Biology

10.5 - Energy Content of Food (Calorimetry)

4 min

note 1 of 3

The experiment

Aim

To compare how much energy different foods store, by burning a food sample and using the heat it gives off to warm a known amount of water.

Background

Food stores chemical energy. When food burns, that energy is released as heat.
Fatty foods store a lot of energy per gram. A peanut burns for a long time and warms the water sharply, but a piece of pasta warms it far less.
You can't measure the energy stored in a food directly, but you can use the heat from the burning food to warm a measured amount of water, and measure the temperature rise.
So in this practical you burn each food under a tube of water, and use the temperature rise and the mass of the food to work out the energy released per gram.

Variables

Independent variable - The type of food, for example a crisp, a peanut, or pasta.
Dependent variable - The energy released per gram of food, in joules per gram (J/g).
Control variables - The volume of water, its starting temperature, the distance from the flame to the tube, and the way the food is held and lit.

Method

Measure the water - Put 25 cm³ of water into a boiling tube and record its starting temperature.
Weigh the food - Find the mass of the food sample on a balance.
Light the food - Hold it on a mounted needle, set it alight, and hold it under the boiling tube straight away.
Keep it under the tube - Until it stops burning, relighting it if it goes out before then.
Record the highest temperature - Note the highest temperature the water reaches, and work out the temperature rise.
Repeat for each food - Use the same volume of fresh water each time.

Safety

Naked flame - Wear eye protection, tie back long hair, and burn the food over a heatproof mat with nothing flammable nearby.
Hot boiling tube - It can scald, so hold it in a clamp rather than your hand, and let it cool before you move it.
Mounted needle - Keep it pointed away from you.
Allergies - Don't use nuts if anyone in the room has a nut allergy.

A boiling tube of water clamped to a stand with a thermometer in it, a burning food sample on a mounted needle held just below, and a heat-proof mat underneath.

note 2 of 3

Worked example - Energy content of food

A 0.5 g peanut is burned under a boiling tube holding 25 cm³ of water. The water's temperature rises from 18 °C to 38 °C. Find the energy released per gram of peanut. (25 cm³ of water has a mass of 25 g.)

Step 1 - Find the temperature rise

38 - 18 = 20 °C

Step 2 - Find the energy given to the water

Use energy = mass of water × 4.2 × temperature rise. The 4.2 is the energy in joules needed to warm 1 g of water by 1 °C.

Energy = 25 × 4.2 × 20 = 2,100 J

Step 3 - Find the energy per gram of food

Divide the energy by the mass of food burned.

$\begin{matrix} Energy per gram & = \frac{2,100}{0.5} \\ = 4,200 J/g \end{matrix}$

Step 4 - Compare the result with the true energy content

This peanut released about 4,200 J for every gram. The true figure is higher, because not all the heat from the burning peanut reached the water.

note 3 of 3

Common exam questions

Question 1

Suggest two ways to make the result more accurate.

Any two of these.

Shield the tube from draughts, or insulate it.
Hold the burning food closer to the tube.
Use a proper calorimeter instead of a boiling tube.

Question 2

Why is the same volume of water used for each food?

A different amount of water would warm by a different amount for the same energy, so the temperature rises couldn't be compared.

Question 3

Why is the energy given per gram rather than per sample?

So foods can be compared fairly, even when the samples burned have different masses.

end of notes