ScienceCBSE Class 8 · Exploring the Investigative World of Science

Asking a Scientific Question and Choosing Variables: Class 8 Science

Updated 17 October 2026 · Fact-checked

A scientific question is one that an experiment can answer. To investigate it, you list the things you are free to change (for example the oil temperature or dough thickness when frying a puri) and the things you can observe or measure (does it puff, and how many seconds does it take). The change is deliberate, the observation is the result.

A scientific question links something you choose to change (oil temperature) with something you can observe or measure (does it puff, how many seconds).

Illustration drawn for QuestionBuddy.

Understand Asking a Scientific Question and Choosing What to Change and Observe

Chapter 1 of Curiosity (Grade 8) is about learning how to find new facts, not only how to learn old ones. It begins with an everyday puzzle: why does a puri puff up in hot oil, and why is one side thinner than the other? Scientists start in exactly this way, by turning a wonder into a question that can be tested.

What makes a question scientific? A question such as 'Why is the puri so nice?' cannot be settled by an experiment, because 'nice' is an opinion. A question such as 'Does the temperature of the oil change whether the puri puffs?' can be settled, because you can fry puris at different oil temperatures and look at what happens. A good scientific question therefore names something you can change and something you can observe.

Things we can change or control. The textbook lists, for the puri: the thickness and size of the rolled dough, the kind of flour (atta, maida and so on), the temperature of the oil, and the way the dough is put into the oil (dropped vertically, slid at an angle, or slid slowly). Each of these is a factor you can deliberately choose.

Things we can observe or measure. Some observations have only a yes or no answer, for example 'does the puri puff up?'. Others give a number, for example 'how many seconds does it take to puff?'. You can also observe whether the puri puffs evenly or whether one side bulges more than the other. Numbers are often more useful than yes or no, because they let you compare two trials more finely.

Notes matter. The textbook also advises writing down everything you see and sense during an experiment: did the oil splatter, smell or smoke? These extra notes often give you the next question, such as 'Do puris puff better from fresh dough or stored dough?' or 'What if I prick a hole in the puri first?'.

So the working steps are: notice something, ask a focused question, decide what to change, decide what to observe or measure, and keep notes. The textbook adds that even this common kitchen event is not completely understood by scientists today, which shows that simple questions can lead to open problems.

The next topic in this chapter, fair testing, shows how to change these factors one at a time.

Asking a Scientific Question and Choosing What to Change and Observe: formulas and key facts

Parts of an investigation: Question
Factor to change
Observation or measurement
Notes
New questions; Every plan should name each of these before the experiment begins.
Types of observation: Yes/no answer (does it puff?) | Number with a unit (time in seconds); A measurement allows finer comparison than a yes/no answer.
Puri example: factors you can change: Dough thickness and size
Type of flour
Oil temperature
Way of dropping into oil; These four are the ones listed in the chapter.

How to solve Asking a Scientific Question and Choosing What to Change and Observe questions

Use this method when you are asked to frame a question or plan what to change and observe.

1Read the situation and state the puzzle in one line.
2Rewrite it as a question that begins 'Does ... change ...?' or 'What happens to ... when ...?'.
3List every factor you could change. Choose the one factor you want to study.
4State what you will observe. Say whether it is a yes/no result or a number, and give the unit if it is a number.
5Say which other factors will not be changed.
6Note what extra things you will record (smell, sound, splatter, time, temperature).

Asking a Scientific Question and Choosing What to Change and Observe in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Turning a kitchen wonder into a testable question

Suppose Meera notices that her rice takes longer to cook on some days than others. She wonders why, and decides to investigate with a small pot, a kitchen timer and a measuring cup.

(a) Write one testable question for Meera. (b) Name two factors she could change. (c) State what she will measure, with its unit. (d) Is 'Which day is the rice tastiest?' a good scientific question? Give a reason.

Try it first, then show the solutionSolution

A testable question names a change and an observation: 'Does the amount of water change the time rice takes to cook?'
Factors she could change: amount of water, size of the flame, soaking the rice before cooking, or the type of rice.
She will measure the time to cook in minutes with the timer; she can also note yes/no whether all grains are soft.
'Tastiest' is a matter of opinion and cannot be measured in a fair way, so it is not a good scientific question.

