Food Energy Practical: Reliable Measurements

Explained

Burning food to measure its energy, and being honest about the answer

Burn a piece of food under a tube of water and the energy released warms the water. Measure the temperature rise and you can work out how much energy came out.

The experiment is easy to do and famously inaccurate, and both halves of that are examinable. Questions ask you to calculate a value and then to explain why it is lower than the true one.

The method

Measure a known volume of water into a boiling tube, usually 20 cm cubed, and record its starting temperature.

Weigh the food. This is the step most often rushed, and the mass is in the final calculation, so it needs to be accurate.

Set the food alight in a Bunsen flame and hold it under the tube immediately, keeping it there until it stops burning. Relight it if it goes out before it is consumed.

Record the highest temperature the water reaches, stirring so the heat is spread through it rather than staying at the top.

Weigh whatever is left of the food. The mass burned is the starting mass minus the final mass, not the starting mass, because some of the food never burned.

The calculation

Energy transferred to the water equals the mass of the water multiplied by 4.2 multiplied by the temperature rise, giving an answer in joules.

The 4.2 is the specific heat capacity of water in joules per gram per degree Celsius. The mass of water in grams is the same number as its volume in cubic centimetres, so 20 cm cubed of water is 20 g.

Then divide by the mass of food burned to get the energy per gram, which is what lets you compare foods.

A worked version: 20 g of water rises by 15 degrees, so 20 times 4.2 times 15 gives 1260 J. If 0.4 g of food burned, the energy per gram is 1260 divided by 0.4, which is 3150 J per gram.

Why the answer is always too low

Most of the energy never reaches the water, and a good answer names where it went rather than saying heat was lost.

Heat goes into the air around the flame, into the glass of the boiling tube, into the clamp and stand, and it radiates away in every direction that is not upwards into the tube.

The food often does not burn completely, so some of its energy is still in the unburned remains. Weighing the residue accounts for that, but only if you actually do it.

And the food may go out and need relighting, during which time it is heating the room rather than the water.

Every one of these makes the measured value smaller than the true one, which is why the answer is an underestimate and never an overestimate. That direction is often a mark of its own.

Improving it

Shield the apparatus from draughts with a screen, and hold the food as close to the tube as possible so less heat escapes sideways.

Insulate the boiling tube, or use a copper calorimeter, which conducts heat into the water rather than absorbing and holding it.

Burn the food in pure oxygen in a sealed container, which is what a laboratory calorimeter does and is why published food energy values are far higher than school results.

Stir continuously and use a thermometer with finer divisions, or a temperature probe, so the peak reading is not missed.

Making it reliable and fair

Use the same volume of water, the same starting temperature, the same distance from the tube, and the same apparatus for every food.

Use the same mass of each food if you can, though the calculation per gram is what makes different masses comparable, and that is the point of dividing by the mass at all.

Repeat each food at least three times and take a mean. One burn is a single reading of a process with a great deal of variation in it, and repeating is the only way to know whether a difference between two foods is real.

Wear eye protection, tie hair back, keep the burning food away from your face, and expect the tube to get hot. Foods with a high fat content burn vigorously and can flare.