5.1.2 Solid, liquid and gas

Okay so most of this is GCSE stuff which is nice but we need to recap a little bit of it...

The kinetic model

This describes how all substances are made up of atoms or molecules which are arranged differently depending on the phase/substance...

• Solids: Molecules are regularly arranged and packed closely together with strong electrostatic forces of attraction between them which hold them in fixed positions. They have kinetic energy as they vibrate. when a slid is heated the particles gain for kinetic energy and vibrate more. If they break away from the solid structure and become free moving they are now part of a liquid.
• Liquids: atoms are ore spaced out but still very close together (all touching). They have more kinetic energy than solids and can therefore change position/flow past each other. Liquids flow and have no fixed shape. When a liquid is heated the molecules gain kinetic energy and some may gain enough to break away from the other particles - these become a gas.
• Gases: The molecules have more kinetic energy than liquids and are much further apart (not touching). There are negligible electrostatic forces between them (provided they don't collide with each other/the walls of a container). They move with different speeds in different directions (basically, they move randomly).

Usually the more solid a substance is the more dense it is (because the spacing between the particles is smaller). However, ice (solid water) is less dense than liquid water. How does this work I hear you exclaim. Well. water freezes in a regular crystalline pattern held together by strong electrostatic forces between the molecules. In this structure the molecules are held a little bit further apart than in liquid water.

Brownian motion
Okay so basically there was this guy called Robert Brown. He wanted to observe some pollen grains so suspended them in water but he couldn't see them because they kept moving around. He didn't understand why they weren't staying still even when there was no wind and thee air was completely still - no one did tbh. It wasn't until a lot lot later (180 years later) that Einstein explained the motion in terms of collisions between the pollen grains and tiny water molecules. These collisions were elastic and resulted in a transfer of momentum from the water molecules to the pollen grains causing them to move.

Nowadays in a lab, we can observe Brownian motion with smoke particles. Smoke particles can be observed under a microscope and move in a random way due to air molecules constantly striking them. The air molecules themselves are also moving in a random motion and the mean kinetic energy of the air molecules is the same as the smoke particles (however the air molecules are much smaller so move much faster).


Internal energy
The internal energy of a substance is the sum of the randomly distributed kinetic and potential energies of atoms/molecules within the substance.

At absolute zero, the kinetic energy of all the atoms/molecules is zero. However, internal energy is not zero as the substance still have electrostatic potential energy stored between the particles. If you increase the temperature of a body you will increase it's internal energy (because you increase it's kinetic energy). however, when a substance changed phase it's internal energy is changing. When going from liquid to gas although it's temperature is not changing the electrostatic potential energy increases which in turn increases the internal energy. When changing phase the energy transferred to the substance goes into increasing the electrostatic potential energies of the atoms/molecules not their kinetic energy. Different phases have different electrostatic potential energies...

• Solid: very large electrostatic forces between atoms/molecules so electrostatic potential energy has a large negative value (the larger the electrostatic potential energy the more negative it is - the negative value just means energy has to be put in to break the atomic/molecular bonds).
• Liquid: the electrostatic forces are more negative than 0 (probably since zero isn't negative, duh) but less negative than in a solid of the same substance
• Gas: the electrostatic potential energy is zero as there are negligible electrical forces between atoms/molecules.