S1 Mix it Up! Summary Notes

Solubility - Mixtures

1 – To develop our understanding of particles in substances.

A pure substance contains only one type of particle.

We can see in this extremely close up photo of gold

that all the particles are identical so gold can be

considered a pure substance.

A substance may also contain a mixture of

particles, just like different sweets can be found

together in a mixture. This is not considered a

pure substance as not all the particles are

identical.

The diagram above shows two different substances being added

together to form a mixture. Notice that..

..although they are mixed together they have not chemically joined.

..the smaller particles are able to fit in the spaces between the

larger ones.

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Solubility – Investigating Solubility

2 – To investigate solubility and identify the definitions of solute, solvent and solution.

When you add sugar to tea it gives it a sweet taste. You can’t see the sugar anymore but it is still there as you can taste it. The sugar has dissolved. When a substance dissolves, the particles mix with the particles of the liquid. As sugar dissolves in water to make a solution, sugar is said to be soluble.

In a solution, the substance that is dissolved is called the solute. The solvent is the substance which does the dissolving. When the solute dissolves in the solvent, the mixture produced is called a solution.

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A solution containing a lot of sugar is called a concentrated solution. A solution containing a lot of solvent is called a dilute solution.

If you put lots of sugar in your cup of tea or coffee, it may not all dissolve. If the solution cannot dissolve more solute it is called a saturated solution. If a substance is not soluble (it doesn’t mix) it is

insoluble.

Solubility – Investigating Separation Techniques

3 – To investigate separation techniques.

The individual substances in a mixture can be separated using different

methods, depending on the type of mixture.

Filtration – Separating an Insoluble Solid from a Liquid

If a substance does not dissolve in a solvent, we say that it is insoluble. For example, sand does not dissolve in water – it is insoluble.

Filtration is a method for separating an insoluble solid from a liquid.

When a mixture of sand and water is filtered:

the sand stays behind in the filter paper (it becomes the residue)

the water passes through the filter paper (it becomes the filtrate)

Evaporation – Separating a Soluble Solid from a Liquid

Evaporation can be used to

separate a soluble solid from a

liquid. For example, salt is soluble

in water. During evaporation, the

water evaporates away leaving

salt behind.

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Distillation – Separating a Solvent from a Solution

Distillation is a method for separating the solvent from a solution.

For example, water can be

separated from a salt solution

by simple distillation.

This method works because

water has a much lower boiling

point than salt. When the

solution is heated, the water

evaporates.

It is then cooled and condensed

into a separate container. The

salt does not evaporate and so

it stays behind.

This process can also be used to separate a mixture of liquids with

different boiling points.

Acids & Alkalis - The pH scale

4 – To develop our understanding of the pH scale

Acids and alkalis are very common substances in a science lab and at

home. The pH scale measures the acidity and alkalinity of a substance

in a solution. (The substances must be dissolved in water)

There are different pieces of equipment for measuring the pH of a

substance.

The pH meter gives a digital value between 0 and 14 which tells us how

acidic or alkaline a substance is.

The pH paper and universal indicator will show a colour when tested

with a substance. This can then be compared with the pH scale to

determine how acidic or alkaline the substance is.

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pH meter pH paper Universal indicator

Acids & Alkalis - Measuring pH of a substance

5 – To explore the uses of acids and alkalis in everyday life

The diagram below shows how we can easily measure the pH of any

substance with a piece of pH paper.

Below are some examples of everyday acids and alkalis and their pH:

Name of Substance Laboratory or

Household

Colour of pH

paper

pH

number

Acid, Alkali

or Neutral

hydrochloric acid lab red 1 acid

sodium hydroxide lab blue 14 alkali

vinegar house red 3 acid

baking soda house blue 9 alkali

nitric acid lab red 1 acid

ammonia solution lab blue 11 alkali

pure water house green 7 neutral

soda water house red 5 acid

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Substance to be

tested

dimple

tile

small piece of

pH paper

dishwasher powder house blue 10 alkali

lime water lab blue 9 alkali

cola house red 5 acid

lemon juice house red 4 acid

sulphuric acid lab red 1 acid

oven cleaner house blue 13 alkali

lemonade house red 4 acid

salt water house green 7 neutral

bleach house blue 13 alkali

Common Household Acids Common Household Alkalis

vinegar dishwasher powder

lemon juice oven cleaner

cola bleach

Laboratory Acids Laboratory Alkalis

hydrochloric acid sodium hydroxide

sulphuric acid ammonia solution

nitric acid calcium hydroxide

Acids & Alkalis - Dilution of acids

6 – To investigate the effect of dilution on acids and alkalis

A concentrated acidic solution is one that contains a lot of acid

compared with water. A dilute solution contains more water than acid.

