Question

A student placed a few ice cubes in a drinking glass with water. A few minutes later she noticed that some of the ice cubes were fused together. Explain what happened.

Short answer

Ice cubes melted slightly, and the water between them refroze, fusing them together.

Step-by-step solution

Understand the Problem

Begin by observing that ice cubes are solid and when placed in a liquid, they can undergo changes in temperature. The phenomenon of ice cubes sticking together indicates that there is a physical change occurring.

Consider the Melting and Refreezing Process

When ice cubes are placed in water, the surface of the ice starts melting due to the warmer temperature of the water compared to the ice. However, the surrounding environment is still cold.

Explore Heat Transfer between Ice Cubes

The water between ice cubes can partially melt, creating liquid water. When adjacent ice cubes come into contact, the water in between may refreeze. This happens because the water cools rapidly from the contact with the surrounding ice, thus causing it to solidify and fuse the cubes together.

Identify the Role of Latent Heat

Latent heat is the heat required to change the state of a substance. As the ice cubes absorb heat from the water, some of this heat is used to melt a small layer of ice. Later, when this heat dissipates, it causes the liquid between the cubes to refreeze, fusing the cubes together.

Key concepts

Melting and Refreezing

When you pop ice cubes into a glass of water, you might notice a fascinating phenomenon: the ice cubes can sometimes stick together. This occurs due to a cycle of melting and refreezing. Initially, the warmer water surrounding the ice cubes causes their surfaces to start melting into liquid water. This happens because the ice seeks to balance the temperature difference with its environment, so it absorbs some heat and transforms from solid to liquid at the edges.
As time passes, if the temperature conditions remain cold enough, the light film of water around the adjacent cubes can refreeze. This happens because the ambient cooler temperatures of the remaining ice quickly pull heat away from the liquid, causing it to solidify once more. As a result, when these refrozen molecules form, they act like glue, sticking the ice cubes together. This ongoing cycle of melting and refreezing creates a seamless bond between the cubes.

Latent Heat

Latent heat is like nature's hidden lever operating in the background when a substance changes from one state to another, such as from solid to liquid. It's crucial in understanding the melting and refreezing of ice cubes. Even when temperatures hover around freezing, the ice absorbs latent heat from the surrounding water, necessary to make the transition from solid to liquid without changing temperature.
When the heat is absorbed, it breaks down the internal molecular bonds within the ice, leading to the formation of liquid water as a thin layer. But interestingly, when the ice cubes start refreezing, this latent heat that was absorbed is gradually released back into the remaining ice and surrounding water. The release allows the formerly liquid layer to lose energy, cool down, and refreeze, binding the cubes together. Hence, latent heat makes this physical state change possible, while barely perceptible in thermal shifts.

Physical Change

Turning from ice into water and back into ice is what we call a physical change. It is a transformation where the substance's form changes but its core identity—its chemical composition—stays the same. When you see ice cubes melting and fusing together in a glass of water, it's due to this type of change.
Unlike chemical changes, where new substances form, physical changes are all about shifts in state. Here, we're simply looking at water molecules rearranging themselves; their H2O essence remains constant. The temporary liquid water film around the ice doesn't mean we've changed the water chemically, it’s still the same molecules in a different state.
Physical changes are usually reversible, which means after melting, the process can swing back, allowing the water to refreeze and ice cubes to fuse. This dynamic back-and-forth dance in temperature is a hallmark of physical processes in action.