Answer: (a) Does the amount of water change the cooking time of rice? (b) Amount of water; soaking time (or flame size, rice type). (c) Cooking time in minutes. (d) No, because taste is an opinion and not something measured.

Memory hook: A scientific question has a handle to turn (what I change) and a dial to read (what I observe).

Yes/no versus number observations

Imagine a class tests whether three kinds of paper towels soak up water. Group A only records 'soaked' or 'did not soak'. Group B pours 50 mL of water on each towel and measures how many millilitres are left in the tray.

(a) Which group can say which towel absorbs more? (b) If the towels leave 8 mL, 20 mL and 35 mL in the tray, how much water did each absorb? (c) Which towel absorbed most?

Try it first, then show the solutionSolution

A number lets you rank the towels; yes/no does not separate towels that all soak some water. So Group B.
Water absorbed = 50 mL minus water left: 50 − 8 = 42 mL, 50 − 20 = 30 mL, 50 − 35 = 15 mL.
The towel that left only 8 mL in the tray absorbed the most, 42 mL.

Answer: (a) Group B; (b) 42 mL, 30 mL and 15 mL; (c) the towel that left 8 mL absorbed most (42 mL).

Memory hook: Whenever you can, measure with a number: it lets you compare, not just describe.

Tricks to remember Asking a Scientific Question and Choosing What to Change and Observe

Short memory aids. Say each one aloud once, then use it on a question the same day.

Handle and dial
Handle = the factor you turn or change; dial = the reading you observe or measure.
Use it when: Sorting factors into 'to change' and 'to observe' in a question.
Q-C-O-N
Question, Change, Observe, Notes: the four things to write before starting.
Use it when: Planning answers to 'plan an investigation' questions.
Opinion words are warning words
If the question has words like nicest, best, prettiest, it cannot be tested until you replace them with a measurement.
Use it when: Deciding whether a question is scientific.

Asking a Scientific Question and Choosing What to Change and Observe: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Identify what can be changed in the puri investigation · 2 marks · Short answerBoard level

List four factors a student can change when investigating how a puri puffs up in hot oil, and name two things that can be observed.

Try it first, then show the solutionSolution

Factors: thickness and size of the rolled dough; type of flour (atta or maida); temperature of the oil; the way the dough is dropped in (vertically, at an angle or slowly). (1)
Observations: whether the puri puffs up (yes/no); time taken to puff (in seconds). (1)

Answer: Change: dough thickness, flour type, oil temperature, way of dropping. Observe: puffing (yes/no) and time in seconds.

Example 2: Choose the scientific question · 1 mark · MCQBoard level

Which of the following is a scientific question that can be investigated by an experiment?
(a) Which colour of balloon is the prettiest?
(b) Does the size of a balloon change the time it takes to fall to the floor when dropped from the same height?
(c) Why do people like balloons at parties?
(d) Is flying a kite the best hobby?

Try it first, then show the solutionSolution

(a), (c) and (d) depend on opinion or preference and cannot be measured by an experiment.
(b) names a factor to change (balloon size) and an observation (time to fall) so it can be tested.

Answer: (b)

Example 3: Turning a puzzle into a plan · 5 marks · Long answerHarder board level

A student notices that some seeds in a tray sprout earlier than others and wants to find out why.
(a) Write a testable question about the amount of water. (1)
(b) List three other factors that could be changed in other investigations. (1)
(c) State what the student will observe and measure, with units. (2)
(d) Name two extra things the student should note down besides the main measurement. (1)

Try it first, then show the solutionSolution

(a) Does the amount of water given each day change how many days the seeds take to sprout? (1)
(b) Any three: amount of light, temperature of the room, type of soil, depth at which seeds are planted, type of seed. (1)
(c) Observe: whether a seed has sprouted (yes/no) each day. Measure: the number of days to sprout, and the number of seeds sprouted out of the total. (2)
(d) Any two: the colour or smell of the soil, signs of fungus or insects, the height of the shoots, the room temperature on each day. (1)

Answer: (a) Does the amount of water change the days seeds take to sprout? (b) Light, temperature, soil, depth. (c) Days to sprout and number of seeds sprouted. (d) Any two extra notes such as fungus on soil or shoot height.

