Pressure and Phase Changes

1. Pick a substance. Choose Neon, Argon, Oxygen or Water.
2. Heat or cool it. Drag the slider toward Heat or Cool. It springs back when you let go. You can also press a temperature button, or type a number and press Go.
3. Watch the tools. The thermometer shows the temperature. The gauge shows the pressure. The red dot on the phase diagram shows if it is a solid, liquid or gas.
4. Squeeze it. Drag the purple handle on the lid, or use the volume buttons. Squeezing a gas also warms it, so cool it back before you record.
5. Add molecules. Drag the pump handle up and down, or press Pump in 4.
6. Be careful! If the pressure goes over 300 atm, the lid pops off and the gas escapes. Press Return lid to put it back.
7. Collect data. Press Record this row. Your row shows up in the table and on the graph. Use Pause and Step to freeze the molecules. Press Reset to start over.

Pressure is how hard the molecules push on the walls. Every time a molecule hits a wall, it gives the wall a tiny push. Lots of pushes add up to pressure.

Hotter means more pressure. Hot molecules move faster. They hit the walls harder and more often.

Smaller means more pressure. When you push the lid down, the molecules have less room. They hit the walls more often.

More molecules means more pressure. The pump adds molecules. More molecules make more hits.

Squeezing warms a gas. When the lid pushes down on moving molecules, it speeds them up. Faster molecules mean a higher temperature. That is why the temperature jumps when you squeeze.

A phase diagram is a map. It shows which state a substance is in at each temperature and pressure. Temperature goes across. Pressure goes up.

The lines between the colors are where two states are together, like boiling water with steam above it. When the dot sits on a line, two states are in the container at once.

The triple point is where solid, liquid and gas can all be together. Past the critical point, liquid and gas stop being different. That area is called a supercritical fluid.

For a gas, the pressure is P = 5.5 × (T ÷ 150) × (N ÷ 60) ÷ V. T is the temperature in kelvin. N is the number of molecules. V is the volume in containers. So 60 molecules at 150 K in a full container give 5.5 atm.

Below the boiling point, most of the substance turns to liquid or solid. Then the pressure drops to a very small value.

Squeezing follows T × V^γ−1 stays the same. Gamma (γ) is a number for each gas. The lid pops off above 300 atm.

The molecules on screen are a picture of the idea. They are not an exact model of real molecules.