In this lesson, students investigate how the freezing point of water changes when salt is added. Many students know that water freezes at 0 °C, and many believe that all substances freeze at 0 °C. The investigations in this lesson help students understand why salt is spread on roads in cold and snowy conditions.
By the end of this lesson, students should be able to:
- Measure amounts of water (solvent) and salt (solute) carefully to prepare solutions.
- Describe how the addition of salt affects the freezing point of water.
- Predict what will happen when a saltwater solution is placed in an environment below 0 °C.
- Apply the results of a freezing point depression experiment to understand the purpose of spreading salt on snowy roads in winter.
This lesson supports students’ understanding of the following topics in chemistry:
- States of matter
- Freezing point
- Freezing point depression
Engage: 10 minutes
Explore Activity 1: 30 minutes
Explain Activity 1: 15 minutes
Explore Activity 2: 25 minutes
Explain Activity 2: 30 minutes
Explore Activity 1:
For each group
- 2 paper towels
- 2 ice cubes
- Table salt in a small cup
Explore Activity 2:
For each group
- 3 clear plastic cups, 8 oz.
- 3 plastic spoons
- Permanent marker
- Measuring cup (¼ or ½ cup)
- 3 thermometers that can measure below -20 °C
- Measuring spoon (Tablespoon)
Extension materials for each small group:
- Ice melt, sugar, or other solute to test freezing point depression
- Remind students not to taste or drink the ice, water, or salt solutions that they prepare.
- Freezing point
- Boiling point
- Freezing point depression
water, molecule, states of matter, phase, solute, solvent, freezing point, freezing point depression
Logistics and Tips
- To get the most from these lessons, it is important that students discuss their results and how their findings relate to the real-life problems being investigated before they move on to the next experiment.
- This activity can be coupled with Pothole Science, which also examines winter road phenomena.
Freezing Point Depression
The temperature at which water turns from liquid to solid or from solid to liquid is known as the freezing point or melting point (they happen at the same temperature; freezing happens when thermal energy – heat— is removed from the system, melting happens when thermal energy is added to the system). Pure water at normal atmospheric pressure has a freezing/melting point of 0 °C (or 32 °F). When a solute such as salt is added to water, a solution forms. The solution has a lower freezing point than the solvent. In the case of salt and water, saltwater freezes (and melts) at a lower temperature than fresh water. The lower freezing point occurs because of the interaction between the solute and solvent. In the solution, water molecules interact with other particles as well as with other water molecules, and so they slow down and form a solid at lower temperatures. Roads are salted in winter to melt snow or prevent water from freezing on the road. However, if temperatures are significantly lower than 0 oC, this method won’t prevent roads from freezing.
Begin by asking students what they know about roads in winter: In what ways can driving be dangerous during the winter? What happens to wet roads and sidewalks during the winter? What can happen overnight? Guide them to think about the effects of weather on roads and driving. [Roads get slippery, snowy, and icy.]
Next, discuss what they’ve seen road crews do to keep people safe on slippery roads. Ask them: What do you see people doing to help prevent icy roads? What kind of equipment or vehicles have you seen used? What were they doing? [snow plows scraping snow and slush off roads, trucks spreading salt] After the discussion, show the students this video (2:12) of snow plows on the road.
Students may also have spread salt on sidewalks for the same reason. The question that will be answered next is: What happens when salt is applied to icy roads and why is it used?
Explore Activity 1
Spreading Salt on Icy Roads Model (hands-on)
Experimenting with this model will show students what happens when salt is spread on icy roads and sidewalks: salt lowers the freezing point of water, which causes the ice to melt.
- Give each group of students two paper towels and ask them to label them “SALT” and “NO SALT.” What do you think will happen if you allow the ice cubes to sit on the table? What will happen if you sprinkle salt onto one of the ice cubes?
- Give each group two ice cubes of equal size to place on the paper towels.
- Give each group some table salt and ask them to sprinkle some on the ice cube labeled “SALT.”
- Ask the students to watch the ice cubes over time and make observations. Which ice cube melted fastest?
Explain Activity 1
Have students use their observations to answer the following question:
Why do cities and towns spread salt on icy and snowy roads and sidewalks during the winter?
Students should work together to answer this question, backing up their answer with evidence from the experiment, other observations they have made, and reasoning. Explain to each group that they should be prepared to answer the following additional questions:
- How does salt affect ice and snow?
- At what temperature do snow and ice usually melt?
- What does adding salt do to the melting point of ice and snow?
- How do you know?
Groups can present their explanations in the form of a group discussion or a written product.
Once groups have presented, make sure that all students understand the main idea: Adding salt to ice or snow decreases the melting point of snow and ice: that is, it melts at temperature lower than 0 °C.
Go on to discuss the application further:
- How do your results connect with what happens in real life? [trucks will spread salt on roads and the ice will begin to melt]
- What happens when cars drive over the salted ice? What do you think it looks like? [the cars will crush it into smaller pieces; slush]
Remember that during winter, the ice and snow may melt during the day. What do you think will happen at night when the temperature falls below freezing again? The next set of experiments will help answer this question.