Example 4: Design thinking: from one question to the next · 4 marks · Case-basedHarder board level

Read the passage and answer.

A group fries three identical circles of dough, A, B and C, in oil. A is dropped vertically, B is slid in at an angle and C is slid in slowly. A and B puff, but C does not. The group also notes that the oil splattered when A was dropped and that C sank slightly before it floated up.
(a) What factor were the students changing, and what did they observe? (1)
(b) Why is it useful that they noted the splattering and sinking? (1)
(c) Write two new questions their notes could lead to. (1)
(d) A student says 'Slow dropping stops a puri from puffing.' Why is this conclusion too strong from one trial of each? (1)

Try it first, then show the solutionSolution

(a) The way the dough was put into the oil (vertical, angled, slow). They observed whether each circle puffed (yes/no). (1)
(b) Extra notes can hint at other causes or give new questions, for example sinking may mean the oil cooled where the dough went in. (1)
(c) Any two: Does the oil temperature drop more when dough goes in slowly? Does the speed of dropping change the time to puff? Does a hole pricked in the dough change puffing? (1)
(d) Only one trial of each was done; the result may be due to chance or another unnoticed difference, so more trials are needed before such a statement. (1)

Answer: (a) Changed: way of dropping; observed: whether the dough puffed. (b) Extra notes suggest causes and new questions. (c) E.g. does oil temperature drop; does speed change the time to puff. (d) One trial each is too little evidence.

Example 5: Which investigation can give a number · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

Four students want to study how a candle burns. Asha asks 'Is a candle beautiful?'. Bilal asks 'Does the length of a candle change the time for which it burns?'. Chitra asks 'Do all candles smell nice?'. Dev asks 'Does a candle burn faster in a draught of air?'.
(a) Name the two students whose questions are scientific. (1)
(b) For Bilal, what is the factor to change and the quantity to measure, with a unit? (1)
(c) Rewrite Chitra's question so it can be tested. (1)
(d) For Dev's question, say one thing that should be recorded other than the burning time. (1)

Try it first, then show the solutionSolution

(a) Bilal and Dev: each names a factor that can be changed and something that can be measured. (1)
(b) Change: length of the candle; measure: time of burning in minutes. (1)
(c) Example: 'How many of 10 students say candle X smells stronger than candle Y when each is lit for 2 minutes?' It turns a feeling into a count. Another correct rewrite is acceptable. (1)
(d) For example the flame colour or height, how the flame flickers, the length of candle consumed, or smoke. (1)

Answer: (a) Bilal and Dev. (b) Length of candle; time in minutes. (c) Replace 'nice' by a counted observation, e.g. students choosing the stronger smell. (d) Flame behaviour, smoke or length burnt.

Where students lose marks in Asking a Scientific Question and Choosing What to Change and Observe

The slips examiners see most often, and what to write instead.

Mistake: Writing a question about opinion as a scientific question.
Students pick words like 'best' or 'nicest' that cannot be measured.
Wrong working
Which flour makes the best puri?
Right working
Does the type of flour change the time a puri takes to puff, when the oil temperature is the same?
Do this: Replace opinion words by something you can measure or count.
Usually costs: 1 mark in a framing question.
Mistake: Mixing up what to change and what to observe.
Both appear in the same sentence of the question.
Wrong working
Factor changed: time taken to puff. Observed: oil temperature.
Right working
Factor changed: oil temperature. Observed: time taken to puff.
Do this: Ask 'Which did I choose?' (change) and 'Which did I read off?' (observe).
Usually costs: 1 to 2 marks.
Mistake: Writing a measurement without a unit.
Rushing the plan.
Wrong working
Measure the time taken.
Right working
Measure the time taken in seconds with a stopwatch.
Do this: Always add the unit and the instrument.
Usually costs: ½ mark.
Mistake: Changing several factors in one trial.
Students want a quick answer.
Wrong working
Use thick dough in cold oil and thin dough in hot oil, then compare.
Right working
Use the same oil temperature and compare thick and thin dough.
Do this: Change one factor and name the ones kept the same.
Usually costs: 1 mark in a planning question.

