution Description]
Dependent variables are the outcomes that are measured in an experiment. In the puri experiment, whether the puri puffs up (yes/no), time taken to puff up, and thickness of the puffed side are dependent variables because they depend on changes made to independent variables like oil temperature or dough thickness.

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: Making Observations

b) Increased molecular motion due to heat
[Solution Description] The rate at which a solute (sugar) dissolves in a solvent (water) increases with temperature because higher temperatures provide more energy to break the bonds between solute particles and allow them to mix more freely with the solvent molecules.

Key Concept: Scientific Questions, Observations

c) Recording the height of plants daily using a ruler
[Solution Description]
Quantitative data involves measurable numerical values. Measuring the height of plants daily provides specific, repeatable data that can be analyzed statistically, unlike qualitative descriptions like "healthy" or "green."

Key Concept: Variables in Experiments

c) Type of flour used
[Solution Description]
A controlled variable is one that is kept constant during the experiment. In studying puri puffing, factors like type of flour and oil temperature are examples of controlled variables when changed intentionally between trials.

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: Variables in Experiments

d) To isolate the effect of the changed variable
[Solution Description]
Changing only one variable at a time ensures that any observed effect can be attributed solely to that variable. This helps in identifying cause-and-effect relationships accurately.

Key Concept: Observation in Science

c) Sensory details like smell, sound, and visual changes
[Solution Description]
During a scientific investigation, meticulous recording of sensory observations such as smell, sound, visual changes (e.g., puffing), oil splatter, and temperature changes is crucial. These details help in analyzing the results and forming conclusions.

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 Investigation Steps

c) To identify the effect of one variable and ensure reliable results
[Solution Description]
In scientific investigations, changing only one variable at a time helps in isolating the effect of that specific variable on the outcome. If multiple variables are changed simultaneously, it becomes difficult to determine which variable caused the observed change. This approach ensures accurate and reliable results.

Key Concept: Careful Observation in Scientific Inquiry

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that a puri puffs up when fried due to steam formation inside it, which is correct. The reason states that careful observation helps understand such phenomena, which is also true and directly explains why we can conclude the assertion. Hence, both are true, and the reason correctly explains the assertion.

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: Types of Observations

c) The puri took 30 seconds to puff up
[Solution Description]
Quantitative observations involve numerical measurements, such as counting or using instruments to obtain measurable data, unlike qualitative observations which describe characteristics without numbers.

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 Investigation, Variables

a) Thickness of the dough and oil temperature
[Solution Description]
In a controlled experiment where the effect of oil temperature is being tested, other variables must remain constant to isolate the impact of temperature. The thickness and size of the dough, type of flour ($atta$ or $maida$), and how the dough is dropped into the oil should all be kept the same. Only the oil temperature should vary.

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: Controlled Experiments

c) To isolate the effect of that variable
[Solution Description]
In a controlled experiment, changing only one variable at a time helps to isolate its effect on the dependent variable while keeping other factors constant. This ensures that any observed changes are due to the manipulated variable alone.

Key Concept: Controlled Experiments, Iterative Process

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion claims that changing multiple variables simultaneously speeds up finding causal relationships, which is false because doing so introduces ambiguity in identifying which variable caused the observed effect. The Reason is true because controlling one variable at a time isolates its impact, eliminating other influences.
Therefore, the Assertion is false, but the Reason is true.

Key Concept: Experimental Design, Variables

b) Whether the puri puffs up or not
[Solution Description]
The dependent variable is what is measured or observed in response to changing the independent variable (flour type). Here, successful puffing (a measurable outcome) is the best choice because it directly reflects the effect of flour type. Other options are either irrelevant or represent controlled variables.

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 Observation

b) They might lead to new scientific questions
[Solution Description]
The syllabus states that unexpected observations may lead to new questions. Detailed recording helps refine understanding and guides further investigation, which is fundamental to scientific inquiry.

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: 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: 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: Observation and Questioning in Scientific Inquiry

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 when fried leads to scientific questions, which is true as observation is the first step in scientific inquiry. The Reason explains that scientific inquiry starts with curiosity-driven observations, which is also true and correctly explains why the Assertion occurs. Both statements are true, and the Reason logically explains the Assertion.

Key Concept: Scientific Investigation Steps

b) Ask a scientific question
[Solution Description]
The first step in scientific investigation is to ask a scientific question based on observations. According to the syllabus, it involves identifying what you want to explore, such as why a puri puffs up when fried or how different variables affect this process.

