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, Experimental Design

c) Systematically varying one factor while keeping others constant
[Solution Description]
A good scientific investigation requires systematic testing of variables while controlling others. The best approach would be to methodically test each potential factor (dough thickness, oil temperature, etc.) independently while keeping other conditions constant, recording observations carefully to identify causal relationships.
Step 1: Identify all possible factors that might affect puri puffing (thickness, oil temp, flour type, dropping method).
Step 2: Design experiments where only one variable is changed at a time.
Step 3: Maintain consistent observation methods for all trials.
Step 4: Record quantitative data where possible (e.g., puffing time).
Step 5: Compare results systematically to draw conclusions.

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: 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: The Importance of Curiosity in Science

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that scientific investigations often start with simple observations and questions related to everyday occurrences. This is true because many scientific discoveries have originated from such observations. The reason mentions that questions like why a puri puffs up when fried stimulate curiosity, which is fundamental to scientific inquiry. This is also true since curiosity drives scientists to explore and understand natural phenomena. Moreover, the reason correctly explains the assertion because curiosity, triggered by such questions, leads to systematic investigation, forming the basis of science.

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: Curiosity in Science

d) It drives questioning, observation, and deeper exploration of natural phenomena.
[Solution Description]
Curiosity motivates scientists to ask "Why?" and "How?" questions about natural phenomena, leading to observations, hypotheses, and experiments. The syllabus explicitly states that science begins with wonder and simple questions, which evolves into structured investigations. Thus, curiosity is essential for initiating scientific inquiry.

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: Curiosity and Wonder in Science

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that science starts with curiosity and wonder, which is true as mentioned in the syllabus. The reason explains that asking "Why?" and "How?" questions aids scientific exploration, which is also true and directly supports the assertion. Therefore, both are correct, and the reason correctly explains the assertion.

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 Investigation, Controlled Experiments

c) Preparing puris with different thicknesses but using the same oil type, temperature, and frying time
[Solution Description]
A controlled experiment requires changing only one variable (dough thickness) while keeping all other conditions (oil type, oil temperature, frying duration) constant. Option c) describes an experiment where dough thickness varies, but all other factors remain unchanged, ensuring that any observed differences in puffing are due to dough thickness alone.

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 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: 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: Designing Experiments

c) Angle of the ramp
[Solution Description] The independent variable is the factor that is deliberately changed in an experiment. Here, the angle of the ramp is manipulated to observe its effect on the speed of the ball. Therefore, the angle of the ramp is the independent variable.

Key Concept: Variables, Controlled Experiments

a) Keeping the oil temperature constant throughout the experiment
[Solution Description]
The student must control all other variables that could affect the puffiness of puris, such as the oil temperature and dough thickness, while only changing the amount of baking soda. This ensures that any observed changes in puffiness are solely due to the variation in baking soda.

Key Concept: Designing experiments and making observations

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 ensures the observed effect is due to that variable alone, which is true as it follows the principle of controlled experiments. The reason correctly explains this by stating that this method isolates the cause by removing interference from other variables.
Step-by-step explanation:
1. In designing experiments, controlling variables is essential to determine causality.
2. Changing one variable while keeping others constant (controlled) ensures any change in the outcome can be attributed to the altered variable.
3. This prevents confounding effects where multiple variables could influence the result simultaneously.
4. Thus, the reason provides the correct explanation for the assertion.

Key Concept: Observation and Questioning

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 is an example of scientific investigation, which is true because it involves noticing a natural phenomenon. The Reason explains that scientific investigation starts with noticing details and asking testable questions, which is also true and directly explains why the observation of the puri puffing is part of scientific investigation. Therefore, both Assertion and Reason are true, and Reason correctly explains Assertion.

Key Concept: Scientific Inquiry, Observation and Experimentation

c) To isolate the effect of each variable
[Solution Description]
Changing only one variable at a time helps in identifying the exact cause of the observed effect. If multiple variables are changed simultaneously, it becomes difficult to determine which variable caused the change in the outcome.

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: 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: 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: 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: Observation, Hypothesis Formation

c) Vary the moisture content of the dough and observe swelling behavior
[Solution Description]
To test the hypothesis that steam causes the swelling, the experiment must manipulate moisture content. One approach is to prepare dough with varying moisture levels (e.g., dry, normal, and extra moist) and observe if swelling changes. Another method is to measure the internal temperature of the phulka during cooking to confirm steam formation. The key is ensuring only moisture content varies while keeping other conditions consistent.

Key Concept: Experimental Design

d) All of the above except thickness variation
[Solution Description]
To isolate the effect of dough thickness on puri puffing, variables like oil temperature, type of flour, and method of dropping the dough must remain constant. Changing only one factor at a time ensures accurate observations.

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 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: 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: 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: Analysis and Conclusion

c) $t_{puff} \propto \frac{1}{T_{oil}}$
[Solution Description]
The given relationship shows that $t_{puff}$ is inversely proportional to $T_{oil}$. As $T_{oil}$ increases, $t_{puff}$ decreases, following the hypothetical equation: $t_{puff} \propto \frac{1}{T_{oil}}$. Hence, the correct choice reflects this inverse proportionality.

