States of matter

Every substance is either solid, liquid or a gas. In each state, particles have a certain amount of energy. Increasing the temperature gives molecules more energy, converting them into another state of matter.

Solids, liquids and gases

• Solid — particles are organised in a regular arrangement. The atoms vibrate in a fixed position and are physically touching the other atoms. These particles are low in energy.

• Liquid — atoms are still touching but are slightly further apart and can slide over one another. Each atom is free to move and has a moderate amount of energy.

• Gas — the atoms are arranged randomly and are far apart from one another. They are moving quickly and have a high amount of energy.

State Symbols

There are four state symbols: solid (s), liquid (l), aqueous (aq) and gaseous (g).

• Solids – these will include most metals (except mercury) and insoluble compounds (like silver chloride and lead chloride).

• Liquid – these will include water, ethanol and molten ionic compounds.

• Aqueous – these include anything in solution e.g. sodium chloride dissolved in water

• Gaseous – elements that are gaseous at room temperature include the noble gases, fluorine, oxygen and nitrogen

Changing State

Melting and boiling

To convert a substance from a solid to a liquid, or from a liquid to a gas, bonds between the particles need to be broken. Energy needs to be added, in the form of heat, to cause the bonds to break. The energy of the particles increases. The conversion of a solid to a liquid is called melting and the conversion of a liquid to a gas is called boiling.

Condensing and freezing

To convert a substance from a gas to a liquid to a solid, the bonds between particles need to form. This is done by lowering the temperature which decreases the energy of the particles. The conversion of a gas to a liquid is called condensation and the conversion of a liquid to a solid is called freezing.

Experiment to show the movement of liquids and gases

To prove that the particles of a liquid are constantly moving, we can add some potassium manganate to a beaker of water. We use potassium manganate because it has a deep purple colour so we can easily see it. Over time, the potassium manganate slowly spreads out through the liquid because the water molecules that surround it are constantly moving and force themselves between the potassium manganate particles. This causes the potassium manganate to become distributed through the liquid.

To show that particles of a gas move, we can set up a test tube containing two balls of cotton wool on either side. One of the balls of cotton wool has been soaked in aqueous ammonia and releases ammonia gas. The other ball has been soaked in hydrochloric acid and releases hydrogen chloride gas. These gas molecules diffuse through the tube until they meet, at which point they will react together to form ammonium chloride. Ammonium chloride is a white solid and appears as a ring of white powder inside the tube. The ammonium chloride does not form right in the centre of the tube because ammonia and hydrogen chloride diffuse at different speeds. Ammonia is the lighter gas so is able to move faster, resulting in the ring of ammonium chloride forming closer to the cotton ball that was soaked in hydrochloric acid.

Solutions

Solutions are formed when a solid or a gas (the solute) are dissolved in a liquid (the solvent). Different solvents are better than others at dissolving things and when they have dissolved as much solute as possible they are said to be saturated.

How well a substance can dissolved is described as its solubility which is measured in grams of solute per 100 grams of solvent. For example, the solubility of table salt (sodium chloride) is 36 g per 100 g of water. This means that we can dissolve a maximum amount of 36 grams of sodium chloride in 100 grams of water at room temperature. At this point the solution will become saturated and no more sodium chloride will be able to dissolve.

Solubility curves

The ability of a solute to dissolve in a solvent improves with increasing temperature. The solubility of a substance at different temperatures can be plotted on a solubility curve.

From the solubility curve on the left, you can see that the four solutes have very different solubilities.