## Lab: Dry Ice and Water Ice: Expected Student Responses

### Part I: Does Heat have to be Hot?

Your teacher will show you a demonstration in which water ice is placed on two different surfaces.

The main this here is to have students discuss aloud their ideas and to expose them to the concepts. It’s important to listen to what they have to say and to validate their ideas while leading them to a better understanding of what is going on.

1. There is a container with a mixture of ice and water. What is the temperature?

The measured temperature should be near or below 4°C. Mention that water freezes at 0°C.

2. There are two black squares. One is heavy and one is light. Describe the heavy one. Pay particular attention to how warm or cold it feels.

The heavy one is made of solid aluminum. It will feel cold to the touch due to its high conductivity for heat. It is heavy and smooth.

3. Describe the lighter black square. Also, is it warm or cold?

The light one is made of a plastic foam, which can be seen on the sides. The surface is textured and it feels warm to the touch.

4. Your teacher will measure the temperature of the two squares. Record those temperatures here.
Heavy:      Light:

Both should be room temperature, about 22°C

5. Did the measured temperatures match what you thought they might be? Why or why not?

Students probably expected the heavy square to be colder than the lighter one. Explain that since they are in the same room in contact with the same air they can be expected to have the same temperature.

1. Make a prediction based on what you have observed so far. What will happen when an ice cube is placed on each square? Will one melt faster than the other? Explain why you think so.

Students will likely anticipate that there is some trick. Get them to predict which square’s ice cube will melt faster when they ignore the feeling that there is something funny going on. Elicit some thoughts and explanations and reflect them back.

2. What actually happened? Describe what happened and explain what you observed.

The ice cube on the aluminum square should have melted completely within a minute. The one on the plastic foam should have melted hardly at all. This is because the aluminum is a good conductor while the foam is an insulator. Heat can move easily into the ice cube on the aluminum but only very slowly into the one on the foam. The point is that it is heat that drives a phase change such as melting, not temperature. Both squares had the same temperature (or nearly) but the ice cube melted faster on the square that could provide the heat faster by conducting it from the surroundings.

3. What is the temperature of the mixture of ice and water? Has some of the ice melted since you last looked at it? Does the temperature have to rise for ice to melt?

The temperature of the ice and water in the styrofoam cooler should still be about the same but the ice has in fact melted a bit: some pieces pulled out are smaller than before. The temperature does not have to rise for ice to melt. The ice just has to absorb some heat, which this ice has done by absorbing it from the air.

4. What melts ice, heat or temperature? Explain.

Heat is what melts ice and causes water to evaporate. Heat has to be added change the temperature of something to its melting point temperature but after that the heat causes the phase change and the temperature does not have to change.

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### Part II: Molecules

The informational section has been left out of this document.

In the spaces below create some molecule drawings for the phases of carbon dioxide (CO2). A molecule of carbon dioxide has one atom of carbon in the middle bonded to two atoms of oxygen and looks like this:

 The drawing should show multiple copies of the carbon dioxide molecule neatly arranged in a regular pattern. (tightly packed, nice and regular) Solid The drawing should show multiple molecules randomly arranged with a bit more space between them. (a bit looser but random) Liquid The drawing should show just a few molecules widely spaced apart and randomly arranged. (very few and far apart) Gas

### Part III: Dry Ice

Deliberately leaving out the safety information because it is on the main handout.

Your teacher will show you how to do each of the demonstrations below. You will have a chance to try each of them and record your observations.

#### Singing Spoon

Hold a piece of dry ice with a pair of tongs pressed against the table top. Press a warm spoon against the dry ice.
1. Describe what you see and hear.

The spoon, if it is warm, will bounce on the surface of the dry ice and make a loud squealing noise.

2. What do you think happens to the molecules of CO2 when you press the spoon against the dry ice?

The CO2 is turning from a solid into a gas.

3. Why do you think it makes that noise?

The spoon gets pushed away by the escaping gas but it being pressed against it so it bounces back and forth at high speed.

4. Try putting dry ice on the ice melting blocks. How does it behave? Why?

The dry ice on the aluminum square will disappear faster than the one on the foam, for the same reason that the water ice melted faster.

