The properties of water (part 2)

Changing state

It's a hot, humid summer day. To cool down, you put some ice cubes in a glass and cold water. Is there anything unusual about this scene? Surprisingly, yes! You are interacting with water in three different states, or forms; solid, liquid, and gas. The ice is solid, the water is a liquid, and the water vapor in the air is a gas. In terms of chemistry, this is a remarkable situation. Water is the only substance on Earth that commonly exists in all of its different states.

If you have ever boiled water or made ice cubes, water can change from one state to another. Most other substances require extremes of hot or cold to change state. A steel car door doesn't melt in a July heat wave. In fact, steel would remain solid even inside your kitchen oven. The air you breathe remains a gas whether the weather is hot or cold. Water, however, can change states within the range of Earth's average temperatures.

Melting

To understand how temperature is related to change of state, start by thinking about an ice cube. The ice is solid. It has a regular shape because its molecules are arranged in a rigid structure. Suppose that the temperature of the ice is - 10 Celsius. What does the temperature tell you? Temperature is a measurement of the average speed of the molecules. Although you can't see them, all the molecules in a substance are constantly moving. At - 10 Celsius, the molecules in the ice cube are vibrating back and forth, but they are not moving fast enough to break free of their structure.

Now Suppose that you put the ice cube in a pan on the stove. As heat energy is added, the molecules in the ice start moving faster. The temperature rises. When the temperature reaches 0 Celcius, the solid ice melts and becomes liquid water.

Boiling and Evaporation

As you know, liquid water looks very different from solid ice. The liquid flows and takes the shape of the pan. This is true because the molecules in liquid water have more energy than the molecules in ice. The molecules move more freely, bouncing off each other.

What happens if you continue to heat the water on the stove? As more energy is added to the temperature rises. At 100 Celsius, the water boils and another change of state occurs. The molecules have enough power to escape the liquid and become invisible water vapor. The molecules in a gas move even more freely than those in a liquid. They spread out to fill their container - in this example, your whole kitchen! 

Another way that liquid water can become a gas is through evaporation. Evaporation is the process by which molecules at the surface of a liquid absorb enough energy to change to a gaseous state. If you let your hair air-dry after going swimming, you are taking advantage of evaporation.

Condensation

As water vapor cools down, it releases some of its energy to its surroundings. The molecules slow down and the temperature decreases. As the temperature of the gas reaches the boiling point, the water vapor begins to change back to the liquid state. The process by which a gas changes to a liquid is called condensation. When you fog up a window by breathing on it, you are seeing the effects of condensation. The invisible water vapor in your breath is cooled by the window and forms visible drops of liquid water.