Key Concept: Hypothesis and Controlled Variables

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that increasing oil temperature speeds up puffing due to higher energy transfer, which aligns with the principle that heat accelerates cooking processes. The Reason correctly explains that controlling other variables ensures only temperature affects the result. Since both statements are true and the Reason logically justifies the Assertion, the correct choice is option (a).

Key Concept: Observation, Data Analysis

d) Higher temperatures generally improve puffing success rates
[Solution Description]
The data shows a higher success rate of puffing at 200°C compared to 160°C. This suggests a positive correlation between oil temperature and puffing success. However, since it's an observational result, causation cannot be definitively established without considering other variables like dough consistency or experimental errors.

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 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: Scientific Inquiry, Experiment Design

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that maintaining constant dough thickness while varying oil temperature isolates the effect of oil temperature on puri puffing. This is correct because controlling other variables ensures that only the independent variable (oil temperature) affects the outcome. The reason explains why controlling variables is important - to establish a clear cause-effect relationship by eliminating confounding factors. Since both the assertion and reason are true, and the reason correctly explains the assertion, the correct answer is option a).

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: 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: 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: Observation Recording

b) To keep track of all changes and findings
[Solution Description]
Recording observations helps in analyzing the results and drawing conclusions. It includes noting details like whether the puri puffed up, time taken, oil temperature effects, etc., which are crucial for refining questions and understanding patterns.

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: 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, Observation

c) Calculating the ratio of final expanded volume to initial dough volume
[Solution Description]
Quantitative measurement requires numerical data. Among the options, measuring the final volume ratio (puffed vs unpuffed state) provides the most direct and objective quantitative measurement of puffing degree.
Calculation approach:
1) Measure initial dough ball diameter ($D_i$)
2) After frying, measure puffed puri height ($H_p$) and width ($W_p$)
3) Volume ratio = \[
R=
\frac{\pi \left(\frac{W_p}{2}\right)^2 H_p}
{\frac{4}{3}\pi \left(\frac{D_i}{2}\right)^3}
\]
4) Higher ratio indicates better puffing
Other options either lack quantification (visual inspection) or measure indirect aspects (time).

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: 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: Investigating Variables

a) Temperature of the hot oil
[Solution Description]
A controlled variable is what we intentionally change during an experiment while keeping other factors constant. In the puri puffing experiment, examples include the thickness of the dough, type of flour, oil temperature, and method of dropping the dough.

Key Concept: Oil Temperature Impact

c) Moderate temperature allows one side to become thinner while puffing
[Solution Description]
The temperature of the oil determines how quickly water in the dough turns to steam. If the oil is too cold, the dough absorbs oil without puffing. If too hot, it burns before proper puffing. Optimal temperature causes quick steam formation, leading to puffing with one side thinner as steam escapes unevenly.

Key Concept: Identifying variables, experimental observation

b) Oil temperature affects puffing rate but has an upper limit for optimal results
[Solution Description]
The controlled variables in this experiment are dough thickness and oil quantity, while the independent variable being changed is oil temperature. The observations show a relationship between temperature and puffing time (higher temp = faster puffing), but also reveal an upper limit where excessive temperature causes burning. This demonstrates that while temperature affects puffing rate, there's an optimal range beyond which quality degrades.

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: Heat transfer

b) Oil temperature
[Solution Description]
The rate of heat transfer $Q = mc\Delta T$ depends on mass (m), specific heat capacity (c), and temperature difference $\Delta T$. Among the given options, oil temperature directly affects $\Delta T$, which is the driving force for heat transfer.

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, variables control

c) Dough thickness and type of flour
[Solution Description]
To isolate the effect of oil temperature on puffing time, all other variables must be controlled (kept constant). These include:
1. Dough thickness - affects heat penetration rate.
2. Type of flour - changes dough elasticity and water content.
3. Method of dropping dough - influences initial heat distribution.
Only oil temperature should be varied as per the hypothesis.

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 Error and Reproducibility

b) Not maintaining the same oil temperature for each trial
[Solution Description]
Inconsistent results often arise due to uncontrolled variables. Here, not maintaining the exact oil temperature across trials would lead to variation in puffing times. Other options (measuring device, hypothesis, or flour type) are less likely if they were kept constant as required. Thus, option b) 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: Identifying Variables and Experimental Design

c) Using different types of flour while keeping oil temperature, dough thickness, and frying method the same
[Solution Description]
To test the effect of flour type, the independent variable must be the type of flour (e.g., whole wheat, all-purpose), while dependent variable is puffing time. Control variables include oil temperature, dough thickness, and frying method. Only option c) follows this principle correctly by changing only the flour type while keeping other conditions fixed.

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: what can be controlled and observed

(b) Type of flour and oil temperature
[Solution Description]
To ensure accuracy in this experiment, only one variable (dough thickness) should change while keeping all other factors constant. The controlled variables include type of flour, oil temperature, and method of dropping dough. Changing any of these would affect the observed outcome (puffing). The correct answer is the option that lists these variables as needing to remain unchanged.