Exam tips

Start a planning answer with the testable question, then list 'Change', 'Observe' and 'Keep the same'.
Say whether each observation is yes/no or a number, and add units to numbers.
Use the puri, plant and pendulum style examples from the chapter as models for your own plans.
Mention extra notes such as smell, sound or splatter; the chapter encourages them.
Avoid opinion words in questions; replace them with counts or measurements.

Practice questions on Asking a Scientific Question and Choosing What to Change and Observe

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 2 marks · Very short answerBoard level
State why 'Which sweet tastes the best?' is not a scientific question as it stands, and rewrite it so that it can be tested.
Hint
Taste is an opinion. Think of a count or a vote.
Answer
'Best' is an opinion and cannot be measured (1 mark). Example rewrite: 'How many of 20 students choose sweet A rather than sweet B in a blind tasting?' because it gives a count (1 mark).
Q2 · 1 mark · MCQBoard level
In an investigation on how a puri puffs up, which one of these is something to observe and not something to change?
(a) Type of flour
(b) Temperature of the oil
(c) Time taken to puff
(d) Thickness of the dough
Hint
Which one is a result, not a choice?
Answer
(c) Time taken to puff is a result that is measured; the others are factors the cook chooses.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Measuring the time a puri takes to puff is more informative than only noting whether it puffs.
Reason (R): A number lets two trials be compared by how much they differ, while a yes/no answer cannot separate two trials that both puff.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Does R say why the measurement helps?
Answer
(a) Both are true and R explains A: two puris that both puff look the same in a yes/no record, but their times can differ.
Q4 · 4 marks · Short answerHarder board level
A student wants to find whether the length of a pendulum string changes the time for 10 swings. (a) Write the testable question. (b) Name the factor changed and the quantity measured with its unit. (c) Give two other things she could note down. (d) Why is a time for 10 swings easier to measure than the time for one swing?
Hint
A longer time interval is less affected by reaction delay.
Answer
(a) Does the length of the string change the time taken for 10 swings? (1) (b) Changed: string length (cm); measured: time for 10 swings (s) (1). (c) Any two: how far the bob was pulled aside, the mass of the bob, whether the swing stayed in one line, the room draught (1). (d) A small delay in starting or stopping the stopwatch is a smaller fraction of a longer interval, so the reading is more reliable (1).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Priya says: 'I tried frying one thick puri in cold oil and it did not puff. Another thin puri in hot oil did. So thickness decides puffing.'
(a) Name the two factors that differ between her trials. (b) Explain why she cannot conclude anything about thickness alone. (c) Write a better question about thickness. (d) State what she should record for each puri.
Hint
Two things were different, so you cannot tell which caused the result.
Answer
(a) Thickness and oil temperature. (b) Both factors changed together, so the cause cannot be separated (1 mark each for (a) and (b)). (c) Does the thickness of the dough change whether a puri puffs, when the oil temperature is the same? (d) Whether it puffed, the time it took, and notes on splatter, smell or smoke (1 mark each for (c) and (d)).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What makes a question scientific?Show answer
It can be answered by an experiment because it names something to change and something to observe or measure.
2.Name four factors the textbook lists for the puri investigation.Show answer
Dough thickness and size, type of flour, oil temperature, and the way the dough is dropped in.
3.Give two kinds of observation.Show answer
A yes/no answer such as 'does it puff?' and a measured number such as the time in seconds.
4.Why keep notes of smell and splatter?Show answer
They can lead to new questions and to new explanations.
5.What does the chapter say about scientists and the puffing of a puri?Show answer
That even this everyday event is still not completely understood today, so simple questions can lead to open problems.

Asking a Scientific Question and Choosing What to Change and Observe: frequently asked questions

What is the difference between a question and a scientific question?

A scientific question can be tested. It points to a factor you can change and a result you can observe or measure, for example the effect of oil temperature on the time a puri takes to puff.

Is it better to record a yes/no or a number?

Both are useful. A yes/no result is the first check. A number with a unit lets you compare trials more finely, so it is preferred whenever you can measure.

Why does the chapter use a puri?

Because it is a familiar kitchen event that shows how everyday curiosity becomes an investigation, and because it is a puzzle that is not completely solved.

Do I need a laboratory to investigate?

No. The chapter says science is everywhere and that a kitchen at home is a good place to observe and ask questions.