1.14B Explain how the energy contained in food can be measured using…

The idea

Calorimetry measures the energy released when food is burned. A known mass of food is ignited and the heat released warms a known volume of water. The temperature rise of the water is recorded, and the energy transferred is calculated using the specific heat capacity of water: energy (J) = mass of water (g) × 4.2 (J/g°C) × temperature rise (°C). Dividing by the mass of food burned gives energy per gram, often converted to kilojoules (1 kJ = 1000 J). A simple school calorimeter (burning food under a boiling tube of water) gives an estimate only, because heat is lost to the surroundings and combustion may be incomplete, so the value is lower than the true energy content.

What earns the marks

• Food is burned and the heat released is used to warm a known mass/volume of water.
• The temperature rise of the water is measured with a thermometer.
• Energy transferred is calculated using energy = mass of water × 4.2 × temperature rise.
• Energy per gram of food is found by dividing the energy released by the mass of food burned.
• Results are estimates because heat is lost to surroundings and combustion may be incomplete.
• Units: energy in joules or kilojoules, with 1 kJ = 1000 J.

Common mistakes and how to put them right

• Using the mass of food instead of the mass of water in the energy calculation.

  Put it right: The water's mass and temperature rise give the energy transferred.

• Forgetting to divide by the mass of food burned.

  Put it right: This gives energy per gram, allowing fair comparison.

• Treating the result as exact.

  Put it right: Heat loss and incomplete combustion make it an underestimate.

Exam tips

• Write the equation energy = mass of water × 4.2 × temperature rise before substituting values.
• Convert joules to kilojoules by dividing by 1000 when the question asks for kJ.
• Explain why the measured value is lower than the true value by referring to heat loss to surroundings.