Practical: Energy Content of a Food Sample

• The calculation relies on the First Law of Thermodynamics, which states that energy cannot be created or destroyed, only transformed. We assume the chemical energy lost by the food is equal to the thermal energy gained by the water.

• The formula used to calculate energy transfer is $Q = m \cdot c \cdot \Delta T$, where $Q$ is energy in Joules, $m$ is the mass of water in grams, $c$ is the specific heat capacity ($4.2$), and $\Delta T$ is the change in temperature.

• To compare different foods fairly, we calculate Energy per Gram by dividing the total energy transferred by the starting mass of the food sample. This standardizes the results, allowing us to see that fats typically yield much more energy than carbohydrates or proteins.

• Step 1: Preparation: Measure exactly $25 \, \text{cm}^3$ of water into a boiling tube and record the initial temperature ($T_1$). Weigh the food sample precisely using a digital balance to ensure an accurate denominator for the final calculation.

• Step 2: Combustion: Secure the food sample on a mounted needle and ignite it using a Bunsen burner. Once lit, immediately hold the food $2 \, \text{cm}$ below the boiling tube, ensuring the flame is centered to maximize heat transfer.

• Step 3: Completion: If the flame goes out but the food is not fully charred, relight it immediately. Once the food is completely burned and no longer relights, record the final temperature of the water ($T_2$) to determine the temperature rise.

Comparison of Calorimetry Methods

• Water Mass vs. Food Mass: In the formula $mc\Delta T$, the mass ($m$) must always be the mass of the water, as that is what is being heated. The food mass is only used at the very end to calculate the energy density per gram.

• Temperature Rise vs. Final Temperature: Students must ensure they subtract $T_1$ from $T_2$ to get $\Delta T$. Using the final temperature alone in the calculation is a common procedural error.

• Unit Conversion: Always verify if the question asks for the answer in Joules (J) or kilojoules (kJ). Divide the final result by $1000$ to convert from J to kJ.

• Consistency Check: If your calculated energy for a lipid-rich food (like a nut) is lower than for a carbohydrate (like popcorn), re-examine your data for excessive heat loss or incomplete burning.

• Reliability: Always suggest repeating the experiment three times for each food type. Calculating a mean temperature rise helps to identify and ignore anomalous results, increasing the overall reliability of the findings.

Key Takeaway: In exam questions about 'improvements', the most common answers are using a calorimeter to reduce heat loss and using pure oxygen to ensure complete combustion.

• Thermometer Placement: Do not allow the thermometer to touch the bottom of the boiling tube. It should be suspended in the water to measure the fluid's temperature, not the temperature of the glass being heated directly by the flame.

• The 'Same' Rule: In the CORMS evaluation for this practical, 'Same' variables (controls) must be distinct. For example, use the same volume of water AND keep the distance between the food and the tube constant ($2 \, \text{cm}$).

• Incomplete Combustion: Students often assume the food has finished burning when the flame goes out. However, if the sample can be relit, it still contains stored energy that must be released for an accurate measurement.