boiling point condensation freezing point Phase transitions salt water
Age Group
14 15
Expected time
1 period
Grit on the streets.PDF
beaker symbol

Grit on the streets

What’s the mystery? A common misconception is that gritting the sidewalk causes the temperature to increase and thus melt the ice. However, it is actually the other way around. Adding salt to a water-ice mixture lowers the freezing point of the solution, causing the ice to become liquid even though the temperature is below 0◦C. This is the reason why we put grit on the streets and one of the reasons why ice rarely forms in oceans in temperate climates.
Domain(s) Chemistry
Subdomain keywords Phase transitions, salt, water, freezing point, boiling point, condensation.
Age group 14 to 15 years old.
Expected time for the mystery Approximate time for teacher preparation: 30 min.Approximate time in classroom: one hour.
Safety/supervision Supervision is recommended.Disclaimer: the authors of this teaching material will not be held responsible for any injury or damage to persons or properties that might occur in its use.
Preparation and list of materials The night before: fill milk cartons with water and put them in the freezer. Equipment: hammer, towel, thermometer, 1kg salt (NaCl), large 400ml beakers, butane burners, tripods, matches.
Learning objectives Revision of phase transitions and the concepts of condensation and chemical bonds. The students should have prior knowledge about the particle model before starting this activity.