The diagram below shows a bowl used for making ice cream. The walls of the bowl contain a liquid coolant. The bowl is cooled to -20^circ C before the mixture is put in the bowl. The bowl causes the mixture to cool down and freeze. (c) Explain why the different thermal conductivities of metal and plastic are important in the design of the bowl. __ disappointed Motal (d) The liquid coolant has a freezing point below -20^circ C Explain one other property that the liquid coolant should have.

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The diagram below shows a bowl used for making ice cream. The walls of the bowl contain a liquid coolant. The bowl is cooled to -20^circ C before the mixture is put in the bowl. The bowl causes the mixture to cool down and freeze. (c) Explain why the different thermal conductivities of metal and plastic are important in the design of the bowl. __ disappointed Motal (d) The liquid coolant has a freezing point below -20^circ C Explain one other property that the liquid coolant should have.

Answer 1

The difference in the thermal conducts of metal and plastic are important in the design of the bowl because the metal, with its high thermal conductivity, efficiently draws heat away from the ice cream mixture and into the coolant causing the mixture to freeze rapidly. The plastic, with its low thermal conductivity, prevents the loss of cold from the bowl to the outside environment, ensuring the sub zero temperature is maintained. The liquid coolant should have a freezing point beneath \( -20^{\circ} \mathrm{C} \), be non-toxic and possess a good thermal stability.

Answer 2

## Step 1: Thermal conductivity is the rate at which heat passes through a specified material. Metals generally have high thermal conductivities and plastic has a low thermal conductivity. ## Step 2: In an ice cream making bowl, the purpose of having a metal and a plastic interfaces serves different role. The metal part which has high thermal conductivity, facilitates the transfer of heat effectively from the ice cream mixture to the coolant. It allows the ice cream mixture to cool down and freeze at a fast rate. ## Step 3: The role of plastic which has lower thermal conductivity becomes important to minimize the loss of cold to the external environment. It acts as an insulator preventing the surrounding heat to get into the system, helping maintain the temperature of the ice cream in the bowl. ## Step 4: A suitable liquid coolant should have a much lower freezing point than \( -20^{\circ} \mathrm{C} \) to ensure it remains in the liquid stat at that temperature. The coolant should also be non-toxic and have good thermal stability so it does not break down or decompose under extreme thermal conditions.

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