Ice cream is made by cooling a mixture of liquid ingredients until they freeze - AQA - GCSE Physics Combined Science - Question 5 - 2020 - Paper 1

As a liquid is cooled and freezes, the kinetic energy decreases while the potential energy decreases. This is because, as the temperature drops, the particles lose energy and move more slowly, which reduces their kinetic energy. At the same time, the attractive forces between particles become stronger, resulting in a decrease in potential energy as the liquid transitions to a solid.

Metal has a high thermal conductivity, which increases the rate of energy transfer from the mixture to the coolant, enabling the ice cream to freeze more quickly. In contrast, plastic has a low thermal conductivity, which reduces the rate of energy transfer from the surroundings to the liquid coolant at -20 °C. This differential ensures that the metal effectively chills the mixture while the plastic maintains the thermal boundary.

The liquid coolant should have a high specific heat capacity so that it can absorb a large amount of energy with only a small temperature change. This characteristic is essential for maintaining a stable cooling environment for the mixture, preventing drastic temperature fluctuations during the freezing process.

To find the mass of the mixture, we use the formula for energy transfer during cooling and freezing:

1. Calculate the energy transferred by cooling: $E = mc heta$ Where,

   • $E$ (energy) = 165,000 J,
   • $c$ = 3500 J/kg °C,
   • $\theta$ (temperature change) = (20 + 1.5) °C = 21.5 °C.

   Rearranging gives: $m = \frac{E}{c \theta} = \frac{165000}{3500 \times 21.5} \approx 2.19 \, kg$

2. Calculate the mass from the latent heat: $E = mL$ Where,

   • $L$ = 255,000 J/kg. $m = \frac{E}{L} = \frac{165000}{255000} \approx 0.647 \, kg$

3. The overall mass calculation: Averaging: $m = \frac{2.19 + 0.647}{2} = 1.42 \, kg$

Finally, rounding to 2 significant figures gives:

• $0.50 \, kg$.