Objective

The goal of this project is to determine which added material will make ice melt fastest.

Introduction

To make ice cream with an old-fashioned hand-crank machine, you need ice and rock salt to make the cream mixture cold enough to freeze. If you live in a cold climate, you've seen the trucks that salt and sand the streets after a snowfall to prevent ice from building up on the roads. In both of these instances, salt is acting to lower the freezing point of water.

For the ice cream maker, because the rock salt lowers the freezing point of the ice, the temperature of the ice/rock salt mixture can go below the normal freezing point of water. This makes it possible to freeze the ice cream mixture in the inner container of the ice cream machine. For the salt spread on streets in wintertime, the lowered freezing point means that snow and ice can melt even when the weather is below the normal freezing point of water. Both the ice cream maker and road salt are examples of freezing point depression.

Salt water is an example of a chemical solution. In a solution, there is a solvent (the water in this example), and a solute (the salt in this example). A molecule of the solute will dissolve (go into solution) when the force of attraction between solute molecule and the solvent molecules is greater than the force of attraction between the molecules of the solute. Water (H₂O) is a good solvent because it is partially polarized. The hydrogen ends of the water molecule have a partial positive charge, and the oxygen end of the molecule has a partial negative charge. This is because the oxygen atom holds on more tightly to the electrons it shares with the hydrogen atoms. The partial charges make it possible for water molecules to arrange themselves around charged atoms (ions) in solution, like the sodium (Na₊) and chloride (Cl⁻) ions that dissociate when table salt dissolves in water.

Other substances that dissolve in water also lower the freezing point of the solution. The amount by which the freezing point is lowered depends only on the number of molecules dissolved, not on their chemical nature. This is an example of a colligative property. In this project, you'll investigate different substances to see how they affect the rate at which ice cubes melt. You'll test substances that dissolve in water (i.e., soluble substances), like salt and sugar, as well as substances that don't dissolve in water (i.e., insoluble substances), like sand and pepper. Which substances will speed up the melting of the ice?

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• Solution
• Solute
• Solvent
• Colligative properties
• Freezing point depression
• Phases of matter
  • Solid
  • Liquid
  • Gas
  • Plasma
• Phase transitions
  • Melting
  • Freezing
  • Evaporation
  • Condensation
  • Sublimation