Science shelf · Class 8
Which water bottle keeps water cool?
Four bottles were tested in a hot room. One kept water coolest, but one row in the table cannot be compared with the others. Which, and why?
What you will learn: Compare products fairly from test results, explain why only one thing should change at a time, and say what the evidence shows and what it does not.
What should I know first?
If this feels new, try this first. It is fine to go back.
Small test at the end: scroll down to the end of this page for five short questions that check what you have learnt. Tap an answer, then press Check answer. Go to the questions ↓
Jump to: Check yourself · For grown-ups · For teachers
Safety first
Use the results on this page. No hot water, and a grown-up handles glass.
Keep it safe:
- Use the made-up results on this page. If you test bottles yourself, use cool tap water only, never hot water, in clean, unbroken containers. Do not drink the test water.
- Use a digital thermometer, never a mercury one. A grown-up handles any glass.
- Keep bottle caps and small parts away from babies: nothing goes in the mouth.
You will need
- Paper and a pencilFor the results and your plan.
- A calculator (optional)For the rises.
- Two bottles and a thermometer (optional)To try a fair test at home with a grown-up.
Time: About 40 minutes.
What do you think?
The clay bottle's water went down by 3 °C. The flask's went up by 4 °C. So is clay the best?
Make a prediction and write one reason for it. Then tap the guess closest to yours to see how to test it. It is fine to change your mind after checking.
Yes, clay is best
Look at where each one started. Can you compare a bottle that started at 25 °C with ones that started at 8 °C?
No, the flask is
Compare only the bottles that started at the same temperature. Which kept its water coolest? And what about clay?
We cannot tell yet
What would you need to change in the test to find out?
Not sure
That is fine. Check where each bottle started, and which ones can fairly be compared.
Need a hint?
Check that everything except the bottle was the same for every row.
The story
It is May, and the children's water is warm by lunch. Dadi's stall sells four kinds of bottle, and someone has tested them: each filled with 750 mL, lid on, in the same shaded room. "The clay one is best," says Tanmay. "Look, it went down, not up!" Tara reads the label on the table of results. "But it started at 25 degrees. The others started at 8." Dhruv frowns. "Then which one really keeps water cool?"
The idea
A fair test changes one thing (here, the bottle) and keeps everything else the same: start temperature, amount of water, room, time and thermometer.
If two things change at once, you cannot tell which one caused the difference.
In this made-up test every bottle held 750 mL with its lid on, in the same shaded room, read with the same digital thermometer. It compares whole bottle designs, not just materials.
When the start temperatures match, ranking the end temperatures or ranking the changes gives the same order. The clay bottle (*) started at 25 °C, so its change of −3 °C cannot be compared fairly.
One trial is not enough to be sure. A difference of 2 °C, like plastic and steel here, could be within measuring error; repeat the test.
A good conclusion says what the evidence shows, for which conditions, and what is still unknown.
Worked example
Writing a careful conclusion
Write one conclusion from the table that the evidence really supports.
- Use only the rows with the same start: plastic, steel and the flask.
- Compare the rises: 18, 16 and 4 degrees.
- Say the conditions: 6 hours in a 30 °C room, starting at 8 °C.
- Say what is unknown: how clay compares, and how each bottle lasts over a whole day.
"Starting at 8 °C, the flask kept water coolest over 6 hours in a 30 °C room. We cannot compare the clay bottle from this test."
Try it
Work out the rises.
For each bottle that started at 8 °C, take the start from the end temperature.
Find the unfair row.
Which row was not tested the same way? What was different?
Rank the fair rows.
Put plastic, steel and the flask in order, from smallest rise to biggest.
Plan a fair test.
Write what you would keep the same, what you would change, and what you would measure, to include the clay bottle.
Think about cost.
The flask costs ₹900. Write when it is worth it, and when the steel bottle might be a better choice.
Write your conclusion.
Write one sentence that says what the evidence shows, under which conditions, and one thing still unknown.
What you make (paper or offline)
Your fair-test plan
Make a fair-test plan for comparing two bottles or pots at home.
List what stays the same, the one thing you change, and what you measure.
Add one thing your test still could not tell you.
Make it on paper. Keep it with you. Nothing to upload.
What did we find out?
Read this after you have tried it. Look back at your guess and your reason first.
Plastic, steel and the flask started at the same temperature in the same room for the same time, with the same amount of water and the same thermometer, so they can be compared: the flask kept water coolest in this test (a rise of only 4 °C).
The clay bottle started at 25 °C, so its row is not a fair comparison. Clay can cool water by evaporation, but this test cannot show how it compares.
