# Heat and Phase change: latent heat

1. Jun 19, 2007

### scw287

1. The problem statement, all variables and given/known data

A woman finds the front windshield of her car covered with ice at -12.8°C. The ice has a thickness of 4.60 10-4 m, and the windshield has an area of 1.25 m2. The density of ice is 917 kg/m3. How much heat is required to melt the ice?

2. Relevant equations
Q=mL or change temp=Q/constant*mass

3. The attempt at a solution
:surprised Volume = Area * thickness<<then found the density
Q=mct Energy = mass* specific heat* cahnge in temp
heres the numbers that I plugged in: Q=(917)*(5.75*10^-4)*(12.8)*(22.6*10^5)<this last number is the latent fusion for water

the answer I got was wrong. not sure if I'm make a simple mistake or completely on the wrong track!

2. Jun 19, 2007

### Staff: Mentor

Try this. Think in terms of two separate steps:
(1) You must raise the temp of the ice to the melting point
(2) You must melt the ice

Find the heat needed for each step and add them up.

3. Jun 19, 2007

### scw287

1.if i raise the temp of the ice to the melting pt. it would need to be at its boiling pt 100 degrees celcius. heat rises

2. now must melt the ice at 0 degrees celcius

I'm sorry I really don't know what I'm doing just learned this for the first time today along with four other physics chapter.

could I use m*L<heat gained
and c*m*change in temp.< heat lost
and set the two equal?
?

4. Jun 19, 2007

### Staff: Mentor

What temperature must the ice be in order to melt? Just melt, not boil!

5. Jun 19, 2007

### scw287

well the melting point is at 0. degrees celcius

6. Jun 19, 2007

### scw287

is the latent fusion the same thing as the temperature for which ice needs to be in order to melt?

7. Jun 19, 2007

### Staff: Mentor

Right. So how much heat is needed to raise the temperature of the ice? (Hint: Specific heat)

The latent heat of fusion tells you how much energy you need to melt something when it's already at the melting point.

8. Jun 19, 2007

### scw287

Specific heat for water=4186

9. Jun 19, 2007

### scw287

So I need to use the formula Q=mass*specific heat*change in temp for the first part right?

10. Jun 19, 2007

### Staff: Mentor

What's the specific heat of ice?
Right.

11. Jun 19, 2007

### scw287

good after I find that I need add that to melting the ice. How does the area and the density come into play?

12. Jun 19, 2007

### Staff: Mentor

It will allow you to find out how much ice you need to melt.

13. Jun 19, 2007

### scw287

I'm still not sure how to find the melting of the ice, do i need to use latent heat constant?
ahhh so add the two values together and then multiply by the area*the density?