Principle
You burn a known mass of food directly below a known volume of water. The heat from the burning food warms the water. By measuring the temperature rise, you can calculate the energy released, using the specific heat capacity of water (4.2 J for each gram of water raised by 1 °C).
Apparatus
- A boiling tube and clamp stand
- Measuring cylinder
- A thermometer (reading to at least 0.1 °C)
- Mass balance
- A mounted needle (to hold the food sample)
- Bunsen burner (to set the food alight)
- Small samples of food to test (popcorn, peanut, crisp, pasta, etc.)
Method
- Use the measuring cylinder to add 25 cm³ of cold water to the boiling tube. Clamp the boiling tube vertically.
- Record the starting temperature of the water.
- Find the mass of the food sample on the balance.
- Push the food sample onto the mounted needle.
- Set fire to the food with the Bunsen burner, then quickly hold the burning sample about 2 cm under the boiling tube. Keep it there until the food has finished burning.
- Stir the water gently with the thermometer and record the highest temperature the water reaches.
- Repeat for at least two more pieces of each food to get a mean.
Calculation
energy per gram of food (J/g) = (mass of water in g × temperature rise in °C × 4.2) ÷ mass of food in g
Where:
- The 4.2 is the specific heat capacity of water (4.2 J/g/°C)
- 1 cm³ of water has a mass of 1 g, so 25 cm³ of water = 25 g
Example — A student burns a 0.40 g piece of peanut under 25 cm³ of water. The water rises in temperature from 19.5 °C to 38.0 °C. Calculate the energy per gram of peanut.
- Step 1: temperature rise = 38.0 − 19.5 = 18.5 °C
- Step 2: energy gained by water = 25 × 18.5 × 4.2 = 1943 J
- Step 3: energy per gram of food = 1943 ÷ 0.40 = 4858 J/g (about 4.9 kJ/g)
Limitations and how to improve
- Heat is lost to the air, not all of it reaches the water. The measured energy is always lower than the true energy in the food.
- Use a calorimeter (a copper container with insulation around the water) or wrap aluminium foil around the boiling tube to reduce heat loss.
- The food may not burn completely, especially if it goes out before all the energy is released.
- Relight the food repeatedly until it will no longer catch fire.
- Different foods catch fire at different rates, so the time held under the tube is hard to control.
- Use a fixed apparatus (such as a clamped sample holder) to standardise the distance and position.