The flask was also the most expensive. "Best" depends on what you need: cool water, low price, low weight.
Check yourself
Five questions about what this page teaches. Choose an answer and press Check answer, or open "See the answer and why". Nothing is saved or sent: your choices stay on this screen and disappear when you leave or reload the page.
How do you want to answer? You choose, and you can change at any time.
See the answer and why
- A. Clay, because its water cooled down Not this one. Clay started at 25 °C, so it was not tested the same way.
- B. Steel, with a rise of 16 °C Not this one. Steel did better than plastic, but the flask rose far less.
- C. The flask, with a rise of 4 °C Correct. Its water rose from 8 °C to 12 °C, the smallest rise.
- D. Plastic, with a rise of 18 °C Not this one. Plastic had the biggest rise of the three.
See the answer and why
- A. 8 °C Not this one. That is where it started.
- B. 16 °C Correct. 24 − 8 = 16.
- C. 24 °C Not this one. That is where it ended, not how much it rose.
- D. 32 °C Not this one. That adds the start and the end. A rise is end minus start.
See the answer and why
- A. It was tested in a different town Not this one. The table says the same room. The difference is the start temperature.
- B. Its price is lower than the flask's Not this one. Price does not change how a test was run.
- C. Its temperature fell instead of rising Not this one. A fall can be measured (−3 °C). The problem is that it started at a different temperature.
- D. Its water started at 25 °C, not 8 °C Correct. A different start is a second thing changed, so the test is not fair.
See the answer and why
- A. She should have tested three bottles instead of two Not this one. More bottles would not fix the two extra changes.
- B. She only needed to wait a few hours longer Not this one. Waiting longer does not fix changing two things.
- C. She changed two more things, not just the bottle Correct. The start temperature and sun or shade both changed as well, so she cannot tell which caused the difference.
- D. Nothing is wrong: it is a fair test of the bottles Not this one. Water start and place are different, so it is not fair.
See the answer and why
- A. Clay keeps water cooler than steel Not this one. Clay was not tested the same way, so this cannot be claimed.
- B. Starting at 8 °C, the flask kept water coolest in this test Correct. It names the conditions and claims only what was measured.
- C. Plastic bottles always warm up fastest everywhere Not this one. One test in one room cannot show "always" and "everywhere".
- D. The flask is the best bottle for everyone Not this one. Price is listed, but weight, strength and how each suits different uses were not tested.
Think back
Why does changing two things at once spoil a test?
When is the "best" product not the right one to buy?
Investigate further
- Plan a fair test of whether a wet cloth around a steel bottle keeps water cooler. What stays the same?
- Find a product claim on a package at home. What test would you need to check it?
Can you use it here?
An advert says "Our bottle keeps water 10 °C cooler!" Cooler than what, for how long, starting where? What would you ask?
Ask someone at home
Ask at home which bottle or pot the family uses for water in summer, and why. Is the reason backed by a test or by habit?
No one free to join in? That's fine. You can do it by yourself with drawings, and ask a grown-up to help with the materials.
For grown-upsTap a heading to open it. The safety notes are at the top of the page.
Before you start
- Paper and a pencil. If you test bottles, use cool tap water only, and handle any glass yourself.
- Keep caps and small parts away from babies; nothing in the mouth.
Questions that help
- "Was everything else the same?"
- "What did the test not measure?"
- "Best for whom?"
What's really going on
A fair test is the backbone of science: change one thing, keep the rest the same, measure the result.
Children (and adverts) often compare end results without checking the starts. The clay row is there to catch that.
A careful conclusion names its conditions and its limits. That habit protects against misleading product claims.
Other ways to do it
- Use two cups of water, one covered and one not, on the same table.
- Discuss the table only, with no testing.
For teachersLearning aims, classroom ideas and what to look for.
Learning aims
- Identify the variable changed, the ones controlled and the one measured.
- Explain why changing two variables at once weakens a conclusion.
- Write a conclusion that matches the evidence and states its limits.
Subject links
- Science: heat transfer and insulation; fair tests.
- Class 6 "Which paper towel soaks up most? A fair test".
- Class 8 "Read an advert like a detective" (product claims).
These links are general. They have not been checked against a specific board syllabus.
In the classroom
- Groups get the table and must find the unfair row before ranking the bottles.
- Each group plans a fair test to include the clay bottle and swaps plans for checking.
What to look for
- Spots the different start temperature without prompting.
- Names the controlled variables.
- Writes a conclusion with conditions and an unknown.