The Heat Lab
The required practical on temperature change, taught as a set of decisions rather than a recipe. Which variable you are testing, what every other step is there to prevent, and how to read a result you did not expect.
Get the method right under pressure
Free interactive practice on the steps that lose marks under exam pressure.
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One variable at a time
You already know from the energy module which reactions warm their surroundings and which cool them. This one is the required practical, and it is easy to learn as a recipe and then be unable to answer anything about it. Here is the idea that makes it answerable instead. The investigation changes ONE thing and measures what happens to the temperature. Every other instruction in the method is there to stop something else changing at the same time. Insulating the cup is not tidiness, it is stopping heat leaving to the room. Using equal volumes is not neatness, it is stopping the amount of solution becoming a second variable. If you can say what each step is preventing, you can answer questions about versions of this practical you have never seen.
Words for the practical
Six terms. The middle two are the ones an exam answer usually has to name.
Variable to what it tests
- acid and a metal
- acid and a carbonate
- neutralisation
- displacement
- a metal reacting with an acid, with gas given off as it goes
- a carbonate reacting with an acid, also producing a gas
- an acid reacting with an alkali to produce a salt and water
- a more reactive metal taking the place of a less reactive one in a solution
Which one should change?
A student investigates whether the concentration of an acid affects the temperature change during neutralisation. Which of these should be the only thing that differs between runs?
- The concentration of the acid
- The volume of acid used in each run
- The starting temperature of the solutions
- The kind of cup the reaction is carried out in
Work out the temperature change
In one run the mixture starts at 18 degrees Celsius and reaches a highest temperature of 29 degrees Celsius. Work out the temperature change, in degrees Celsius.
The mean of three runs
The student repeats the same conditions three times and records temperature changes of 11, 13 and 12 degrees Celsius. Work out the mean temperature change, in degrees Celsius.
Why the cup is polystyrene
Almost every mark lost on this practical comes from describing a step without saying what it prevents. The reaction is carried out in an insulated cup with a lid, and the reason is not that it is convenient. Heat passes between the mixture and the room, and any heat that escapes is heat your thermometer never sees, so the temperature change you record is smaller than the reaction actually produced. Insulation reduces that loss. The lid reduces it further, because warm air leaving the top carries energy with it. When a question asks why the cup is insulated, the answer is not to keep the heat in. It is to reduce heat transfer to the surroundings so the measured change is closer to the true one.
A hotter result
The student repeats the investigation with a more concentrated acid. The mixture starts at 18 degrees Celsius and reaches 35 degrees Celsius. Work out the temperature change, in degrees Celsius.
Match each step to its reason
- using an insulated cup with a lid
- using the same volume of acid every time
- stirring the mixture before reading the thermometer
- repeating each set of conditions three times
- stops heat passing to the room and being missed by the thermometer
- stops the amount of solution becoming a second variable
- stops one part of the mixture being warmer than the part being measured
- makes an odd result visible instead of being taken as the answer
The method in order
Put the steps of the required practical into the order you would carry them out.
- Measure a fixed volume of acid into an insulated cup and put the lid on
- Record the starting temperature of the acid
- Add the second substance and replace the lid
- Stir, and record the highest or lowest temperature reached
- Work out the temperature change by subtracting the starting temperature
- Repeat with the same conditions, then change only the variable being tested
The practical in a paragraph
The one thing deliberately changed between runs is the _____ variable, and the temperature change you measure is the _____ variable. Everything held the same, such as the volume of acid, is a _____ variable. The reaction is carried out in an insulated cup so that less heat passes to the _____. Each set of conditions is run more than once so that a mean can be worked out and an odd _____ can be spotted.
A conclusion from RP4
Question: a student measured mean temperature changes of 12 degrees Celsius with one acid and 17 degrees Celsius with a more concentrated one. What can they conclude, and how confident can they be? Model answer: the more concentrated acid produced a larger temperature change, which suggests that increasing the concentration increases the energy released in this reaction over the range tested. Confidence rests on how well the other variables were controlled. The same volume of acid was used each time, the same insulated cup, and the same starting temperature, so the concentration is the only thing that differed. Each set of conditions was repeated three times and the individual results were close to one another, so no single reading is carrying the conclusion. Two limits are worth stating. Only two concentrations were compared, so nothing follows about concentrations between or beyond them. And some heat will still have passed to the surroundings despite the insulation, so both measured changes are likely to be smaller than the true ones. Notice that the answer names what was controlled before claiming anything, which is what turns a result into a conclusion.
Which step is wrong
Three of these steps belong in a well-designed version of this practical. Select the ONE that would spoil the investigation.
- Use the same volume of acid in every run, measured with the same equipment.
- Use a larger insulated cup for the runs with the more concentrated acid.
- Record the starting temperature of the acid before adding anything to it.
- Stir the mixture before reading the highest temperature reached.
Running the investigation
You are carrying out the required practical and have to make the decisions it forces. Work through them.
- You want to test whether the mass of metal affects the temperature change. What must stay the same?
- Your three repeats give changes of 11, 13 and 25 degrees Celsius. What should you do?
- A classmate says the insulated cup keeps the heat in so the reaction gets hotter. What is wrong?
- You want to state a conclusion. What has to be true before you can?
Design the investigation
Describe how you would investigate whether the volume of acid affects the temperature change when it reacts with a carbonate, and explain how you would know your result could be trusted.
- State which variable you would change and which you would measure
- Name two variables you would keep the same, and say what each one would otherwise affect
- Explain why the reaction is carried out in an insulated cup with a lid
- Describe how you would work out the temperature change from your readings
- Explain why you would repeat each set of conditions
- State one thing that would limit how far your conclusion could be taken