The Changing States of Solids, Liquids, and Gases

When a substance goes from one state of matter — solid, liquid, or gas — to another state of matter, the process is a change of state. Some rather interesting things occur during this process.

Melting point as a chemistry concept

If you measure the temperature of a chunk of ice, you may find it to be –5° Celsius (23˚ Fahrenheit) or so. If you take temperature readings while heating the ice in a pot on your stove, you find that the temperature of the ice begins to rise as the heat from the stove causes the ice particles to begin vibrating faster and faster.

After a while, some of the particles move so fast that they break free of the crystal lattice (which keeps a solid solid), and the lattice eventually breaks apart. The solid begins to go from a solid state to a liquid state — a process called melting. The temperature at which melting occurs is the melting point (mp) of the substance. The melting point for ice is 32° Fahrenheit, or 0° Celsius.

If you watch the temperature of ice as it melts, you see that the temperature remains steady at 0°C until all the ice has melted. During changes of state (phase changes), the temperature remains constant even though the liquid contains more energy than the ice (because the particles in liquids move faster than the particles in solids).

Boiling point of water

If you heat a pot of cool water, the temperature of the water rises and the particles move faster and faster as they absorb the heat. The temperature rises until the water reaches the next change of state — boiling. As the particles move faster and faster, they begin to break the attractive forces between each other and move freely as steam — a gas.

The process by which a substance moves from the liquid state to the gaseous state is called boiling. The temperature at which a liquid begins to boil is called the boiling point (bp). The bp is dependent on atmospheric pressure, but for water at sea level, it’s 212°F, or 100°C. The temperature of the boiling water will remain constant until all the water has been converted to steam.

You can summarize the process of water changing from a solid to a liquid to a gas in this way:

ice→water→steam

Because the basic particle in ice, water, and steam is the water molecule, the same process can also be shown as:

Here the (s) stands for solid, the (l) stands for liquid, and the (g) stands for gas. Unlike water, most chemical substances don’t have different names for the solid, liquid, and gas forms.

Freezing point of a substance

If you cool a gaseous substance, you can watch the phase changes that occur. The phase changes are:

• Condensation — going from a gas to a liquid

• Freezing — going from a liquid to a solid

The gas particles have a high amount of energy, but as they’re cooled, that energy is reduced. The attractive forces now have a chance to draw the particles closer together, forming a liquid. This process is called condensation. The particles are now in clumps, but as more energy is removed by cooling, the particles start to align themselves, and a solid is formed. This is known as freezing. The temperature at which this occurs is called the freezing point (fp) of the substance.

You can represent water changing states from a gas to a solid like this:

Sublimation

Most substances go through the logical progression from solid to liquid to gas as they’re heated — or vice versa as they’re cooled. But a few substances go directly from the solid to the gaseous state without ever becoming a liquid. Scientists call this process sublimation. Dry ice — solid carbon dioxide — is the classic example of sublimation. You can see dry ice particles becoming smaller as the solid begins to turn into a gas, but no liquid is formed during this phase change.

The process of sublimation is represented as:

In addition to dry ice, mothballs and certain solid air fresheners also go through the process of sublimation. The reverse of sublimation is deposition — going directly from a gaseous state to a solid state.