Solids Liquids and Gasses

How to tell them apart. A thought Experiment.

To Do and Notice

At first, the distinctions between solids, liquids, and gasses seem easy.

Consider water.

Ice is a solid, make an ice cube in a freeze [ (H2O)3 ?] and it keeps its cubic shape shape when removed from its container. It doesn't flow. Ice is a mineral and usually is found as a crystalline solid.

Liquid water adapts to the shape of its container, it can fill only a part of a container (unlike gas), when removed from one container it changes shape to fit the next container. Liquid water surrounded by Earth surface air has a surface where density changes abruptly by nearly three orders of magnitude, from that of air to water. Liquid water is a fluid that flows.

Water vapor fills all available space in a container and has a density close to that of the surrounding air. Water vapor is a fluid that flows.


Now let's consider the differences a little more deeply.

1. Ice flows under pressure

Solid ice can flow. Consider a glacier. On earth glaciers occur in ice at temperatures between -45 °C and O °C. At the coldest temperatures in this temperature range ice will flow under 10 Atmospheres of pressure. This is the pressure beneath 100 meters of ice. Thus, glaciers flow. However, even though solid ice can flow it is still considered a solid because it flows slowly. (Note that ice is a crystalline solid that can flow, consider how this relates to whether glass is a solid or liquid.)

In general to be considered a liquid a substance must flow out of a wide mouth jar in about two minutes under earth gravity.

2 Gasses and liquids

If gasses expand to fill all of space how come there is any atmosphere left on earth? Because of gravity. In our original definition we ignored the effects of gravity. The sun is entirely gaseous, and yet it is held together by its own gravity, well actually it is a completely ionized gas called a plasma, but that's another story.

3. Dense gasses

Dense gasses like carbon dioxide can fill the bottom part of a container, they can slosh about and be poured from one container to another. However, remove gravity and they will fill the entire container while a liquid will still fill only part of the container.

So, the real distinction between a liquid and a gas is easiest to make when the effects of gravity are removed, as they are in freefall in an orbiting spacecraft.

The skylab astronauts made balls of liquid water several inches in diameter. These spherical balls of liquid water held together in the atmosphere of the space station. If they were made of water vapor they would have rapidly mixed with the air of the station until they had spread to fill the station. Eventually, however the balls of water evaporate and the resulting water vapor fills the station.

4. The Critical point

At a high pressure and temperature water vapor gets denser while water liquid gets less dense until at a temperature and pressure known as the critical point the density of the liquid and the gas become identical. There is no longer a surface between the liquid and gas forms of water. Instead of liquid or gas we have a fluid. On Jupiter there is a place deep in the atmosphere where the hydrogen and helium atmosphere is a fluid which is denser than rocks on earth. At pressures and temperatures above the critical point water becomes a supercritical fluid.

5. The Triple Point

At 0 °C and 6 mbar pressure water can exist in a closed container as a solid, liquid and gas at the same time. This is called the triple point pressure of water. In other words at this low pressure, which is the average pressure on the surface of Mars, water can boil and freeze at the same time. Mountaineers know that the boiling point of water decreases with altitude, that is the boiling temperature decreases as the atmospheric pressure decreases. Decrease the atmospheric pressure enough and the water will boil at 0°C. Below the triple point pressure, water can only exist as a solid and a gas, not as a liquid for any length of time.

6. Boiling and evaporation

Liquid or solid water can evaporate into a gas at any temperature and pressure. When I hung out my wet clothes to dry in the subfreezing temperatures in Boston they first froze solid, then dried as the ice sublimed and became a gas leaving me dry clothing. In the following discussion let's consider liquid water. There is a precise boiling point for liquid water at each pressure. Evaporation is a process that happens at the surface of liquid water. While boiling occurs within the liquid. While some molecules at the surface may have enough energy to leave at any temperature. There is a precise temperature at which molecules within the liquid can form a bubble of water vapor, this is the boiling point.

7. Superheating

It is hard to start to form a bubble. When you blow up a balloon it is hardest to start to blow up the balloon. Then once the balloon is larger the pressure needed to inflate the balloon decreases. In the same way it is difficult to form a small bubble inside a liquid. If there is a small air bubble in the liquid aleady, or even a small air bubble in a crack in the wall of the vessel holding the water then it is easy for water to inflate this starter bubble. Notice how the gas comes out of champagne or beer, long chains of bubbles form rising from one imperfection, a scratch, on the bottom or inside wall of the glass. While this is outgassing not boiling the principle is the same, a seed bubble makes boiling easier to initiate. Chemists often add boiling beads, which are beads that are made with surface cracks, to allow fluids to form bubbles easily and therfore to boil.

8. A microwave disaster.

If you heat water in a cup in the microwave to boiling then the cracks in the walls f the cup will serve as nucleation sites for bubbles and will become filled with water vapor rather than air. If you forget the cup in the microwave and let it cool down the water vapor in these cracks will condense into a liquid and there will then be no air filled nucleation sites in the cup. If you then re-boil the water it can super heat to a temperature higher than 100 °C withot boiling. If you then reach into the mcrowave and jiggle the cup so air bubble are introduced into the cup or even if dirt is dislodged from the walls of the cup then the entire contents of the cup can erupt into boiling, spewing boiling water drops high into the air. Never reheat water in a microwave.

9. Supercooling.

It is also difficult to create a small ice crystal. So you can put a bottle of soda into the freezer and cool it down to a temperature below its freezing point and yet have it reamain liquid. However if you open the bottle you can trigger the release of dust particles into the liquid to act as seed crystals which can cause the entire bottle to freeze. See the Exploratorium exhibit, watch water freeze. Water drops in the atmosphere are often so pure that they have no seed crystals to help the formation of ice. These water drops can be cooled to -40 °C without freezing. However as soon as they hit a surface they will freeze. This produces the phenomenon of freezing rain which coats everything with a layer of ice. The temperature of -40°C is called the homogeneous nucleation temperature. Cool a liquid water drop to below -40 °C and it will crystallize without the aid of a seed crystal.


What's Going On?

Physicist Richard Feynman once said that if he knew that all scientific information was going to be lost, but that he could pass on one sentence to the future it would be a summary of the atomic theory: All the world is made of atoms, small indivisible particles that repel each other when close together and attract each other when a little ways apart.

Water molecules at higher temperatures have more kinetic energy of motion. When the attraction of the water molecules as measured by their binding energy, the energy necessary to pry them apart, is greater than their kinetic energy of motion due to their temperature then they will stay together as a solid or a liquid. When their kinetic energy is large enough to allow the individual atoms to slide past each other on a large scale then the material can flow and is a liquid. When the kinetic energy is greater than the binding energy then the material is a gas.


Water molecules at the surface have a spread of kinetic energies. Some have enough energy to escape the binding energy due to the attraction of their neighbors. These molecules evaporate, they leave the liquid. When the fastest molecules leave the slower molecules are left behind. This means that the average energy of the molecules in the liquid is decreased by evaporation so that the liquid cools due to evaporation.


Water molecules in a vapor collide with a liquid surface and lose kinetic energy to the bulk liquid due to their collision. The molecules cannot escape immediately. The liquid gains energy. Condensation is a heating process.


When molecules of water are in a crystal it takes energy to remove them from the crystal and return them to the liquid. Thus when liquid freezes energy is released and the surrounding environment warms. Freezing is a heating process.

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Scientific Explorations with Paul Doherty

© 2003

1 November 2003