IDS 102 Name______

Change of Phase

When substances change from solid to liquid or liquid to vapor, we call this a “change of phase.” On the atomic level, we can describe a change of state as a change in the number of bonds holding the atoms together. Generally speaking, the number of bonds between atoms in a solid is greater than the number of bonds between atoms in the corresponding liquid. The number of bonds between the atoms also decreases dramatically when you move from a liquid to a gas.

In our study of heat and temperature, we found that if you mix warm and cold water (H2O) together you get a final temperature that is in between the starting temperatures and dependent on the mass of the warm and cold water. We also found that solid rocks and metal absorb heat at a different rate than liquid water.

But what about solid H2O (aka ice)? In the following experiments we will examine how heat is absorbed by both the liquid and solid forms of H2O (in other words, how water and ice compare).

Ø  Make a prediction. (Recall that your answer this does not have to be correct. It is important that we think about our initial ideas before we can hope to correct any mistaken views.) If you combine 30g of 20 deg C water and 30 g of ice at 0 deg C, what will be the resulting temperature? Explain your logic.

On the cart, find:

·  a Styrofoam cup and lid

·  a thermometer

·  some ice

·  paper towels

·  graduated cylinder

·  an electronic scale

Put the Styrofoam cup and lid on the scale and press the “Tare” button until it measures zero. (A nice feature of the electronic scales is that the since you got the “tare” weight of the cup as zero, the mass on the scale is the mass of the water).

Tare the scale again – this time without the Styrofoam cup. Put a piece of paper towel on the scale and grab put about 90- 100grams of ice on the towel. Record the mass of the ice on the table below, move the ice into the Styrofoam cup, and dry the scale.

Measure out an amount of tap water into the cup that is approximately equal (within a few mg) to the mass of the ice. Record the mass of the water in the table below.

Determine the temperature of the water in the cup. Record the temperature of the water in the data table.

Add to the water to your ice. Put the lid on the cup and record the mass of the cup, lid, water and ice. Gently stir the water in the cup and monitor the temperature. Record the lowest temperature you observe in the scale below.

Mass of ice / Mass of water / Initial temperature of the water / Final temperature of water plus ice
Trial # 1
Trial # 2
Trial #3

Now use the information above to fill in the table below:

Initial Temperature of ice / Initial Temperature of water / DTice / DTwater
Trial # 1 / 0°C
Trial # 2 / 0°C
Trial #3 / 0°C

What was the average change of temperature for the water (= average DTwater)?

What is the average heat lost by the water? (Show your work)

What is the average heat gained by the ice?

What was the average change of temperature for the ice (= average DTice)?

How does the change in temperature for the ice compare with the change in temperature for the water?

Check with your instructor before continuing.

In the previous experiment, you should have found that the change of temperature for the ice differed from temperature for the water. This is because heat energy can also be used to change the phase, thus as ice absorbs heat it melts. Only after all of the ice melts will it start to change temperature.

Think of the ice melting as a two-step process:

1st step: There was the heat required to melt the ice (in this case 80 calories for every gram of ice). The heat required the change something from a solid to a liquid is called the “heat of fusion.” The heat of fusion varies for different types of materials.

The temperature of the water produced in this fist step of melting is 0oC.

2nd step: the rest of the heat will raise the temperature of the ice-water until it reaches the final value.

If we could study the temperature of the warm water and the ice separately. We would see a relatively constant decrease in temperature while the ice remains the same temperature as the heat transferred from the warm water melts the ice and then the after all the ice is melted, the water that is a part of the ice changes temperature to the final temperature.

Summary: When ice melts it absorbs heat from the environments. The process also happens in reverse: when ice freezes, it releases heat into the environment.

A similar process happens when water (or any substance) changes from a liquid to a vapor. This heat is called “heat of vaporization.”

Change of Phase

End of Module / Concept Review Question:

Many animals (including humans) stay cool in the heat by sweating. (This process is called evaporative cooling). Why does the evaporation of water from your skin cause you to feel cool? Explain this in terms of heat absorption and heat of vaporization.

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