Journal 9/30/15

If you’ve ever gone to the beach, the sand is crazy hot and the water is still kind of cold. Shouldn’t both be the same temperature?

Objective Tonight’s Homework

To learn how energy moves as an object changes phase

p 88: review 1, 3, 8Facet: 4 WDYL: 3, 5

Phase Change

If you’ve ever boiled water, you know that it takes a lot of time to get the water to evaporate. Same with freezing water. It takes a while. Why?

Phase Change

If you’ve ever boiled water, you know that it takes a lot of time to get the water to evaporate. Same with freezing water. It takes a while. Why?

It has to undergo what we call “phase change”. When something changes from solid to liquid or liquid to gas, there’s extra amounts of energy we have to deal with.

Phase Change

Let’s take a look at this extra energy. Ice needs 2.060 J of energy to heat up by 1 degree C. (for reference, 1 joule is about the energy you’d get from dropping an apple from 3 feet up)

Phase Change

When our ice reaches 0°, it starts to melt. Before it heats up even a single degree more, it must melt. If we have 1 gram of ice, it will take an extra 335 J to get it to melt. This is called the Heat of Fusion or Enthalpy of Fusion.

Phase Change

As our 1 gram of liquid water heats up, it takes 4.186 J for each degree it heats up. To get it from 0 to 100 degrees will take 418.6 J of energy.

Phase Change

To turn our liquid water into steam we have to add a lot more energy. For our 1 gram of water, it will take a whopping 8,260 J to get it to just turn into steam. We call this the Heat of Vaporization or Enthalpy of Vaporization.

Phase Change

As our steam now heats up, it takes 2.020 J of energy for 1 gram to heat up by 1 degree.

Phase Change

Example (fairly hard):How much energy does it take to heat up 50 grams of water from 30 degrees C to 120 degrees C?

Phase Change

Example (fairly hard):How much energy does it take to heat up 50 grams of water from 30 degrees C to 120 degrees C?

From 30 degrees to 100 degrees:(4.186 J /g●C)(70 C)(50 g) = 14,651 J

Heat of Vaporization:(8,260 J)(50 g) = 413,000 J

From 100 degrees to 120 degrees:(2.020 J /g●C)(20 C)(50 g) = 2,020 J

= 429,671 J

Practice

1) How much energy does it take to heat up 33g of water from 0° to 68°?

2) How much energy does it take to heat up 33g of ice from 0° to 68°?

3) How much energy does it take to heat up 1 kg of ice from -12° to 144°?

4) How much energy is released by cooling 58 mg of water from 104° to 22°?

5) How much energy does it take to heat up 1g of water from 23° to 24°?

Exit Question

Do you think all materials have a triple point?

Probably most of themProbably not many of themIce cream!