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

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 is essential in scientific investigation, which is true as per the syllabus which mentions "Changing one variable at a time while keeping others constant". The reason given is that changing multiple variables makes it difficult to determine causation, which is also true and directly explains why controlling one variable is important. Therefore both are true and reason correctly explains the assertion.

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: Oil temperature, Puffing time

b) The puri absorbs excess oil and fails to puff properly
[Solution Description]
If the oil temperature is too low, the puri will absorb more oil before the outer layer sets, making it greasy. The moisture inside the dough will not evaporate quickly enough to create sufficient steam for puffing. This results in either partial puffing or no puffing at all. The ideal temperature ensures rapid vaporization of moisture, forming steam that expands the dough evenly.

Key Concept: Effect of Dough Thickness

c) Moderately thin dough results in one side being thinner than the other
[Solution Description]
The puffing of a puri depends on how steam gets trapped inside the rolled dough. A thicker dough may not allow uniform expansion, while a thinner one puffs more easily due to rapid vaporization of moisture and thin layers separating quickly. Therefore, an extremely thick dough might not puff properly compared to a moderately thin one.

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: Observation

d) The thin side expands due to trapped steam pressure
[Solution Description]
When a puri is dropped into hot oil, steam forms inside due to trapped moisture. The steam pushes upwards against the thinner side first, causing it to inflate more while the thicker side remains denser. This makes one side appear thinner than the other.

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

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

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: Measuring Changes

c) The upward-facing side expands more, becoming thinner.
[Solution Description]
When the puri is placed in hot oil, steam forms inside the dough, causing it to puff. The side facing upwards expands more due to direct exposure to heat, making it thinner compared to the lower side, which remains slightly thicker.

Key Concept: Hypothesis formation, Experimental variables

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion states that uniform dough thickness prevents puffing due to no steam pressure differential. This is false because puffing can still occur if steam accumulates uniformly inside and inflates the dough without escaping preferentially from one side. The Reason correctly explains why puffing happens in typical cases where one side is thinner—steam escapes unevenly, creating pressure differences. Thus, the Assertion is false, but the Reason is true.

Key Concept: Variables in Experiment

c) Independent variable
[Solution Description]
In an experiment, the independent variable is the one that is intentionally changed to study its impact. For example, changing the oil temperature while frying puris to see how it affects puffing time.

Key Concept: Experimental design, pressure-temperature relationship

b) Pressure becomes half
[Solution Description]
Given $P \propto \frac{T}{V}$:
1. Temperature ($T$) is constant (given)
2. Final volume $V_f = 2V_i$
3. Thus pressure ratio $\frac{P_f}{P_i} = \frac{T/V_f}{T/V_i} = \frac{V_i}{V_f} = \frac{1}{2}$
The pressure reduces to half its initial value when volume doubles at constant temperature.

Key Concept: Hypothesis Formation

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that the hypothesis about uneven steam escape can be tested by rolling dough uniformly. This is true because varying thickness is an independent variable, and observing puffing tests the hypothesis. The Reason explains the purpose of controlled experiments, which is valid. However, the Reason does not directly explain why rolling dough uniformly tests the hypothesis; it instead provides a general principle of experimentation. Thus, both are true, but the Reason does not correctly explain the Assertion.

Key Concept: Analysis and Conclusion from Data

a) Puffing time increases with increasing dough thickness
[Solution Description]
The table shows that as dough thickness increases, puffing time also increases. Thin dough takes 5 seconds, medium takes 10, and thick takes 20. This suggests a direct proportional relationship ($\text{Puffing Time} \propto \text{Dough Thickness}$). Thus, option a) is correct.

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: Variables in an Experiment

b) Oil temperature
[Solution Description]
The independent variable is the factor that is deliberately changed or manipulated in an experiment to observe its effect. Here, the experiment aims to study how changing oil temperature affects puffing time. Therefore, the oil temperature is the independent variable.

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: Independent and Dependent Variables

b) Dough thickness
[Solution Description]
According to the key principle, when studying the effect of one independent variable (oil temperature), all other variables must be kept constant. Therefore, factors such as dough thickness, flour type, and dropping method must remain unchanged during the experiment.

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: Identifying Variables

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The temperature of the hot oil is listed as a controlled variable in the syllabus, meaning it can be adjusted during the experiment. The reason correctly defines controlled variables as those that can be changed, which aligns with the assertion.

