Adding water to ice

 What happens when water   is added to ice.

When you mix water and ice, what happens depends entirely on the initial temperatures of both the ice and the water, the amount of each you have, and the environment they are in. Over time, the mixture will always move toward a state of thermal equilibrium (where everything reaches the same temperature). Here is a breakdown of what happens during this process under different conditions:

1. The Ideal Equilibrium:          Exactly 0°c (32F).

If you have a mixture of ice and water in a perfectly insulated container (like a high-quality thermos) and it has stabilized, a fascinating dynamic occurs:

Dynamic Equilibrium: 

The temperature of both the water and the ice will rest at exactly 0°c. At this temperature, water molecules are freezing onto the ice at the exact same rate that ice molecules are melting into the liquid.  

No Net Change: 

To the naked eye, nothing changes—the ice stops melting, and the water stops freezing.

2. When the Water Wins            (Melting): 

If you add ice to a glass of room-temperature water:

The Process: 

Heat from the warmer water flows into the colder ice.  

Latent Heat of Fusion: 

As the ice reaches 0°c, it begins to melt. Interestingly, while the ice is melting, the temperature of the water-ice mixture will stay right around 0°c because all the absorbed heat energy is being used to break the hydrogen bonds holding the ice together, rather than raising the temperature.

The Result: 

Once all the ice has melted, any remaining heat from the room will begin to warm the liquid water back up to room temperature.  

3. When the Ice Wins                 (Freezing):

If you add a small amount of liquid water to a massive, deeply frozen block of ice (say, straight out of a deep freezer at -18°c:

The Process: 

The sub-zero ice will rapidly absorb heat from the liquid water.  

The Result: 

The liquid water will quickly cool down to 0°c and then freeze solid, joining the ice block. The overall temperature of the system will remain below freezing.

4. What Happens to the Water Level?

A classic physics question is: If you have a glass of water with ice floating in it, does the water level rise as the ice melts?

The Answer: No, the water level stays exactly the same.  The Physics: 

According to Archimedes' Principle, a floating object displaces a volume of water equal to its own mass. Because ice is less dense than liquid water, it expands by about 10% when it freezes and floats. When that ice melts, it shrinks back down, perfectly filling the exact volume of water it was previously displacing.

Written on 1st October 2011

Author David I Birch