#### Air Hockey

Use tongs to place a piece of dry ice on the table. Let it sit awhile: look at it closely while you wait about a minute. Once the bottom of the piece of dry ice has flattened out, try hitting it from the side to make it slide.
1. Describe what you see and hear.

The dry ice acts like the puck on an air hockey table and moves without friction.

2. What do you think happens to the molecules of CO2 where the dry ice is touching the table?

The molecules of CO2 turn rapidly into a gas.

3. Why do you think it moves so smoothly?

The gas produced by the dry ice creates a cushion underneath it that is enough to hold it up off the surface of the table a tiny bit.

#### Pop-top Film Canisters

Use tongs to place a small piece of dry ice in one of the film canisters. Seal it up (this is safe because the top will pop off before the container builds up enough pressure to explode).
1. Describe what happens.

After a brief wait the lid pops off the film canister with a loud noise and a high speed.

2. Try putting a quarter inch of warm water in the canister with a piece of dry ice before you seal it. Describe what happens and whether it is different from the experiment done without water.

With a little warm water in the canister the dry ice sublimes more quickly and the top pops off a lot sooner.

3. What is causing the lid to pop off?

As the dry ice turns from a solid to a gas the pressure builds up inside until it is large enough to break the seal and pop off the lid.

4. Why is it different when warm water is in the canister?

The warm water speeds up the production of gas.

#### Fog

Use tongs to place one or two pieces of dry ice into a cup of cool water. Use tongs to place one or two pieces into a cup of very warm water.
1. Describe what you see.

Bubbles of CO2 rise up from the bottom of the cup and fog appears at the top. The warm water produces a lot more fog than the cool water.

2. What is the fog made of?

Students will immediately say it’s carbon dioxide but it is really tiny droplets of condensed water vapor.

3. Why is there a difference in the amount of fog from each cup?

The warm water produces more water vapor than the cool water and so as the cold CO2 rises up there is more vapor to condense to make the fog.

4. Why are the pieces of dry ice always surrounded by a little bit of fog?

Dry ice is always surrounded by a mysterious-looking fog because the air always has a little bit of water vapor in it. The cold CO2 causes it to condense so we can see it.

Questions

Students may or may not have any further observations or questions but this could be a fun part of a post-activity discussion.

2. What is the term used to describe a solid turning into a gas? Use it in a sentence to describe something you observed today.

The term is sublimation. A solid sublimes when it turns directly into a gas without melting first.

3. Did you see a liquid phase for CO2? Explain.

The carbon dioxide never melted so we could not see it in its liquid phase. At some point the teacher may mention (now or earlier) that in order to be a liquid carbon dioxide needs to be at a very high pressure: over 5 times atmospheric pressure. Equivalent to being 53 meters (173 ft) under water.

4. Explain how you know that a solid does not always have to melt before it turns into a gas.

We observed this very thing as we played with the dry ice.

5. Do you think water ice can turn directly from a solid to a gas? Why or why not?

Students may think not since it is difficult to observe and probably none of them ever have. Two examples that may resonate are snow banks and frosty freezers. Snow banks can diminish in size when the sun shines on them without melting. This is because the ice is subliming. Freezers get all frosty inside over time as water sublimes from food and ice cubes and then freezes again on other surfaces.

6. Is it an exchange of heat that causes a phase change or a change in temperature that causes a phase change? Why do you think so?

This is really an advanced question that today’s experience may not give students the necessary experience to answer properly. However, it is a good idea to plant seeds of ideas so that they question assumptions in the future.

Heat exchange is what causes phase changes. In order to overcome the weak stickiness that affects all molecules heat must be absorbed to make the molecules break away from each other. The heat that breaks those connections can’t make the temperature rise so phase changes often happen at one constant temperature. The ice melted in the first demonstration while the temperature of the aluminum went down. This is because heat is used up in melting the ice. The ice and water in the cooler at the beginning remained about the same temperature while the ice cubes melted because they absorbed heat to cause the melting and there was none left to raise the temperature.

Last Updated: Feb 01, 2019
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