Think of diluting orange juice:-

More orange and less water = concentrated solution Less orange and more water = diluted solution

Diluting an acid by adding more water increases the pH of the solution

towards 7 making it less acidic.

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As the acid is diluted the pH increases towards 7

Diluting an alkali by adding more water decreases the pH of the

solution towards 7 making it less alkaline

It is not possible to dilute an acid so that its pH increases to greater than

7 nor an alkali so that its pH decreases below 7. This is because we are

diluting them with water which is a neutral substance (with a pH of 7) so

adding more and more water will simply maintain the pH of the diluted

acid or alkali at 7.

Acids & Alkalis - Neutralisation

7 – To investigate the reactions of acids and why these reactions are useful. 8 – To investigate how changing certain variables affects the rate of the reaction between an acid and calcium carbonate

Acids and alkalis neutralise each other to form a salt and water.

Remember that the acid has a low pH and the alkali a high pH, but when

they are mixed in precise quantities they form water (a neutral

substance) plus salt.

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As the alkali is diluted the pH decreases towards 7

When performing a neutralisation experiment it is important not to add

too much alkali to the acid as the pH will increase beyond 7 creating an

alkaline solution.

Remember that neutralisation is not the same as dilution. In dilution

water is added to the acid so the pH of the solution cannot increase

above 7 but in neutralisation it is an alkali that is being added so it is

quite easy to go beyond 7 and produce a high pH alkali.

Everyday examples of neutralisation:

Alkali

Indigestion tablets help with

heartburn by neutralising

stomach acid

Neutralisation with indigestion powder

Acids and alkalis neutralise each other to form a salt and water. Other

substances also neutralise acids to form a salt.

This time we see that the red hydrochloric acid turns green as it is

neutralised but we also notice that a gas is given off.

Indigestion powder is a form of metal carbonate which means it will give

off carbon dioxide gas as well as salt and water during neutralisation.

spatula stirring rod

10cm3 hydrochloric acid + 3

drops of universal indicator

Indigestion

powder In this experiment,

indigestion powder is

used to neutralise

hydrochloric acid just like

an alkali neutralises an

acid.

Toothpaste neutralises acid from food and so helps to prevent tooth decay

Acid Rain

9 – To develop our understanding of the causes of and resultant problems with acid rain. 10 – To investigate how we can reduce acid rain.

Non-metal elements can react with air or oxygen to form compounds

called oxides.

carbon + oxygen carbon dioxide

sulphur + oxygen sulphur dioxide

nitrogen + oxygen nitrogen dioxide

Non-metal oxides which can dissolve in water (like those above)

produce acids.

e.g. Sulphur dioxide dissolves in water to produce sulphuric acid

Nitrogen dioxide dissolves in water to produce nitric acid

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Acid Rain is an environmental problem which can:

damage buildings made from carbonate rocks (e.g. marble)

corrode metals structures made of iron and steel (e.g.

bridges)

affect plant life by changing the pH of soil (e.g. less crops

grown)

affect animal life by changing the pH of rivers/lakes (e.g. kills

fish)

Acid rain could be reduced if the gases that cause acid rain were

reduced:

Carbon dioxide is released when fossil fuels like coal, petrol and

gas are burned

If we burn less fossil fuels, then less carbon dioxide will

be released

Sulphur dioxide is released when fossil fuels like coal are burned

Use low-sulphur fuels instead of high sulphur fuels

Remove sulphur dioxide fumes before release into

atmosphere

Nitrogen dioxide is produced in car engines by the sparking of air

with petrol to set the petrol on fire.

Fit catalytic converters to cars to remove nitrogen

dioxide from car exhaust fumes.