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: 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: Observing Outcomes

c) Time taken for the puri to puff up
[Solution Description]
Observable or measurable outcomes are the results that are tracked during an experiment to analyze the effect of changes made to controlled variables. Examples include whether a puri puffs up, time taken to puff up, and final shape and texture.

Key Concept: Controlled Experiment, Independent vs Dependent Variables

d) Keeping all conditions identical except dough thickness
[Solution Description]
A controlled experiment requires changing only one independent variable (dough thickness) while keeping all other factors (flour type, oil temperature, dropping method) constant. This allows the observed changes in puffing (dependent variable) to be solely attributed to variations in dough thickness. Randomly changing multiple variables would make it impossible to determine which factor caused the observed effects.

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: Variables in Experiments

d) The variable that is deliberately changed by the experimenter
[Solution Description]
An independent variable is the variable that is deliberately changed or manipulated in an experiment to observe its effect on the dependent variable. For example, in the puri puffing experiment, the thickness of the dough is an independent variable because it can be changed to see how it affects puffing.

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: Identifying Variables

b) To keep all conditions the same except for one factor being tested
[Solution Description]
Changing only one variable at a time helps isolate its effect on the outcome. This ensures that any change observed is due to that specific variable and not influenced by others.

Key Concept: Recording Observations in Experiments

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that keeping detailed records of experimental steps and observations is important for scientific research, which is true. The reason given is that accurate records help ensure experiments can be repeated by other scientists, which is also true and directly explains why keeping records is important. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

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: Recording Data

c) They provide additional data that may reveal new patterns or problems
[Solution Description]
Unexpected observations can lead to new insights or questions, helping refine the scientific understanding and design of future experiments.

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: Scientific Principles

b) To ensure clear and reliable results
[Solution Description]
Changing only one variable ensures that any observed effects can be directly attributed to that variable, maintaining accuracy and reliability of results.

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

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: 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: Controlled Variables

b) To avoid changing multiple variables at once
[Solution Description]
In a controlled experiment, only one variable (independent variable) should be changed at a time while keeping all others constant (controlled variables). If the thickness of the dough varies, it could affect the puffing independently of the oil temperature, making it difficult to isolate the effect of temperature alone.

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: Measurement and Observation

b) Ask a scientific question based on observations
[Solution Description]
The first step in conducting a scientific investigation is to ask a scientific question based on observations. This helps in identifying what needs to be studied or tested.

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: 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: 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: Measurement Precision, Variable Isolation, Consistency

b) To ensure that differences in puffing are only due to dough freshness.
[Solution Description]
Keeping oil temperature and frying method constant ensures that any observed differences in puffing are solely due to the dough's freshness. If these variables change, the effects of freshness cannot be isolated, making the comparison unreliable and the conclusions invalid.

Key Concept: Observation Importance

b) The sound of oil splattering and puffing of puris
[Solution Description] Observations in experiments include sensory details like splattering, smell, or smoke, which lead to deeper questions.

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: 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: Variables in Scientific Inquiry

b) Type of flour used for the dough
[Solution Description]
A controlled variable is a factor that remains constant during an experiment to ensure that any changes observed are due to the manipulated variable only. In the context of frying a puri, controlled variables include factors like thickness and size of the dough, type of flour, and method of dropping the dough into the oil.

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: Scientific Investigation

c) To isolate the effect of one specific factor on the outcome
[Solution Description]
Controlling variables helps isolate the effect of one specific factor on the outcome. By changing only one variable at a time while keeping others constant, we can accurately determine how that single variable influences the puffing of the puri. This ensures reliable and valid results.

Key Concept: Observation, Scientific Inquiry

b) Steam is seen escaping from the phulka as it swells.
[Solution Description]
Scientific hypotheses are based on observable phenomena that can be tested. The formation of steam inside the phulka due to heat causing expansion is a measurable and testable observation, unlike subjective experiences like taste or unrelated factors like utensil material.

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: 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: Electricity and Technology

b) Using the heating effect of electric current
[Solution Description]
Electricity produces heat through the heating effect of electric current, which is utilized in devices like heaters and electric blankets to provide warmth during cold weather.

Key Concept: Health and Medicine

c) To boost immunity against specific diseases
[Solution Description]
Vaccines are developed using scientific methods to protect individuals from infectious diseases by stimulating the immune system to recognize and combat pathogens without causing the disease itself.

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: 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: Phases of the Moon

c) Less sunlight reflects off the Moon as it orbits
[Solution Description]
After a full moon, the Moon's illuminated portion decreases as it moves in its orbit around Earth. This makes the Moon appear smaller (waning phase) until it reaches a new moon. The change is due to the relative positions of the Sun, Earth, and Moon.

Key Concept: Particles and States of Matter

c) Particles in solids vibrate but cannot move freely, while gas particles move freely due to weaker forces and larger spaces.
[Solution Description]
In solids, particles are tightly packed in a fixed arrangement, held by strong intermolecular forces, restricting their movement to small vibrations. In gases, particles are far apart with weak intermolecular forces, allowing them to move freely and rapidly in all directions.