Dry Ice (Catherine Nicholas)

Sublimation and Fog

Principles:

This lesson related to many California science standards, particularly those for 5th grade science, 8th grade science and chemistry. You can relate this lesson to changes in states of matter, specifically sublimation, or to the weights of gasses, or to weather, including fog and cloud formation.

Materials:

• Clear plastic bowl (I got mine at the Dollar Tree)
• Plastic bottle or plastic flask (I used a spray bottle, but a water bottle would work too, it is best if it is see through)
• 2 balloons (helium quality are best)
• Safety goggles
• Gloves (ones that will protect against heat will also protect against cold)
• Hammer
• tongs
• Warm water
• Dry Ice (I buy mine at Ralph's, you have to ask someone, because they keep it locked up, you must be 18 to purchase)
• Pie tin (this is a good place to break up the dry ice)

Procedure (with explanation and discussion points):

• CAUTION: DO NOT TOUCH DRY ICE WITH YOUR BARE HANDS!
• Put on your safety gear: goggles, gloves, and an apron or lab coat if you would like
• Carefully break up the dry ice in the pie tin with a hammer, you want some large pieces and some small pieces
• Explain that Dry Ice is -79^oC and that at standard temperature and pressure on earth that it will sublimate (go from solid to gas, skipping the liquid phase)
• Place many small pieces of the ice into the plastic bottle using tongs or gloves
• Stretch one balloon over the bottle and set it aside to be observed later
• As the balloon begins to fill, you can explain that dry ice is made of carbon dioxide and ask what the balloon is being filled with (if you let the ice sit in the bottle for a minute before placing the balloon on, then CO₂ will be what is filling the balloon, otherwise the heavier CO₂ will just be pushing the air that was in the bottle up into the balloon)
• Next, put warm water in your clear plastic bowl and place dry ice into the water, you should put a large piece or two into the water
• The students will be excited, so let them be excited about the fog for a minute, and then talk about what is happening
• Point out that the bubbles coming off of the ice are clear and that the fog is white, so these cannot be the same thing (explain that the bubbles are the sublimating carbon dioxide and that the smoke or fog looking stuff is literally fog or a cloud, the cold gas coming off of the ice is condensing the water vapor in the air and creating a cloud)
• Set the bowl aside and go back to the bottle. The balloon should be getting larger, you can shake up the ice to cause heat and make the balloon inflate a little more. Tie off this balloon.
• Take a balloon of a different color and blow it up to about the same size and tie it off.
• Let some students feel that the dry ice balloon is heavier, and then you can talk about the make up of the gasses in the air and talk about or calculate that CO₂ is heavier than O₂ and N₂.

Student's Necessary Prior Knowledge:

It is best if students are learning about state of matter and changes in states. Sublimation is a special case and is best understood after learning about the basics about the states of matter. If you are going to be comparing the mass of the different gasses, then students should be comfortable with the periodic table and calculating the masses of compounds.

Explanation:

Dry ice is very cold, -79^oC. Regular, water ice is zero degrees Celsius. Dry ice is formed from carbon dioxide and on earth it sublimates. Sublimation is the process when matter goes from a solid straight to a gas and skips the liquid phase. When the dry ice is put into the bottle and is allowed to sit open for a minute, the dry ice will sublimate and the bottle will fill with carbon dioxide and push the lighter air out. Once the balloon is placed onto the bottle, the balloon will begin to inflate with carbon dioxide, as this is the gas that is being released from the dry ice. Many students may guess that it is helium or air, but you must explain that the block of ice is made of only carbon dioxide, so the only thing that is available in the bottle to fill the balloon is carbon dioxide and that this must be what is in the balloon.

When dry ice is placed in to warm water, sublimation will continue to occur as it does in air, but since the water is warmer than the air, it will happen more quickly. You will see the gas coming off the ice in the form of bubbles. You will also see fog on top of the water. The fog is white, but if you look carefully you realize that the bubbles are clear, this should be the first hint to the students that the fog and the bubbles are not made of the same stuff. The bubbles must be made of carbon dioxide, again this is the only thing available to be coming off the block of dry ice. The stuff on top is fog or a cloud, but students will tell you it looks like smoke, lead them away from this and into thinking about what else it could look like. The cold carbon dioxide gas is condensing the water vapor that is in the air and literally making a cloud. You can explain that this is how fog is formed, that cold air, especially near the water will condense the water vapor that is in the air and form fog.

When you compare the weight of the balloon with carbon dioxide and the balloon you blew up, you can compare the weights of the gasses that are in each. The air contains 78% nitrogen gas, 21% oxygen gas, and the last 1% is made of other gasses (when you exhale, your breath, like you are using in the balloon is still 78% nitrogen gas, 17% oxygen gas, 4% carbon dioxide gas and 1% everything else). Carbon dioxide has a mass of 44 g/mol, nitrogen gas has a mass of 28 g/mol and oxygen gas has a mass of 32 g/mol. This should make it clear to the students that the balloon with carbon dioxide will be heavier and then you can prove it, because with two balloons of the same size, the carbon dioxide balloon will be heavier.

Questions and Answers:

1) What is filling up the balloon?

The balloon is being filed with carbon dioxide gas as this is what is sublimating from the dry ice which is carbon dioxide solid.

2) How do you know that the bubbles in the water are not the same thing as the white stuff on top of the water?

The bubbles in the water are clear and the stuff above the water is white. The bubbles are made of the carbon dioxide that is coming off of the dry ice, while the white stuff is actually water vapor condensing when it is cooled by the carbon dioxide, but in the case of the white part, we cannot see the carbon dioxide, as it is clear.

3) What is the white stuff floating over the bowl? How is it formed?

The white stuff over the bowl is fog or a cloud. It is formed when the cold carbon dioxide condenses the water vapor that is in the air and literally forms a cloud.

Applications to Everyday Life:

• This is a great way to learn about how fog and clouds form when the air is cold.
• This can tie into the fact that carbon dioxide sublimates on Earth, due to the pressure here, but on Mars, carbon dioxide can be in a liquid form since Mars has different pressure than the Earth does, this has caused confusion for NASA's Martian missions in the past because they have gotten excited to find liquid water and later realized that it may have been liquid carbon dioxide.
• This is just one example of one phase change, we see the other phase changes in every day life.