Energy Transfer in Ice Melting
Date: Winter 2012-2013
When ice melts in a warm room where does the energy from the air go to melt the ice?
Great question! You are indeed correct that it takes energy to melt ice. Energy can have several different forms, but the energy we are concerned with here is heat energy, and temperature is one way that we can measure heat energy. Temperature also tells us which way heat energy will flow -- heat always flows from warmer temperature to colder.
Let us look at ice. Water can exist in three forms: liquid, solid (ice), and gas (water vapor). Scientists call these forms "phases". If you measure the temperature of pure ice, it can be as high as 0 deg C but no higher. If it was higher, it would not be ice at all but liquid water. So let us say we take an ice cube out of the freezer and put it on the kitchen counter. The temperature of your freezer is probably around -18 deg C, and the kitchen room temperature is about 20 deg C. The first thing that the ice will do is warm up, but it will not melt -- yet. It is going to warm up from -18 deg C all the way to 0 deg C with no observable melting. How does it warm up? Remember that heat flows from warm temperature to cold temperature. The warm air and warm counter top are transferring heat energy to the ice cube which warms it up.
When the ice cube reaches 0 deg C, now something different happens. It starts to melt, but it no longer changes in temperature. Heat energy is still being transferred into the ice cube (remember the temperature difference between the ice cube and its surroundings), but now the heat is accomplishing something else. The heat is making the ice change from the solid phase to the liquid phase (in other words, it melts). This is called a phase change and they require energy to happen. One important thing to remember about phase changes is that they occur at a constant temperature, since the heat energy is being used for changing phase instead of changing the temperature. The heat to melt ice into water has a name.The amount of heat it takes to melt water at constant temperature is called the "latent heat", and it has units of energy per mass of water.
John C. Strong
Although we cannot see them, atoms and molecules are always moving. They either fly around (in a gas), roll over each other (in liquids) or stay put and vibrate (in solids). The hotter molecules get, the faster they move.
In ice the molecules of water are moving so slowly that they join up and form a solid The water molecules stay pretty much in the same place and vibrate just like a bunch of people linking arms, standing out in the snow, shivering.
When heat comes in from the outside of an ice cube (from the air), the molecules warm up and shiver faster but, like those cold people, they still hold on to each other until everyone is just warm enough. Since every molecule has to have just enough energy, ice cubes melt at one temperature (everyone in the crowd lets go at once) instead of getting mushy as they heat up. When that happens, the molecules separate and begin to roll around over each other - the ice cube melts.
The heat from the air goes into moving those molecules faster and faster.
Hope this helps.
Oklahoma State University Institute of Technology
Thanks for the question. When ice melts in a warm room, the air in the room becomes cooler because the heat energy goes from the air into the ice.Go ahead and try this at home with your parent's permission. Use a thermometer to measure the temperature of the ice. You will see the ice stays at 32 degrees F as it is melting. That is because the ice absorbs heat energy to go from the solid form to the liquid form.
I hope this helps. Please let me know if you have any additional questions.
Assuming there is no subsequent heating of the water, the energy in the form of heat from the air goes to converting the ice into liquid water at 0C.
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Update: November 2011