Have you ever wondered why ice cubes in your cold drink become gradually smaller, or why their surface becomes smoother as they melt? Does ice always melt this way? To answer those questions, we will run the experiment below:
Water dropper or syringe
Food coloring, preferably liquid
Workspace that can get wet
Towel to wipe your workspace
Optional: Flash light
All matter is made up of tiny particles, and temperature is a measure of how much these tiny particles move. Even the particles in a solid like ice move, they vibrate, but because they are cold, they do not wiggle much and can hold onto each other. In the case of water, the particles arrange themselves in a regular lattice.
When ice is surrounded by air or liquid at room temperature, it absorbs heat from its surroundings. As a result, the tiny particles in the ice start to vibrate more. Those at the edge might break loose and flow freely over and alongside each other and the ice. At this stage, water and ice coexist. This is an active process: some particles break loose, while others attach to the solid. Because the surroundings are at a higher temperature, more break loose than reattach, so we see the ice melt. For pure water, this coexistence of water and ice happens at 0°C or 32°F. When the surroundings are cooler, heat will flow to the surroundings, cooling down the water particles. As a result, more particles attach to the lattice, and we see the ice grow.
When table salt (or sodium chloride) is dissolved in water, sodium and chlorine particles are added to the water particles. These make it more difficult for water particles to arrange themselves in a regular lattice and solidify into ice. That is why you need to cool salt water well below 0°C or 32°F before it becomes ice. Scientists call this phenomenon freezing point depression.
When pure ice is surrounded by a salt water solution at room temperature, particles at the edge of the ice will absorb heat and start to wiggle more. Some might break loose, and flow freely. At the same time, some water particles might try to reattach, but with the sodium and chlorine particles in the way, it is difficult for the water particles to reattach to the ice. As a result, ice in contact with room temperature salt water melts faster compared