Solids, Liquids and Gases

Substances exist as solids, liquids or gases.

• A solid has a fixed shape and fixed volume
• A liquid has a fixed volume but no fixed shape
• It takes on the shape of its container
• A gas has no fixed volume and no fixed shape
• It takes on the shape of its container and fills its container

When a solid dissolves in a liquid, its particles spread throughout the liquid

Vocabulary:

Solvent a liquid in which something will dissolve

Soluble a material that can dissolve in a particular solvent

Insoluble a material that cannot dissolve in a particular solvent

Solute the material that dissolves

Solution the result of dissolving a solute in a solvent

Saturated A solution in which no more solute can dissolve

• e.g. the solute salt is soluble in the solvent water.
• When you dissolve salt in water you get a solution of salty water.
• If you keep adding salt eventually no more salt can dissolve – you have a saturated salt solution.

Things dissolve faster when:

• They are stirred
• They are ground up:
  • This increases the surface area of the solute and so increases the reaction.
• When the temperature of the solvent is increased.

Separating

Insoluble materials can be separated from water by filtering:

• E.g. mud, chalk and iron filings

Soluble materials can be separated from the water by evaporating the water:

• e.g. salt can be obtained from muddy, salty water by firstly filtering out the insoluble mud then evaporating the water to give salt.

Acids, Alkalis and Neutral

Many substances can be classified as acids, alkalis and neutral

• These are usually liquids with water as a solvent
The unit used is pH The pH can be measured using pH paper or universal indicator:
• Acids are pH 1 – 6
• Neutral is pH 7
• Alkalis are pH 8 – 14

• The colours that universal indicator gives are:

Neutralisation

When an acid is added to an alkali the result is a neutral solution of salt and water. E.g. when sodium hydroxide is added to hydrochloric acid the result is sodium chloride (common table salt) and water:

Acid in your stomach can be reduced by taking an alkali like milk of magnesia Acid rain in lakes or acid soil can be reduced by adding lime (calcium hydroxide, an alkali) to the lake or the soil

Reactivity

Some metals are more reactive than others.

• Some react well with cold water to produce hydrogen and heat
• Some will react withdilute acid
• Some will not react at all

When a metal reacts with oxygen it is said to be oxidised:

Oxidised iron is called rust The order of reactivity (most reactive first) of some metals is:

most reactive	potassium	react well with water	react with acid	corrode when in oxygen oxygen
	sodium
	calcium
	magnesium	react poorly with water
	aluminium
	zinc
	iron
	nickel
	tin
	lead	do not react with water
	copper		do not react with acid
	silver
	gold			do not corrode
least reactive	platinum

Some uses of metals rely on their reactivity:

• Zinc is more reactive than steel so can be used to prevent iron and steel from rusting (galvanising)
• Tin is less reactive than steel so it can be used to coat steel cans to prevent rusting.
• Aluminium is more reactive than steel but rapidly becomes oxidised. The oxide layer protects it from further corrosion so it can be used as a lighter, rust proof replacement for steel. It can be used in construction of aeroplanes, cars, window frames etc.

Oxidation is when two atoms are joined together, one loses an electron and is oxidised. In common language it is when a metal corrodes in the presence of oxygen,

• e.g. iron + oxygenrust

Electroplating

It is possible to coat one metal with another by electroplating

• The process is called electrolysis
• The metal is made the negative electrode in an electrical circuit like this:

• Copper coats the nickel electrode after a few minutes
• EPNS – electroplated nickel-silver – is made in this way

Sources of Materials

Metals are mined from the earths crust:

• Reactive metals have to be separated from their compounds.
• Iron is separated from iron ore
• Aluminium is separated from bauxite

• Less reactive metals like copper, silver and gold are found as nuggets of the metal

The speed of a reaction can be speeded up by:

• Powdering the chemicals to increase their surface area
• Increasing the temperature that the reaction takes place at
• Increasing the concentration of the chemicals reacting

E.g. marble chips + hydrochloric acid Carbon dioxide + Calcium chloride

To increase the speed of the reaction:

• the marble should be powdered
• the concentration of the acid should be increased
• the reaction should be heated

Combustion

Combustion means burning

Other materials apart from metals can oxidise:

• Burning is oxidation happening very quickly
• For burning to take place you need oxygen, fuel and heat (the heat is needed to start the fire, after that it will go by its own heat).
• When something oxidises heat is given out.

• We can write about combustion in a word equation like this:
• Coal + oxygen carbon dioxide + water
• Natural gas + oxygen carbon dioxide + water

• Magnesium + oxygen magnesium oxide

Fire extinguishers

Fire extinguishers stop combustion by preventing oxygen getting to the burning material.

There are four common kinds:

• Water
• Powder
• Foam
• Carbon dioxide

For information only – don’t learn this – is a list of the uses of each type:

Fabrics can be protected from fires by fire retardant chemicals:

• This is useful when materials are in danger of combustion
• E.g. furniture, carpets, clothing, car seats etc
• Fire retardant chemicals wash out in time and must be replaced in clothing.

Chemical and Physical reactions

CHEMICAL REACTIONS

In a chemical reaction new chemicals are produced by reacting other chemicals together:

• It is difficult to get the original chemicals back without putting in a lot of energy
• There is an energy change
• The energy change is usually chemical energy to heat energy (see graph)
• E.g. reacting magnesium with oxygen gives out a lot of heat and a white powder (magnesium oxide) is made
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