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Pressure and States of Matter

By Ron Kurtus (6 November 2003)

The state or phase of matter depends on its temperature and the surrounding pressure. We typically see materials change their state at normal atmospheric pressure.

Changing the surrounding pressure changes the temperature at which a material goes from one state to another. The reason is because the molecules causing the pressure inhibit the molecules of the material.

Graphs have been made comparing the values of temperature and pressure needed to have a change of state of various materials. They show special situations where a solid will change directly into a gas, skipping the liquid phase.

Materials expand or contract with the change in pressure and temperature.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion



Effect of pressure

The temperature that a material changes its state is usually given at the average air pressure at sea level. For example, water will boil at 100o C (212o F) at sea level. But if the pressure is changed, the temperature at which the material changes its state also changes. The higher the pressure, the higher the temperature needed to change the state of the material.

Boiling temperature of water changes

Water will boil at a lower temperature at high elevations. In fact, water can be made to boil at room temperature if it is placed in a chamber or container where most of the air has been evacuated—thus greatly reducing the air pressure.

Likewise, if you increased the pressure inside the chamber, the boiling point would increase.

Pressure of surrounding material

This pressure can be air pressure, water pressure or pressure from some other material surrounding the object. Usually it is fluid pressure, but it can be pressure from a solid if the object is completely encased in the solid.

Molecules inhibit changes

The reason increasing pressure will increase the required temperature of changing the state is because the pressure is a result of molecules from the outside material (like air molecules for air pressure) smashing into the object's molecules and preventing them to become free from their own molecular attraction.

The surrounding pressure helps the molecular attraction overcome the kinetic energy of the material's molecule or atoms.

Graph of Pressure versus Temperature

Because the change in state involves both temperature and pressure, you can make a graph of the combinations where the change of state takes place. Surprisingly, there are situations where a material will go directly from solid to gas, completely skipping the liquid state or phase. This is called sublimation. Likewise, it can go from gas to solid at some temperature-pressure combinations, skipping the liquid phase.

The chart below shows the relationship between temperature, pressure and the changes of state for a typical material. At a given pressure, you can draw a line across the chart to see the temperatures at which the material will change from solid to liquid and from liquid to gas.

Water

Click on the Water button, and you will see that at sea level or 1 atmosphere, the melting point is 0o C and the boiling point is 100o C.

Dry ice

At pressures below and temperatures above the Triple Point, the change of state will skip the liquid phase and go directly from solid to gas. This is often seen with Dry Ice, where it will change directly from a solid to CO2 gas. Click the Dry Ice button to see the point where it goes directly from a solid to a liquid.

Note that at pressures greater than 5 atmospheres (5 times the normal air pressure), solid CO2 will become a liquid before boiling into a gas.

Expansion and contraction

Increased pressure will contract gases. The effect of pressure on liquids and solids is considerably less and in some cases considered negligible. On the other hand, most substances expand when heated and contract when cooled. This continues even after they have changed their states.

For example, steam will contract with pressure and cooling. The volume of water is not really affected by pressure, but it does contract with cooling. Once water becomes ice, it actually expands. It is one of the few materials that expand when it changes to the solid state.

Unfortunately, this is the cause of much of the damage to road in the winter. Water seeps into a small crack in the road, and then when it freezes, the ice expands and makes the crack bigger. Cars drive over the crack and loosen the material, so the water and ice can work again to make a pothole in the road.

As the temperature of ice decreases, it starts to contract. But it still has a greater volume than it does in the liquid state.

Summary

A change in the surrounding pressure can change the temperature that will cause a material to go from one state to another. This is because the molecules causing the pressure inhibit the molecules of the material. A graph comparing the values temperature and pressure needed to have a change of state of various materials also shows special situations where a solid will change directly into a gas. Materials expand or contract with the change in pressure and temperature.


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