States of Matter
There are three different states of matter: solids, liquids and gases.
These three states of matter behave differently in space:
- Solids have got a both a fixed volume and fixed shape. It can increase and decrease slightly in size with changes in the temperature.
- Liquids have got a fixed volume but they would take the shape of the container. A change in temperature would have a change in volume.
- Gases have got no fixed volume nor shape and a change in temperature would have a very big change in volume.
Different states of matter would have different energies and thus it can be noted that each particle would be able to react differently to different conditions. This can be explained by the kinetic theory. The kinetic theory states that:
- All matter is made up of particles.
- The particles are moving all the time.
- Heavier particles move slower then lighter particles.
This can then be used to analyse the interactions found in each of the states of matter.
- Solids would have their particles very close together which would be held by very strong forces making it very difficult for individual particles to be able to roam freely, but these particles can vibrate.
- Liquids would still have forces between separate particles, but they would be much weaker, allowing individual particles to roam around in a random way.
- In gases no forces are found between separate particles allowing the particles to roam around freely and at very high velocities.
Diffusion
Diffusion is the movement of a substance from an area of high concentration to an area of low concentration
As discussed in the kinetic theory, particles in liquids and gases are constantly on the move. This would result in the migration of particles from one place to another resulting in the process of diffusion which can be observed as migration of chemicals from a high concentration to a place of a lower concentration. This process of diffusion can be observed as:
Bromine liquid in the bottom glass cylinder is evaporating, producing an orange gas. When allowed into contact with the cylinder filled with air (top), the two tend to try to equalise by molecules moving from an area of higher concentration to one of lower – upward for the bromine and downward for the air.
Rates of Diffusion
The rate of diffusion is related to the mass of the molecules. The bigger the mass, the slower the rate of diffusion. An experiment to show the difference in the rate of diffusion is the following:
The two gases will diffuse at different rates, with the lightest particle moving faster than the heaviest particle. Since HCl is heavier than NH3, the white cloud formed when the two interact will be closer to the hydrogen chloride.
Brownian motion
Particles in both liquids and gases (collectively called fluids) move randomly. This is called Brownian motion. They do this because they are bombarded by the other moving particles in the fluid. Larger particles can be moved by light, fast-moving molecules.
Brownian motion is named after the botanist Robert Brown, who first observed this in 1827. He used a microscope to look at pollen grains moving randomly in water. At this point, he could not explain why this occurred.
Some examples of Brownian Motion are pollen grains on the surface of water and smoke in air.
Changes of states
Matter can easily interchange between different states of matter, with the most important changes being between solids and liquids and liquids and gases, although it must be noted that solids can sublime to form a gas without forming liquid as an intermediate
On heating the energy of the particles would increase and once enough energy is given to the particles they would be able to overcome the strong forces between the particles to melt and produce a liquid. This would be the melting temperature. If the temperature is increased then the particles would obtain even more energy would be taken by the particles and until the energy is high enough for each particles to escape the effects of the other particles to form a gas. This temperature would be the boiling point
When the temperature is reduced the gases would go back to being a liquid while liquids would form a solid, with the processes being known as condensation and solidification respectively.
In some special cases solids can form gases without the formation of liquids, with such an example being Carbon Dioxide. The temperature at which this process occurs is called the sublimation point while the reverse process os called deposition.
Melting and Boiling Point Graph
Each substance has its own unique melting and boiling point. For water the melting/freezing point is 0 oC and the boiling/condensation point is 100 oC.
In fact, for a pure substance the melting and boiling points are always constant, and therefore if you want to check for the purity of water you can check its melting and boiling points. If they are 0 and 100oC respectively then it will be pure water.