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: 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: Scientific Observation

c) How you feel about cooking
[SolutionDescription]
While smell, puff time and oil splatter are relevant observations mentioned in the syllabus, personal feelings aren't part of scientific recording.

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: Role of Observation & Creativity

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

Key Concept: Scientific Curiosity

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

Key Concept: 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 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: 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: 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: 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: 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: Controlled Experiment

d) To establish a clear cause-and-effect relationship
[Solution Description]
Changing only one variable at a time ensures that any observed effect can be directly attributed to that specific variable. This helps in establishing a cause-and-effect relationship and eliminates confusion from multiple variables affecting the outcome simultaneously.

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: 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: 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: Controlled Experiments, Variables

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

Key Concept: 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: 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: 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 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, Controlled Experiments

a) Size of the rolled dough
[Solution Description]
To study the effect of oil temperature (independent variable), all other factors like dough thickness, type of flour, and method of dropping the dough must be kept constant. This ensures that any change in puffing is only due to the oil temperature and not other variables.

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: 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: Scientific Observation

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

Key Concept: Observation, Experimentation

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

Key Concept: Scientific Observation

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: 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: Scientific Questions and Observations

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

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: Controlled Experiments

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

Key Concept: Observation, 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: 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: Scientific process, Iterative investigation

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

Key Concept: Scientific Inquiry Basics

c) Asking a scientific question
[Solution Description]
The first step in any scientific inquiry is to ask a question about the observed phenomenon. Based on the syllabus, science begins with curiosity and careful observation, followed by formulating a scientific question.

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: 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: Investigating Everyday Phenomena

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 due to trapped steam, which is true because heating causes water in the dough to vaporize. The reason explains that this steam creates pressure, leading to expansion, which is scientifically accurate and directly explains the assertion. Both statements are correct and logically connected.

Key Concept: Variables, Heat Transfer

d) The hotter oil puri will start puffing sooner but may not expand as fully
[Solution Description]
Higher oil temperature leads to faster steam generation inside the dough, causing more rapid puffing. However, excessive heat can prematurely set the crust before full expansion occurs.
Detailed explanation:
1) Steam formation rate increases with temperature (higher heat transfer)
2) Faster steam pressure buildup causes quicker puffing initiation
3) But if too hot, outer layer cooks too fast, limiting expansion
4) Optimal puffing requires balanced steam generation and dough flexibility
5) Therefore slightly higher temp puri should puff faster but may achieve similar final size
So the correct answer describes this nuanced temperature effect.

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: 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: Scientific Question

b) How does oil temperature affect puffing time?
[Solution Description]
Scientific questions focus on testable variables like those listed in the syllabus—thickness of dough, oil temperature, or method of dropping the dough. A valid question should aim to explore cause-and-effect relationships.

Key Concept: Phenomenon of Puri Puffing, Heat Transfer

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

Key Concept: 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: 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

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

b) The puri puffs up more quickly but may burn.
[Solution Description]
Increasing the oil temperature causes the water in the dough to evaporate faster, creating steam that makes the puri puff up more quickly. However, if the oil is too hot, the puri may brown too fast or even burn before it puffs properly.

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: 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 approach

c) Alter only one variable while keeping others constant
[Solution Description]
In a controlled experiment, only one variable should be changed at a time while keeping all other factors constant to isolate its effect. This ensures the results are attributable to that specific variable.

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: Data interpretation, multi-variable analysis

c) Higher temperature and lower thickness reduce puffing time
[Solution Description]
Analyzing the data step-by-step:
1. Comparing rows 1 & 2: At same temp (180°C), thicker dough (3mm vs 2mm) takes longer (6s vs 4s) → thickness increases time
2. Comparing rows 1 & 3: Same thickness (2mm), higher temp (200°C vs 180°C) reduces time (3s vs 4s) → temperature decreases time
3. Combined effect shows both variables significantly impact puffing time.

Key Concept: Experimental Design

c) To isolate the cause-and-effect relationship
[Solution Description]
Changing only one independent variable at a time helps ensure that any observed changes in the dependent variable are solely due to the manipulated independent variable and not influenced by other factors. This maintains the reliability of the results.

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: 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: 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: 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: Controlled Experiment Design

b) Keeping all variables except flour type constant
[Solution Description]
To isolate the effect of flour type (independent variable), other variables (like oil temperature, dough thickness, and dropping method) must be controlled (kept constant). This prevents interference with the observed 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: 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: 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: Data Collection, Qualitative vs Quantitative Observations

b) As a categorical variable describing splatter intensity (low/medium/high)
[Solution Description]
Scientific observations can be qualitative (descriptive) or quantitative (numerical). Since "more splattering" is a relative description without exact measurement, it should be recorded as a qualitative observation using standardized descriptors (e.g., low/medium/high splattering) alongside quantitative data like oil temperature. This provides comprehensive evidence for analysis.

