In a household freezer, ice cubes will form most quickly if the trays used are made of which material?

Introduction / Context:
When making ice cubes in a freezer, the material of the tray affects how fast heat can be removed from the water. Everyday observations show that ice forms faster in some trays than in others. This question tests your understanding of thermal conductivity and how different materials transfer heat, which is a basic concept from heat and thermodynamics with very practical applications.

Given Data / Assumptions:

• A household freezer is used to freeze water into ice cubes.
• Different tray materials are considered: rubber, plastic, aluminium and wood.
• The environment inside the freezer is at a temperature below the freezing point of water.
• The goal is to identify the tray material that allows ice cubes to form the fastest.

Concept / Approach:
The rate at which water cools and freezes depends significantly on how quickly heat can flow from the water, through the tray, to the cold air in the freezer. This heat transfer is strongly influenced by the thermal conductivity of the tray material. Metals, such as aluminium, have very high thermal conductivity and allow heat to flow out rapidly. Nonmetals like rubber, plastic and wood are thermal insulators with much lower conductivity. Therefore, water in an aluminium tray loses heat faster and reaches the freezing point sooner than water in trays made from insulating materials.

Step-by-Step Solution:
Step 1: Identify that freezing involves removing heat from water until its temperature falls to 0°C and latent heat is removed.Step 2: Recognise that the tray serves as the path for heat to leave the water and enter the cold air inside the freezer.Step 3: Recall that aluminium is a metal with high thermal conductivity.Step 4: Remember that rubber, plastic and wood have low thermal conductivity and act as insulators.Step 5: Conclude that an aluminium tray allows heat to flow out of the water faster than the other materials.Step 6: Therefore, ice cubes will form most quickly in aluminium trays.

Verification / Alternative check:
Practical kitchen experience supports this conclusion: metal containers feel colder or hotter to the touch than plastic ones at the same temperature because they conduct heat rapidly. Additionally, insulating materials are often used where we want to slow down heat loss, such as in handles of cookware or in thermal flasks, not where we want to speed up cooling. In contrast, metals are used for radiators, heat sinks and cooking pans where efficient heat transfer is desired. This real-world pattern confirms aluminium as the best choice.

Why Other Options Are Wrong:
Rubber is a poor conductor of heat and would slow down cooling and freezing. Plastic also has low thermal conductivity and is often used for insulation or to prevent heat transfer. Wood is similarly a good insulator and is not suitable when rapid heat transfer is required. Therefore, trays made of rubber, plastic or wood will freeze water more slowly compared to aluminium trays.

Common Pitfalls:
Some students may wrongly think that any tray inside a very cold freezer will freeze water at the same rate, ignoring the effect of the tray material. Others might focus only on the temperature of the air and not on the rate of heat conduction. To avoid such errors, always remember that heat transfer rate is influenced by the thermal conductivity of the material through which the heat must flow.

Final Answer:
Ice cubes will form most quickly in trays made of aluminium.