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: Scientific Questioning

c) It compares two distinct conditions and is testable
[Solution Description]
A good scientific question is testable, specific, and leads to further investigation. The given question meets these criteria because:
1) It compares two clear conditions (fresh vs. stored dough).
2) It can be tested through experimentation by controlling other variables.
3) The outcome (puffing behavior) is observable and measurable.
This type of question encourages systematic investigation and deeper understanding.

Key Concept: Scientific Investigation

b) Type of flour used
[Solution Description]
To investigate why one side of a puri is thinner systematically, it is essential to control variables. This means changing only one factor at a time while keeping others constant. Key variables to keep constant include:
1) Type of flour used (e.g., only atta or maida).
2) Size and thickness of the rolled dough.
3) Method of dropping the dough into the oil.
By keeping these variables consistent, we can accurately study the effect of the change in one variable (like temperature of oil) on the outcome.

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: 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: 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: Nature of Scientific Inquiry

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

Key Concept: Curiosity 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 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: 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 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

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

Key Concept: Variables in Experimentation

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

Key Concept: 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: 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: 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: 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: Scientific Questions, Observations

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

Key Concept: 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 in Science

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

Key Concept: Scientific Inquiry Basics

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

Key Concept: Variables in Experiments

d) The temperature of the oil
[Solution Description]
In a scientific experiment about puri puffing, variables that can be tested include dough thickness, oil temperature, type of flour, or pricking holes in the puri before frying. These variables directly influence the puffing behavior and can be systematically tested.

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 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: Careful Observation in Scientific Inquiry

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

Key Concept: Scientific Observation

d) Making careful observations and asking questions
[Solution Description]
The first step in scientific inquiry is developing curiosity and making careful observations about natural phenomena. This leads to asking questions like "Why?" or "How?" which form the basis of further investigation.

Key Concept: Observation and Questioning

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

Key Concept: Systematic Investigation

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

Key Concept: 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: Controlled Experiments

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

Key Concept: 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: 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: Variables identification, Experimental design

a) Type of oil used for frying
[Solution Description]
An extraneous variable is one that could affect the dependent variable but isn't the focus of the study. For this experiment about dough thickness and puffing: The independent variable is dough thickness, the dependent variable is puffing behavior. We need to control variables like oil temperature, cooking time, and flour type that could also affect puffing but aren't being studied.

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: 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: Scientific Inquiry Process

d) Assertion is false, but Reason is true.
[Solution Description]
The assertion states that formulating a hypothesis is the first step in scientific inquiry, which is false because the first step is observation and questioning. The reason states that a hypothesis is a testable prediction that guides experiments, which is true. Therefore, Assertion is false, but Reason is true.

Key Concept: Variables, Scientific Investigation

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

Key Concept: Scientific Investigation

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

Key Concept: Variables, 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: 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: 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: 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: Measuring Changes

c) Whether the puri puffs up or not
[Solution Description]
Dependent variables are outcomes that are measured in response to changes in the independent variable. Here, the dependent variables include whether the puri puffs up, time taken to puff up, and the thickness of the fried puri, as these are the results observed based on oil temperature variations.

Key Concept: Measuring Changes

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

Key Concept: 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: 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: 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: 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: Vapor pressure

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

Key Concept: 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: Designing and Conducting Experiments

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In an experiment, controlled variables are kept constant to isolate the effect of the independent variable (here, oil temperature) on the dependent variable (puri puffing). The assertion states that keeping dough thickness constant is essential, which is correct because it prevents this factor from affecting puffing results. The reason explains why controlling variables is necessary—to ensure only the independent variable influences the outcome. Therefore, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: Variables in an Experiment

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

Key Concept: Identifying Variables

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

Key Concept:  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: 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: Independent and Dependent Variables

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

Key Concept: Controlled variables, Observational outcomes

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that higher oil temperature reduces the puffing time of a puri, which is true because increased thermal energy accelerates the vaporization of water in the dough, leading to quicker expansion. The reason correctly explains this phenomenon by stating that higher oil temperature improves heat transfer, which directly affects the puffing time. Thus, both the assertion and reason are true, and the reason explains the assertion.

Key Concept: Scientific Investigation

c) To keep all conditions the same except for one variable being tested
[Solution Description]
A controlled experiment ensures that only one variable is changed at a time while keeping all other conditions constant. This helps in accurately determining the effect of the independent variable on the dependent variable.

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: Observations and Measurements

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

Key Concept: 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: 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: 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: 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: 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: Observation and Measurement, SI Units

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

Key Concept: Observation and Measurement

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: 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, Quantitative Measurement

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

Key Concept: Systematic investigation

c) To identify which variable causes the observed effect
[Solution Description]
Changing only one variable at a time ensures that any observed effect can be directly attributed to that specific variable. If multiple variables are changed simultaneously, it becomes difficult to determine which variable caused the observed change.
The correct answer highlights this principle: "To identify which variable causes the observed effect."

Key Concept: Measuring responses in experiments

c) Change in the dough’s thickness
[Solution Description]
To determine whether a puri has puffed up completely, we need to observe measurable responses such as whether the puri puffs up (yes/no), the time taken to puff up, or changes in thickness. Among these, checking whether the puri puffed up (yes/no) is a direct observation, while measuring the change in thickness provides quantitative data about the extent of puffing.
The correct answer is "Change in the dough’s thickness" as it gives a measurable response indicating complete puffing.

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: Measurements in scientific experiments

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that measuring time helps understand the effect of temperature on cooking, which is true because time measurement is essential to observe changes during an experiment. The reason correctly explains why time measurement is important in scientific experiments as it allows analysis of changes over a period. Both are true and the reason properly explains the assertion.

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: 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: Effect of Temperature on Puffing

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

Key Concept: Systematic Investigation

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

Key Concept: Controlling Variables

c) Oil temperature, frying time, and dough thickness
[Solution Description] Controlling variables like dough thickness, dropping method, and frying conditions (oil temperature, time) ensures accuracy and helps identify patterns.

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: Recording Observations

c) Maintaining detailed notes of all sensory and measurable changes during the process
[Solution Description]
Detailed and accurate records are crucial for scientific investigations. Observations should include both qualitative (e.g., color change, puffing sound) and quantitative measurements (e.g., dough thickness, frying duration). Recording sensory details like smell or texture also aids in comprehensive analysis.

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 Design

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

Key Concept: Observational Outcomes

b) Whether the puri puffs up (yes/no)
[Solution Description]
Observable outcomes in an experiment are measurable or describable results. For frying a puri, these include whether it puffs up, the time taken to puff up, and the thickness of its sides.

Key Concept: Scientific Method

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

Key Concept: Electricity 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: 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: 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: 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: 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: Investigating everyday phenomena

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Both Assertion and Reason are true. Steam forms inside the puri when heated because the water content in the dough turns into vapor. This steam creates pressure inside the dough, causing it to expand (puff up). Therefore, the Reason correctly explains why the Assertion occurs.

Key Concept: Pressure and Area

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

Key Concept: 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.

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.