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: Observations and Measurements

d) Timer
[Solution Description]
Measuring the time taken for a puri to puff up requires a timing device such as a timer or stopwatch. Rulers are used for measuring length, thermometers for temperature, and weighing scales for mass.

Key Concept: Experimental Variables, Observation, Systematic Investigation

c) Recording the time taken for the puri to puff up and checking if it puffs uniformly
[Solution Description]
To systematically study the effect of oil temperature, the student should change only one variable (oil temperature) while keeping others like dough thickness, flour type, and dropping method constant. The student can measure the time taken for the puri to puff up (quantitative observation) and whether it puffs uniformly (qualitative observation). This ensures a controlled experiment where the change in puffing behavior can be directly attributed to the oil temperature.

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: 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: 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: Data Interpretation, Drawing Conclusions

c) Plant growth is maximum at 25% fertilizer concentration
[Solution Description]
The data shows initial increase in height with 25% fertilizer, but higher concentrations reduce growth. Optimal growth occurs at 25%, suggesting excessive fertilizer inhibits growth.

Key Concept: Variables, Data Analysis

c) Temperature and oxygen production
[Solution Description]
The independent variable is the factor deliberately changed (temperature). The dependent variable is the outcome measured (oxygen production). Control variables could include light intensity, CO$_2$ concentration, and plant species.

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: Observation, Quantitative Measurement

c) The puri puffs up in 3 seconds
[Solution Description]
Observation and measurement show that higher temperatures generally reduce reaction times. Here, increasing the oil temperature from 180°C to 200°C would likely decrease the time taken for the puri to puff up. Thus, the new puffing time will be less than 4 seconds.

Key Concept: Observation and Measurement

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that accurate measurement of oil temperature is essential for determining the puffing time of puris, which is true because temperature plays a crucial role in cooking processes. The reason explains that temperature affects the rate at which moisture turns to steam, leading to puffing, which is also scientifically correct. Furthermore, the reason logically explains why the assertion is true—higher temperatures cause faster steam formation, reducing puffing time, while lower temperatures do the opposite. Hence, both statements are true, and the reason correctly explains the assertion.

Key Concept: Variables in Experiments

b) Dough thickness
[Solution Description]
In this experiment, the independent variable is the factor being deliberately changed (dough thickness), while other variables like oil temperature are kept constant.

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 in Experiments

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that when investigating why a puri puffs up, only one variable should be changed at a time. This is true because scientific experiments require controlled conditions to isolate the effect of individual factors. The reason explains that changing multiple variables at once complicates identifying the exact cause of the observed response, which correctly justifies the assertion. Therefore, both assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Systematic observation, Variable control

b) Thickness of dough, oil temperature, type of flour, and dropping method
[Solution Description]
For a valid experiment, all other variables must be controlled (kept constant) except the one being tested (hole-pricking). This includes thickness of dough, oil temperature, type of flour, and method of dropping the dough.

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: Observation, Controlled Experiment

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 dough affects the puffing time of a puri due to altered heat transfer rate, which aligns with the principle of systematic investigation where varying one parameter (thickness) impacts the observable outcome (puffing time). The Reason correctly explains that heat transfer rate is inversely proportional to thickness, meaning thicker dough leads to slower heat transfer and longer puffing time. Both statements are conceptually correct, and the Reason directly justifies the Assertion.

Key Concept: Effect of Temperature on Puffing

a) Oil is too cold
[Solution Description]
The puffing of a puri depends on the temperature of the oil. If the oil is not hot enough, the water inside the dough does not turn to steam quickly, preventing the puri from puffing. The ideal frying temperature for puris is around 180°C to 200°C, as this ensures rapid steam formation and even cooking. Thus, insufficient oil temperature leads to no puffing.

Key Concept: Systematic Investigation in Experiments

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 during an experiment ensures accurate results, which is true because this approach helps isolate the effect of that specific variable. The reason given is that altering multiple variables simultaneously makes it difficult to determine which change caused the observed effect, which correctly explains why changing one variable at a time is important for systematic investigation. Both the assertion and reason are true, and the reason provides the correct explanation for the assertion.

Key Concept: Controlled Experimentation

b) To ensure the effect is due to that specific variable
[Solution Description]
Changing only one variable at a time ensures that any observed effect can be directly attributed to that specific variable, making the experiment reliable and conclusions valid.

