Please tell me if the following statements are correct…
[ol]
[li]Ice melts at 0C. (Don’t know how to make the degree symbol.)[/li][li]The temperature is supposed to stay constant at 0C until all the ice has melted. [/li][li]The temperature of the water formed by the melting ice is also supposed to stay 0C throughout the melting process.[/ol][/li]
[list=a]
[li]Water evaporates at all temperatures.[/li][li]Water is evaporating at 0C also.[/li][li]Temperature of the water vapour formed from melting ice is 0C.[/li][/list]
[quote]
[li]The temperature is supposed to stay constant at 0C until all the ice has melted. [/li][/quote]
The temperature of the ice will remain at or below 0oC. The temperature of the water can be at equilibroum with the atmosphere and conatiner and will almost always be higher than 0oC.
Nope. The water will gradually achieve equiliborum with the atmosphere and the container. There is nothing keeping it at 0oC.
You can demonstrate this yourself if you like. Heat a light (preferably aluminium) frypan or skillet then remove from the heat and drop in an ice cube. The water that melts off the cube will actually start to spit and boil at the edges while there is still solid ice left unmelted. IOW the water formed form melting is at 100oC while the ice remains at 0oC.
Nyahhhh. Not really.
Water can evaporate from a mass of water at 0oC. But saying that water evaporates at al temperatures is misleading. See below.
No. In order for a water molecule to vaporise it has to be travelling fast enough to overcome the intermolecular bonds within the mass of water. That means it will be travelling at the same speed as the average molecular speed in a mass of boiling water. It will effectively be at 100oC.
The problem is that referring to the temperature of a single molecule is pretty meaningless. Temperature is a measure of the average speed of lot of molecules. When we have only one molecule we can’t have an average speed, we can only have one speed. We can measure the velocity of a single molecule, and we can state what temperature a mass of those molecules would have if they were all travelling at that speed, but we can’t really have a temperature of a single molecule.
If we measured the velocity of a single water molecule flying off an ice cube and then got a cupful of water molecules to do the same average speed the cup of water would be boiling, ie 100oC.
Yes, thats typically the assumption used in applications. It doesn’t necessarily have basis in fact, but for most uses its a good approximation.
Yes, thats typically the assumption used in applications. It doesn’t necessarily have basis in fact, but for most uses its a good approximation.
Yes
Yes
Individual molecules that evaporate from water will be at various temperatures above 100 degrees C. See the phase diagram for water. As you can see, at 0C the pressure would have to be about .005 atmospheres for water to form. What happens is that there are individual water molecules that have enough energy to break away from the water. These individual molecules have a temperature higher than the rest of the water surrounding it.
This is why evaporation cools. The molecules that escape the water are on average the higher energy molecules and so the average energy of the water molecules is lowered which results in a lower water temperature.
This is all true IF the water-ice system is at perfect thermal equilibrium the entire time. If you start with a block of ice and add heat, the heat will go toward warming the ice to 0 degrees. At that point, assuming perfect equilibrium, all the heat will go toward turning the ice into water. Once it’s all melted, the heat will start warming the water.
The example of ice on a griddle is a good example of a system that’s not at equilibrium. Perfect equilibrium is one of those scientific idealizations that’s not really possible in the real world, but it can be approximated. I remember doing an experiment to prove exactly this process in one of my school labs.
One point has been missing. If the ice is melting, you are transferring heat to it. If it’s surrounded by water, you’re transferring the heat through the water. Therefore there is a temperature gradient in the water, and only the surface where the melting happens is at 0. Everywhere else is above 0.
In precision temperature work, the ice/water combination is used as a reference (it is actually very close to 0 C but not exactly). To use it you have to make sure there’s no melting or freezing. An even better reference is ice, water, and water vapor, in an enclosed “triple point cell”. You maintain this in a stable setup to deliver exactly 273.16 K or 0.01 C.
