The Melting Point
In this 15-minute companion activity to That's a (N)ice Temperature!, teams of children ages 8 to 13 predict which ice cube will melt faster, one sprinkled with salt, or one without salt. After making their predictions, the children pour salt on one ice cube and leave the other untouched, then observe for two minutes to see if their predictions were correct. Children learn that adding salt — or other substances — to ice lowers the melting point of ice. Finally, they put their knowledge to the test by making ice cream!
What's the Point?
- Water can exist in different states; ice is the solid state of water.
- The melting point of pure water ice is 32°F (0°C).
- Adding salt — or other substances — to ice lowers the melting point of ice.
For each child
- His/her Ice Investigator Journal
- 1 pencil or pen
- 1 (1-quart) Ziploc baggie
- 1 (1-gallon) Ziploc baggie
- 1 tablespoon sugar
- 1/2 cup milk
- 1/4 teaspoon vanilla
- 2 tablespoons rock salt
- Sufficient ice cubes to almost fill the gallon baggie
- 1 plastic spoon
For each group of 4 to 6 children:
- 1 Styrofoam plate
- 2 ice cubes
- 1/4 teaspoon of salt
- Small cup for the salt
- Paper towel
- Magnifying glass (optional)
For the facilitator:
- Pre-measure the salt and place in small cups
- Consider pre–mixing the ice cream ingredients
- Ask the children to predict what will happen when salt is sprinkled on an ice cube. Have them annotate their predictions in their Ice Investigator Journal.
- Divide the children into teams of four to six and invite them to perform an experiment that will test their predictions!
- Distribute the materials, including the plate, salt, two ice cubes, and magnifying glass to each team.
- Let the melting begin! Invite the children to sprinkle salt on top of one of their ice cubes, being careful not to get any salt on the second cube. Have them observe their cubes for about two minutes and record their findings in their Ice Investigator Journals.
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. Pure water ice melts and changes state from a solid to a liquid (water) at temperatures below this melting point.
- After the children have made and recorded their observations in their Ice Investigator Journals, encourage them to draw conclusions about what they observed.
Facilitator's Note: 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).
- What happened to the ice cube when salt was poured on it? It started to melt.
- Both ice cubes started at the same temperature. Why did one melt faster than the other? The salt caused the ice to melt faster.
- Can they think of any connection this might have to their lives? One important connection for children who live in colder climates is the use of salt — or other substances — on frozen roads.
- Why would salt be poured onto roads that were covered in ice? When you pour salt onto frozen roads the salt dissolves into the liquid water in the ice and lowers its freezing point, so that the temperatures have to get even colder to make the salty water freeze. Salted roads stay ice-free even at temperatures as cold as 15°F (-9°C). Other chemicals, such as calcium magnesium acetate, also are used to melt ice on roads. They are less harmful to the environment than rock salt (sodium chloride).
Facilitator's Note: When salt is added to an icy road, the salt begins to dissolve as it comes into contact with the ice; this makes salt water with a very high concentration of salt! The salty water continues to interact with the ice, lowering its freezing point and melting it. The temperature would have to drop below the freezing point of pure water (32°F, 0°C) in order to freeze the salty water.
Many different salts are used for deicing roads. Sodium chloride (NaCl) is a common road salt — table salt is a higher grade of sodium chloride. At colder temperatures, salts like magnesium chloride or calcium chloride are often used because they reduce the melting point even further.
Facilitator's Note: 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") again once the temperature has dropped to the new melting point.
- Have the children ever made home-made ice cream? How is salt used? Salt and ice are placed in an outer bucket. An inner container that holds the ingredients for ice cream is placed in this salty ice.
- Why would salt be used to help make ice cream? Salt allows the temperature of the ice water around the ice cream container to drop below the melting point of pure water; it makes it colder than just plain ice and freezes the ice cream.
- Invite the children to put their new-found knowledge to the test as they make ice cream in a baggie! Provide each child with a quart-sized Ziploc baggie and the sugar, milk, and vanilla. Have them pour the ingredients into the baggie, seal the baggie, and mix them up.
- Have them fill a gallon-sized Ziploc baggie with enough ice cubes to almost fill the bag, 2 tablespoons of rock salt, and the smaller baggie — well sealed! — with the ingredients.
- Invite the children to shake and roll the bag around until the inner ingredients are frozen (about 20 minutes).
- While everyone enjoys their treats, review what the children learned about how adding salt — or other substances — lowers the melting point of water.
If the children have started to construct a snow mobile, invite them to record any answers they discovered on the appropriate pieces.