Adding salt to ice may seem counterintuitive, as it’s commonly used for melting icy surfaces. However, this simple addition can significantly lower the temperature of melting ice, creating a much colder mixture than ice alone. This phenomenon is rooted in fundamental scientific principles that explain how a common household item can produce surprisingly low temperatures.
The Method for Colder Ice
To create a colder ice mixture, gather a container, ice cubes, and common table salt (sodium chloride). Crush the ice cubes into smaller pieces for a larger surface area, which helps the salt dissolve more effectively. Combine the crushed ice with salt, aiming for a ratio of approximately three parts ice to one part salt by weight. This proportion can achieve temperatures as low as -21 °C.
Stir the mixture thoroughly to ensure the salt dissolves into the thin film of water that naturally forms on the ice surface. As the ice melts and the salt dissolves, the mixture becomes a very cold slush. Exercise caution when handling this mixture, as direct skin contact can lead to frostbite.
The Scientific Principle Behind Salt and Ice
The scientific principle when salt makes ice colder is freezing point depression. Pure water freezes at 0°C (32°F), but when a solute like salt is dissolved, the solution’s freezing point decreases. This occurs because dissolved salt ions interfere with water molecules’ ability to form the crystalline lattice required for ice. More energy must be removed for water molecules to solidify.
As salt dissolves into the thin water layer surrounding the ice, it lowers the freezing point. Since the mixture can be liquid below 0°C, the existing ice begins to melt. This melting process is endothermic, absorbing heat energy from its surroundings. The heat absorbed to change ice from solid to liquid is known as the latent heat of fusion.
The absorption of this latent heat causes the ice-salt mixture’s temperature to drop, often reaching temperatures well below pure water’s freezing point. The continuous melting of ice draws more heat from the environment. This creates a sustained cooling effect until the salt is fully dissolved or the ice has completely melted.
Practical Applications and Important Considerations
Salt’s ability to create cold temperatures has several practical uses. One common application is in making homemade ice cream, where the chilled salt-ice mixture rapidly freezes the ice cream base. This technique is also useful for quickly chilling beverages or creating cold baths in laboratory settings. The method’s effectiveness depends on the type and concentration of salt used.
While common table salt (sodium chloride) is effective, other salts like calcium chloride or magnesium chloride can achieve even lower temperatures. However, these alternative salts may not be food-grade and require appropriate safety measures. Regardless of the salt type, avoid direct contact of the mixture with bare skin to prevent cold burns or frostbite.
The effectiveness of common salt in melting ice diminishes at very low ambient temperatures, typically below -9.4°C (15°F). The cold produced by a salt-ice mixture is temporary, lasting only as long as the ice melts and the salt remains dissolved. Once the ice is gone or the salt is fully diluted, the mixture warms towards ambient temperature.