KS3 · Biology

Measuring energy in food

How much energy is packed into a piece of puffed wheat? Set it on fire under some water and find out.

Run the practical

1Weigh the food2345670.0 °Crise in water temperature

Stage 1 of 7: Weigh the food. rise in water temperature: 0.0 °C. paused

Weigh the food · 1/7scrub through the method →

Put the food sample on the balance and write down its mass. Our example: 2 g of puffed wheat.

Example run: a 2 g piece of puffed wheat. The 20 °C start is an example reading.

The kit

Every piece of kit has one job
balancemeasuring cylinderheat proof matBunsen burnerclamp standthermometerboiling tubemounted needle

Showing 1 layer: Apparatus

Explore

tap the kit ↓

Tap a piece of equipment to see its job and its safety point.

Making it a fair test

Change one thing, measure one thing, keep the rest the same

Which kind of variable is each factor?

Still to sort

Independent variable (0)

The factor you change.

Where the line is: You choose it before the test. The dependent variable is what you find out after.

Dependent variable (0)

The factor you measure.

Where the line is: It is the result you read off, not something you set up.

Control variables (0)

Factors kept the same so the investigation is valid.

Where the line is: Kept the same on purpose. If one of these changed, you could not tell whether the food caused the difference.

5 of 5 still to sort.

Sort each factor into the kind of variable it is in this practical.

Recording results

Spot the slip in this results table

A student burned Food A three times and recorded the results. One line goes wrong. Which one?

A student's results for Food A — which line goes wrong?

From temperature rise to joules

Turn the puffed wheat run into energy per gram

In the example run, 2 g of puffed wheat warmed 30 cm³ of water from 20 °C to 25 °C. How much energy did each gram provide?

  1. Readings: start 20 °C, final 25 °C. Food burned: 2 g. Water: 30 cm³.
  2. missing step
Which line is step 2?

Reading the result

What does your energy-per-gram result really tell you?

You burned Food C and worked out its energy per gram. It came out much higher than Food D.

Which is closest to what you think the result tells you?
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Burn a food, warm some water, and turn the temperature rise into joules.

What you need to know

  • Different foods provide different amounts of energy; food labels show it in kilojoules (kJ) or kilocalories (kcal). You can compare foods yourself by burning each one under some water and measuring the temperature change of the water.
  • Temperature change (°C) = final temperature − initial temperature, so you must read the thermometer before the food is lit and after it goes out.
  • Energy provided by the food (J) = mass of water (g) × rise in temperature (°C) × 4.2, and 30 cm³ of water has a mass of 30 g.
  • Divide the energy by the mass of food burned to get the energy per gram (J/g) — that is the number you compare foods with.

The big picture

Different foods provide different amounts of energy. In this practical you burn a food under 30 cm³ of water and measure how much the water's temperature rises. Energy provided (J) = mass of water (g) × rise in temperature (°C) × 4.2, and dividing by the mass of food burned gives the energy per gram, which is how you compare foods. Your answer can come out lower than expected, because not all the food burns and some energy heats the surroundings instead of the water.

Key points

1Independent variable: the type of food. Dependent variable: the temperature change of the water. Control variables: the mass of water, the mass of food, and the distance between the food and the boiling tube.
2Results table: type of food in the first column, then mass of food, initial temperature, final temperature and temperature change. Units go in the headings only.
3Repeat each food and work out a mean: add the measurements, then divide by how many there are.
4Safety: goggles because you are using glassware, a heat proof mat below the burning food in case it drops, and never touch the equipment because it will be hot.
5Your measured value can be lower than expected: not all the food burns, and some energy heats the surroundings instead of the water.

Worked example

Problem

A student burns 1.5 g of Food B under 30 cm³ of water three times. The temperature changes are 6 °C, 7 °C and 8 °C. Calculate the energy per gram of Food B.

⚠ Watch out

Putting the final temperature into the energy equation instead of the rise. The equation needs how much the water warmed up (final − initial), not the temperature it ended at.

🧠

Memory hook

The food is the fuel; the water keeps the score. Water × Warm-up × 4.2 gives the joules — then share them out per gram of food.

✓

Check yourself

Two foods each warm 30 cm³ of water by 6 °C, but one sample had twice the mass of the other. Which food provides more energy per gram, and how do you know?

Flashcards

(14)
How can you compare how much energy different foods provide?
Burn each food under the same volume of water and measure the temperature change of the water. A bigger rise means the food gave out more energy.
Where is the energy in a food shown, and in what units?
On the food label, in kilojoules (kJ) or kilocalories (kcal).
Independent and dependent variables in the burning-food practical?
Independent: the type of food (what you change). Dependent: the temperature change of the water (what you measure).
Name the three control variables.
The mass of water, the mass of food, and the distance between the food and the boiling tube.
What do the mounted needle and the Bunsen burner each do?
The mounted needle holds the food. The Bunsen burner ignites it.
How do you work out the temperature change?
Final temperature − initial temperature (°C).
Equation for the energy provided by a food?
Energy (J) = mass of water (g) × rise in temperature (°C) × 4.2
What is the mass of 30 cm³ of water?
30 g, because 1 cm³ of water has a mass of 1 g.
How do you find the energy per gram of a food?
Divide the energy provided by the mass of food burned. The unit is J/g.
Where do units go in a results table?
In the column headings only, never in the body of the table.
How do you calculate a mean temperature change?
Add up the repeat measurements, then divide by the number of measurements.
Give two reasons the calculated energy can be lower than expected.
Not all of the food burns (some may fall off the needle), and some energy heats the surroundings instead of the water.
Why must the heat proof mat be below the burning food?
In case the food drops.
What should you think about alongside a food's energy content?
How active the person is and the portion size. High-energy foods are eaten in moderation, as part of a balanced diet and an active lifestyle.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Biologylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 2 October 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.