Question

You drop an ice cube (made from pure water) into a saltwater solution at \(0^{\circ} \mathrm{C}\). Explain what happens and why.

Short answer

When an ice cube made of pure water is dropped into a saltwater solution at \(0^{\circ}\mathrm{C}\), it begins to melt due to the surrounding saltwater solution having a lower freezing point than pure water. This is because dissolved salt ions interfere with the formation of the crystalline ice structure, making it harder for the water molecules to freeze. As the ice cube melts, it dilutes the saltwater solution, causing its freezing point to rise closer to that of pure water. Eventually, thermal equilibrium is reached and the ice cube may dissolve completely, unless the ice cube is too large or the saltwater solution is already at its maximum freezing point depression.

Step-by-step solution

Understanding the Components

Before we proceed, let's understand the two components involved in this scenario. First, we have an ice cube made of pure water, meaning it is frozen water without any contaminants. Second, we have a saltwater solution at 0 degrees Celsius. This means the solution has salt (typically NaCl) dissolved in water, and it is at the freezing point of pure water.

Melting Point Depression

In a saltwater solution, the presence of salt particles interferes with the formation of solid water structures and lowers the freezing point compared to that of pure water. This phenomenon is known as "melting point depression" or "freezing point depression". When salt is dissolved in water, the freezing point of the solution decreases because the dissolved salt ions interfere with the ability of the water molecules to form crystalline ice structures, making it harder for the water to freeze.

What Happens to the Ice Cube

When the ice cube is dropped into the saltwater solution, the surrounding liquid is at 0 degrees Celsius, which is the freezing point of pure water but not the freezing point of the saltwater solution. As a result, the ice cube begins to melt since it is now in contact with a solution that is warmer than its own freezing point.

What Happens to the Saltwater Solution

As the ice cube melts, the water from the ice cube will mix with the saltwater solution. This addition of pure water will dilute the saltwater solution slightly, causing its freezing point to rise closer to that of pure water. However, as long as the salt concentration remains higher than zero, the freezing point of the saltwater solution will still be lower than 0 degrees Celsius.

Equilibrium and Final Scenario

The melting of the ice cube will continue until thermal equilibrium is reached between the ice and the saltwater solution. At this point, the temperature of the saltwater solution may drop slightly due to the melting of the ice and the dilution of the salt concentration. In most cases, the ice cube will eventually dissolve completely in the saltwater solution, unless the ice cube is too large or the saltwater solution is already at its maximum freezing point depression.