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: Variables in Experiments

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that dough thickness is the independent variable in the given experiment, which is true because it is the variable being deliberately altered (e.g., rolled to different thicknesses) to study its impact on puffing behavior. The reason correctly explains why thickness is considered the independent variable—it is the one being manipulated to observe changes in the dependent variable (puffing). Thus, both Assertion and Reason are true, and Reason provides the correct explanation for Assertion.

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: 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: 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 Variables and Conclusions

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

Key Concept: Variable Control, Observation in Experiments

c) It complicates identifying which change affects the outcome
[Solution Description]
Scientific experiments require changing one variable at a time to isolate its effect. Changing both oil temperature and dough thickness simultaneously makes it impossible to determine which variable caused any observed changes in puffing behavior.

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: Tools, Qualitative vs. Quantitative Observation

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

Key Concept: Observation and Measurement

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Observation and measurement are fundamental in science. The assertion states that accurate measurement is essential for valid conclusions, which is true because precise data allows reliable analysis. The reason explains that measurements provide quantitative data for identifying patterns, which is also true and directly supports the assertion. Thus, both are true, and the reason correctly explains the assertion.

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 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: Controlled variables, Observational variables

c) Time taken for the puri to puff up
[Solution Description]
The dependent variable is the one that is observed or measured as a result of changing the independent variable (thickness of dough). Here, the dependent variable would be whether the puri puffs up, time taken to puff up, or changes in texture since these are responses being measured.

Key Concept: Variables in Experiments

c) Time taken for the puri to puff up
[Solution Description]
The dependent variable is what is measured or observed in response to changes in the independent variable. Here, the puffing of puris (whether it puffs up or not, or the time taken to puff) depends on other variables like temperature or thickness of the dough and hence is the dependent variable.

Key Concept: Time Measurement, Observation

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

Key Concept: Controlling Variables

d) All of the above
[Solution Description]
When studying the effect of dough thickness on puri puffing, other variables like oil temperature, type of flour, and method of dropping the dough must be kept constant. Changing multiple variables simultaneously can make it unclear which factor caused the observed result. Consistency in these parameters ensures the experiment isolates the impact of dough thickness alone.

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: 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 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: Analyzing Results

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

Key Concept: Systematic 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: Real-World Application

d) Making careful observations and systematic experiments
[Solution Description]
Careful observation and systematic experimentation are key steps in scientific investigations. They help form hypotheses, test them under controlled conditions, and draw conclusions about everyday phenomena such as puri puffing up.

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: Experimental Control, Data Collection

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

Key Concept: Scientific Method

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

Key Concept: Pressure and Its Effects

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

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

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

Key Concept: Ecosystems and Human Impact, Climate Change

b) Through the release of stored carbon dioxide and reduced carbon absorption
[Solution Description]
Trees absorb carbon dioxide ($CO_2$) from the atmosphere during photosynthesis, which helps regulate the Earth's temperature by reducing greenhouse gases. Deforestation removes these trees, decreasing the amount of $CO_2$ absorbed and increasing its concentration in the atmosphere. $CO_2$ is a greenhouse gas that traps heat, leading to an increase in global temperatures. Thus, deforestation directly contributes to global warming by reducing carbon absorption and increasing greenhouse gas concentrations.

Key Concept: Real-world applications, Pressure

c) Assertion is true, but Reason is false.
[Solution Description]
The puffing up of a puri occurs because the water content in the dough vaporizes into steam when heated, creating pressure inside the dough. This pressure causes the dough to expand, but the reason for uneven expansion is not just the pressure difference—it’s also due to structural weaknesses or thin spots in the dough. Thus, while the assertion correctly identifies the role of steam, the reason incorrectly attributes the uneven swelling solely to pressure differences without accounting for structural variations in the dough. Therefore, Assertion is true, but Reason is false.

Key Concept: Material Behavior, Heat Transfer

c) Time taken decreases
[Solution Description]
The puffing of puri occurs due to the rapid formation of steam inside when water evaporates. Higher oil temperature increases the rate of heat transfer to the puri, causing faster evaporation and reduced time for puffing.
Step 1: Heat transfer rate ($Q$) is directly proportional to the temperature difference between oil and puri ($\Delta T$).
Step 2: Increasing $\Delta T$ by 20°C accelerates heat transfer.
Step 3: Faster heat transfer means quicker formation of steam, reducing the puffing time.
Thus, the time taken decreases as oil temperature increases.

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: 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: 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: Light Reflection, Refraction

b) Focal length increases
[Solution Description]
Focal length of a lens depends on the refractive index contrast between lens material and surrounding medium.
Step 1: Lensmaker's formula: $\frac{1}{f} = (n_{\text{lens}} - n_{\text{medium}}) \left( \frac{1}{R_1} - \frac{1}{R_2} \right)$
Step 2: In air, $n_{\text{medium}} = 1$, so $(1.52 - 1) = 0.52$.
Step 3: In glycerine, $n_{\text{medium}} = 1.47$, so $(1.52 - 1.47) = 0.05$.
Step 4: Since $(n_{\text{lens}} - n_{\text{medium}})$ decreases, $\frac{1}{f}$ decreases, meaning $f$ increases.
Hence, focal length increases when submerged in glycerine.