Explain the determination of the calorific value of a food with a bomb calorimeter, with a calculation.

1. 1 · Aim. To find the gross energy (calorific value) of the given food sample (observed during the lab visit).
2. 2 · Principle. The dried sample burns completely in oxygen at about 25-30 atm inside a bomb surrounded by water. Heat released = heat capacity × temperature rise; the heat capacity is found with benzoic acid.
3. 3 · Requirements. [Diagram: bomb inside the water bucket, stirrer, thermometer, ignition leads, insulated jacket] Oxygen cylinder, benzoic acid, pellet press, fuse wire, balance.
4. 4 · Procedure.
   • Standardise with about 1 g benzoic acid.
   • Pellet about 1 g dried sample; fix fuse wire; fill oxygen.
   • Place in a known mass of water; record steady temperature; fire; record the peak.
   • Release pressure slowly and check for complete burning.
5. 5 · Observation. [Table: Substance | Weight | Initial temp. | Final temp. | Rise] Benzoic acid 1.000 g, rise 2.50 °C; sample 1.000 g, rise 1.74 °C.
6. 6 · Calculation. Heat capacity = 6.32 ÷ 2.50 = 2.528 kcal/°C. Gross energy = 2.528 × 1.74 ÷ 1.000 = 4.40 kcal/g (18.4 kJ/g). Add a line on Atwater's 4-9-4.
7. 7 · Result. The gross energy of the given food sample is ___ kcal/g (___ kJ/g).
8. 8 · Precautions.
   • Fill oxygen slowly; never exceed the rated pressure; no grease on fittings.
   • Dry, firm pellet; check for soot.
   • Release pressure slowly before opening; goggles.

Scoring tip: Show the benzoic acid step separately before the sample. The difference between gross and physiological energy earns easy marks.