# Why does temperature remain constant during melting/boiling?

1. Sep 27, 2013

### sgstudent

When ice melts at 0°C, energy is being absorbed to break the bonds. And so temperature remains constant. However, when some of the atoms enter the liquid state why can't they continue gaining heat and increase in temperature. So that this way, the overall temperature increases instead of remaining the same?

Thanks in advance :)

2. Sep 27, 2013

### Vanadium 50

Staff Emeritus
Let's start from the top. Do you think your ice/water mix is at a single temperature or not?

3. Sep 27, 2013

### sgstudent

I think so? Because in the graph for temperature against time it stays constant for the substance that's being heated during its melting.

4. Sep 27, 2013

### Vanadium 50

Staff Emeritus
OK, so if it is all at the same temperature, how do you bring the ice to above its melting point without melting it?

5. Sep 27, 2013

### sgstudent

I think by increasing the pressure the melting point would increase. But even at normal atmospheric pressure, if we have a piece of ice that's is melting in a 25 degrees environment, won't the melted part of the ice/water be exposed to the 25 degrees environment and get heated up while the ice part would continue melting at 0°C?

So in other words in a 25°C room, the ice part of the ice/water part is still 0°C while the water part can be at a temperature greater than 0?

6. Sep 27, 2013

### Vanadium 50

Staff Emeritus
So are you arguing that the system is all at the same temperature (#3) or not (#5)?

7. Sep 27, 2013

### sgstudent

Hmm I'm thinking when the ice is taken out to a hot environment, it starts to melt. So the ice part of the system is still 0 degrees while the water part that melted from the ice is free to heat up to above 0 degrees?

8. Sep 29, 2013

### Vanadium 50

Staff Emeritus
So now you have to decide whether you are talking about "small" things or "large" things. Small things are in thermal equilibrium, which means they have one temperature. Large things - like the earth - are not in thermal equilibrium, and different parts can have different temperatures. Your confusion stems from trying to take a statement about small things and apply it to large things.

9. Sep 29, 2013

### sgstudent

Hi thanks for the reply

Would the ice/water vase be like the small thing that's in thermal equilibrium? because I was thinking about the distillation case also whereby the thermometer would read the temperature of the gas to be at the bolong point of the gas exactly. But similarly to this case, I thought the evaporated gas can actually heat up to be more than its boiling point. For example in a ethanol water fractional distillation the temperature read is 78degrees initially which is the boiling point of the ethanol. But as the ethanol vaporized I thought it could increase in temperature as well.

Thanks so much for the help

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