Education and
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at the Lunar and Planetary Institute
All About Ice

That's A N(ice) Temperature

Overview

In this 30-minute companion activity to The Melting Point, teams of children ages 8 to 13 experiment to find the melting and freezing points of water and ice.

What's the Point?

Materials

For each child


For each group of four to six children:


For the facilitator:

Preparation

Activity

1. Introduce the activity by discussing the freezing temperatures of water and the melting temperatures of ice.

Ask the children to make predictions about the melting point of ice.

Facilitator's Note: Changes in temperature can cause water to change state, and these changes occur at specific temperatures. Fresh water transitions between the solid and liquid states at 32°F (0°C) at sea level. At temperatures below 32°F (0°C), liquid water freezes; 32°F (0°C) is the freezing point of water. At temperatures above 32°F (0°C), pure water ice melts and changes state from a solid to a liquid (water); 32°F (0°C) is the melting point. For most substances, the melting and freezing points are about the same temperature.


Share with the children that they are about to undertake some experiments to determine the freezing point of water and the melting point of ice. Can they come up with some ideas for an experiment to find this information?

2. Divide the children into groups of four to six and provide them with their Ice Investigator Journals, the cups of crushed ice, thermometers, pencils or pens, and paper towels.

3. Prompt them to place the thermometer so that the bulb is sitting in the middle of the cup of ice, toward the bottom. It is important that the thermometer does not touch the sides or bottom of the cup and that the ice is tightly packed in the cup.

Invite the children to record their predictions in their Ice Investigator Journals.

After the thermometer has been in the ice for a few minutes, the children can attempt to get a temperature of their ice. The readings they collect may be close to 32°F (0°C), but may vary for several reasons. The warmer air in the room can circulate in the air pockets between the pieces of crushed ice, causing their readings to be warmer. Warmer readings may also arise because the thermometer was in contact with the sides or bottom of the cup, or it wasn't buried deeply enough in the ice, or the children may have held it out of the ice for too long before reading it. Colder reading may occur because standard household freezers are generally kept at about 0°F (-18°C) and the ice will also be that cold. In addition, some thermometers read differently.

Remind them that 32° Fahrenheit is the same temperature as 0° Celsius. They are just two names for the same temperature on different temperature scales. Scientists use the Celsius scale.

4. Allow the ice to melt for 5 to 10 minutes. Have the children collect a temperature reading once there is sufficient water to immerse the thermometer bulb in the water.

5. Invite each team to share the temperature readings from the water.

Share with the children that the temperature at which ice melts is called the melting point. The melting point is the temperature at which a solid turns to a liquid. The melting point at which ice — a solid turns to water — a liquid — is 32°F (0°C). Invite the children to record their responses in their Ice Investigator Journals.

6. If there is time, allow the ice to melt for another 5 to 10 minutes.

7. Have the children take another temperature reading.

Facilitator's Note: Contrary to our instincts, the melt water's temperature will not increase as long as ice is present.  The warm room continues to add energy to the cup, but all of that energy continues to go into melting the ice rather than raising the temperature.  Once all the ice has melted, that energy can begin to increase the water's temperature.

Invite the children to record their responses in their Ice Investigator Journals.

8. Provide the children with the larger container, ice cubes, foil, and salt. Demonstrate the different steps of the experiment and have the children repeat what you do.

Cover the entire bottom of the large container with 1/3 of the ice cubes.

Cover the top of the cup that contains crushed ice and water from the previous experiments with foil. Place it in the middle of the container on top of the ice.

Sprinkle half of the salt over the ice cubes. Create another layer of ice over the salt using 1/3 of the ice cubes. Sprinkle this layer with the rest of the salt. Cover it with the remaining 1/3 of the ice cubes. Make sure that the top of the cup with crushed ice is a little above the layers of ice and salt.

Remove the foil from the cup, place the lid on the container, and allow the experiment to sit undisturbed for about 10 minutes.

9. As the facilitator, repeat the experimental design for one additional container, but leave out the salt. This container will hold only ice cubes and the cup of crushed ice and melt. Share what you are doing with the children.

10. Invite the children to predict what will happen to the cups of crushed ice that are in the containers of salt and ice.

Facilitator's Note: Adding salt lowers the melting point of water.

Pure water and ice, kept insulated from the warm outside world, come to equilibrium over time. On a molecular level, water molecules are freezing onto the ice at the same rate as they are melting off of it. The entire water/ice solution is at the melting/freezing point, 32°F (0°C). Adding rock salt or any substance that dissolves in water disrupts this equilibrium. Fewer water molecules are interacting with the ice at any given moment, so the freezing rate is slowed. The salt has no effect on the melting rate, so more melting occurs than freezing — melting "wins" and the ice melts. In doing so, heat energy is used to break the hydrogen bonds that hold the molecules in the ice together. In other words, the ice "uses up" some warmth from the solution and the temperature drops. Melting and freezing again match rates ("tie") once the temperature has dropped to the new melting point.

The greater the amount of salt, the lower the freezing point (to a point; once there is sufficient salt that no more will dissolve, the freezing point no longer decreases). Ocean water is about 3.5% salt; sea water freezes at about 28°F (-2°C). A 10% salt solution freezes at about 20°F (-6°C), and a 20% solution freezes at 2°F (-16°C).


Prompt the children to brainstorm why they think their predictions will be correct.If you have already completed the companion activity, The Melting Point, remind them of their experiences with the salt on the ice cubes.

11. After 5 to 10 minutes, check to see if the predictions were correct! Note that the experiment should be stopped when some of the water has refrozen, not all. Have the children uncover their containers and lift the cup of crushed ice out of the bowl.

Have the children take another temperature reading of the remaining water.

12. Share the results from the container of ice without salt.

13. Invite them to find the temperature of the ice/salt mixture using one of their thermometers. At the same time, have them take the temperature of the crushed ice again.

Have the children continue to make notes about their findings in their Ice Investigator Journals.

14. Discuss their experimental findings.

Conclusion

If the children have started to construct a snow mobile, invite them to record any answers they discovered on the appropriate pieces. Some of the questions that they can answer might include:

Have them annotate any new questions they have or interesting things they learned on the appropriate shapes of either raindrop, cloud, or snowflake.