“Lids on or lids off? – a fun bit of practical science”
Suitable for Key Stage 2 (ages 7 to 10)
Skills: Measurements, Comparisons, Subtraction, Fractions and optionally Multiplication and Percentages.
This is most easily applied to boiling pasta or rice or similar.
It introduces the setting up of a science experiment and the scientific method. It is suitable for being done at home under parental supervision.
Explain hypothesis. Scientists want to test a question.
An experiment is made up of hypothesis or question for testing, then thinking about how to test it (method) and carrying it out taking observations (results). Finally from the results we find out what the answer to the question is (conclusion).
Here we will form the hypothesis as something like “cooking with lids on saves time and saves energy”. The hypothesis doesn’t need to be right but we need to be able to test it. We will work out whether it is right at the end.
To get there ask the child to guess whether cooking with the lid on is better than the lid off. Do we think it will make a difference to speed? Do we think it will make a difference to how much energy we use?
As a minimum we will need one pan with a compatible lid and a stopwatch or a clock plus the usual ingredients: water, a little salt and some pasta (or rice).
It is more exciting if we also measure energy use. If you have a gas hob, do you have access to the gas meter? If you have an electric hob do you have access to the electricity meter? For the duration of the experiment it is best if we are not using other gas appliances or electrical appliances: that way our readings will be accurate. If you don’t have access to the meters or can’t switch off other appliances then simply skip the steps asking for meter readings and approximate the energy use by the time taken (the longer we have the gas on at a fixed flame setting or the electric hob on the fixed number, the energy used goes up linearly with time).
- Measure out around 1L to 1.5L of water to boil pasta (you need to be able to exactly match it for the second boil later). The water should be cold from the cold tap. Rinse the pan first with cold water from the cold tap so we know we are starting with a really cold pan. Start with the pan without the lid. Note: if you don’t normally use much water to boil, then to be safe use a bit more here because we want to reduce the risk of boiling dry (particularly if you are boiling rice).
- Check the gas meter and write down the number including the red numbers after the dot (your gas company is not interested in small quantities of gas, but we are for this experiment)
- Switch on the gas and start the timer (or write the time from the clock with seconds). Don’t use too high a flame as we want to avoid boiling over later. Try and stick with that setting.
- Note down the time when the water just starts to boil
- Add the measured quantity of salt (use a level or heaped teaspoon so you can be consistent later)
- Note down the time when the water starts to boil again
- Add the measured quantity of pasta (use a mug or something like that so you can be consistent later)
- Stir only when necessary, otherwise leave it to cook with as few changes as possible. You’ll have to turn the gas down if it starts boiling over.
- Check when cooked to your normal taste requirements and then switch off the gas and stop the timer.
- Check your gas meter again and write down the full number.
Hopefully your mini scientist has been keeping a good record during this first part.
Once you’ve drained the pasta and it’s on a plate ready to eat, run cold water into the pan and get it ready for the next stage. Follow the same steps as before but this time keep the lid on from the beginning and take it off as little as possible. In step 8 you may need to be extra careful about boiling over, so get ready to turn it down. If you do need to lift the lid to stop it boiling over, try and replace it as soon as it has calmed down.
Your mini scientist will have start and end times and gas meter readings.
If you have used a clock, first ask how long the pan without lid has taken. They should try and subtract the start time from the end time.
For your gas meter readings, subtract the start readings from the end readings. If your scientist has not come across the decimal marker yet, this could be an opportunity to explain what it is (fractions: each digit is number of 1/10ths, 1/100ths and 1/1000ths).
Now your scientist will have time taken and gas used for the pan without lid and time taken and gas used with lid.
Ask them which one is quicker and by how much. Another subtraction here to note how much quicker in minutes and seconds.
Optionally convert both times to seconds by multiplying the minutes by 60 (see if they know how and if not then say first x6 then x10) and adding to the seconds. When the time is in seconds then they can also write it out as a fraction: time taken with lid on divided by time taken with lid off.
Ask the which one used less energy. Another subtraction here to note how many units less.
You can also look at how much less gas was used as a fraction. They may not be able to simplify it, but it helps show another use for fractions. You could help with introducing approximation and then fraction simplification and conversion to a decimal fraction as shown in the example. Alternatives include using percentages.
Finally before your scientist runs away, try and get them to think about the hypothesis and ask them to say what the real answer is. A conclusion can be very short. Something like
“It is better to have the lid on because it is quicker and uses less gas”.