Key Concept: Measurement of Time in Experiments

c) Stopwatch
[Solution Description]
To measure time accurately in experiments, especially short durations like the puffing time of a puri, a stopwatch is the most suitable tool. It provides precise measurements in seconds or milliseconds, making it ideal for tracking fast processes. Kitchen timers may lack precision, and analog clocks are not accurate enough for such small time intervals.

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: Observation and Measurement

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description] The Assertion is correct because systematic recording of observations, including sensory details, helps in maintaining accuracy by providing a comprehensive dataset. The Reason is also correct as sensory details add qualitative insights that can complement and validate quantitative measurements. Moreover, the Reason correctly explains why the Assertion is true, as qualitative data enhances the reliability of observations.

Key Concept: Systematic Investigation

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description] The syllabus states that keeping notes of everything observed during an experiment is essential for systematic investigation (Assertion A). It also mentions that while performing experiments, it is better to change only one thing at a time (Reason R). Both statements are true, but Reason R does not explain why keeping notes is a part of systematic investigation. Therefore, the correct answer is b).

Key Concept: Systematic Investigation

d) Altering one variable at a time while controlling other factors
[Solution Description]
To systematically investigate why puris puff up, a scientist should conduct controlled experiments by altering one variable at a time while keeping others constant. For instance, they can test how thickness or size of the dough affects puffing while maintaining consistent frying temperature and oil quality.

Key Concept: Effect of Dough Thickness on Puri Puffing

c) Assertion is true, but Reason is false.
[Solution Description]
The assertion states that a thinner layer of dough results in a puffier puri when fried in hot oil, which is true because thinner dough cooks faster and creates an even steam pressure distribution, leading to better puffing. However, the reason given is incorrect because thinner dough does not allow steam to escape more easily; instead, it traps steam effectively, causing puffing. Thus, the assertion is true, but the reason is false.

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: Variables, Controlled Experimentation

d) Assertion is false, but Reason is true.
[Solution Description]
First, let's analyze the Assertion: While it's true that thicker dough may initially seem to trap more steam, excessive thickness can prevent uniform heating and lead to uneven puffing or no puffing at all. Thus, the Assertion is not universally correct. Next, the Reason is scientifically accurate—the puffing occurs due to water vaporization creating steam pockets within the dough. Therefore, while the Reason is true, it does not correctly explain the Assertion, as the Assertion itself is false under certain conditions.

Key Concept: Scientific Variables, Experiment Design

c) Using the same dough circles and dropping method but varying the oil temperature.
[Solution Description]
To study the effect of oil temperature alone, all other variables must be kept constant. This means using identical dough circles (same thickness, size, and flour type), dropping them in the same way into oils at different temperatures, and measuring the puffing outcome.

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: 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: Understanding Forces and Pressure

b) Because of the gravitational pull of the Earth
[Solution Description]
A ball thrown upward falls back due to the force of gravity acting downward. Gravity is a fundamental force that attracts objects towards the center of the Earth, causing them to return to the ground after being projected upward.

Key Concept: Electricity and Its Effects, Understanding Forces and Pressure

c) As heat is directly proportional to the square of the current
[Solution Description]
The heating effect of electric current is given by Joule's law: $H = I^2 R t$, where $H$ is the heat produced, $I$ is the current, $R$ is the resistance, and $t$ is the time. Here, heat is proportional to the square of the current. If the current ($I$) is doubled, the heat produced increases by four times because $(2I)^2 = 4I^2$. Thus, increasing the current significantly raises the heat output of the heater when resistance is constant.

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:

Key Concept: Air Pressure, Weather Phenomena

b) Cyclone formation becomes less likely
[Solution Description]
Cyclones form due to differences in air pressure, where warm, moist air rises and creates a low-pressure zone.
Step 1: In a region already experiencing very low pressure, the pressure gradient between surrounding areas and the center reduces.
Step 2: Reduced pressure gradient weakens wind speeds and disrupts cyclone formation.
Therefore, cyclones are less likely to form or intensify in regions with very low atmospheric pressure.

Key Concept: Solutions and Mixtures

c) Salt breaks into $Na^+$ and $Cl^-$ ions
[Solution Description]
When salt (sodium chloride, $NaCl$) dissolves in water, it breaks into ions: sodium ($Na^+$) and chloride ($Cl^-$). This occurs because water molecules surround and separate the salt crystals, allowing them to disperse in the solution.

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 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.