Ice is used as a coolant due to the high specific heat capacities of ice and water and the high specific latent heat of fusion of ice - Leaving Cert Physics - Question 9 - 2021

The high specific latent heat of fusion of ice makes it a good coolant because it takes a large amount of energy to change ice into water without changing temperature. This property allows ice to absorb a significant amount of heat from the environment, maintaining a low temperature and effectively cooling the surrounding area.

1. Better insulator: Hollow plastic traps air, which is an excellent insulator, reducing heat transfer and keeping the contents cooler for a longer time.
2. Lighter: Hollow plastic is lighter than solid plastic, making the picnic box easier to carry while still maintaining durability.

To find the final temperature, we can use the equation:

$m_{c} \Delta T = -m_{i} L_{f}$

Where:

• $m_{c} = 0.25 \, \text{kg}$ (mass of food)
• $c = 17800 \, \text{J kg}^{-1} \text{K}^{-1}$ (heat capacity of food)
• $m_{i} = 0.25 \, \text{kg}$ (mass of ice)
• $L_{f} = 3.3 \times 10^5 \, \text{J kg}^{-1}$ (latent heat of fusion of ice)
• Initial temperature of ice = -18 °C & Initial temperature of food = 10.5 °C.

Now calculating:

1. For the food:

   $\Delta T = T_{final} - 10.5$

2. For the ice:

   $\Delta T = 0 - (-18) = 18$

Combining both:

$0.25 imes 17800 imes (T_{final} - 10.5) = -0.25 imes 3.3 \times 10^5$

$4450(T_{final} - 10.5) = -82500$

$T_{final} - 10.5 = -18.54$

$T_{final} = 10.5 - 18.54$

Calculating gives us:

$T_{final} = 5.04 \, °C$

The heat pump diagram should include the following components labeled:

• Compressor
• Expansion valve
• Condenser
• Evaporator

Arrows should indicate the flow of refrigerant through each stage.

A heat pump operates by transferring heat from one location to another. It consists of:

1. Evaporator: The refrigerant absorbs heat and evaporates.
2. Compressor: The gaseous refrigerant is compressed, increasing its temperature.
3. Condenser: The refrigerant releases heat as it condenses back to liquid.
4. Expansion valve: The refrigerant expands, dropping in pressure and temperature, returning to the evaporator to repeat the cycle.

The student might have observed that:

• The ice melted during the experiment.
• Heat was transferred from the water to the ice, causing the water temperature to decrease.
• The rate of melting would vary based on the temperature difference between the ice and the surrounding water.

The student could conclude that water is a good conductor of heat, as it effectively transfers heat to the ice, facilitating its melting and demonstrating the principles of thermal conductivity.