3rd form revision

Question 1

Indicator colours and the PH scale

Answer 1

Question 2

What ions are in solutions of acids and alkalis

Answer 2

An acid solution is one which contains hydrogen ions H+(aq).
An alkaline solution is one which contains hydroxide ions OH-(aq).
The concentration of ions in pure water is small.

Question 3

Subatomic particles and the structure of the atom

Answer 3

Each atom consists of a nucleus containing protons and neutrons, with electrons arranged around it in energy levels.

Question 4

Flame test colours and the chemical test for the halide ions ( group 7 ions)

Answer 4

Flame tests

Metals change the colour of a flame when they are heated in it. Different metals give different colours to the flame, so flame tests can be used to identify the presence of a particular metal in a sample. This is how you would carry out a typical flame test:

1. dip a clean flame test loop in the sample solution
2. hold the flame test loop at the edge of a bunsen burner flame
3. observe the changed colour of the flame, and decide which metal it indicates
4. clean the loop in acid and rinse with water, then repeat steps 1 to 3 with a new sample

Question 5

Distillation

Answer 5

Distillation is a process of separating the component substances from a liquid mixture by selective evaporation and condensation. Distillation may result in essentially complete separation (nearly pure components), or it may be a partial separation that increases the concentration of selected components of the mixture.

Question 6

Chromatography

Answer 6

Chromatography can be used to separate mixtures of coloured compounds. Mixtures that are suitable for separation by chromatography include inks, dyes and colouring agents in food.

Simple chromatography is carried out on paper. A spot of the mixture is placed near the bottom of a piece of chromatography paper and the paper is then placed upright in a suitable solvent, eg water. As the solvent soaks up the paper, it carries the mixtures with it. Different components of the mixture will move at different rates. This separates the mixture out.

Question 7

Filtration

Answer 7

Filtration use a filter paper or fine porous ceramic to separate a solid from a liquid. It works because the tiny dissolved particles are too small to be filtered BUT any insoluble ‘non-dissolved’ solid particles are too big to go through! Filtration be used to …

• isolate the product of a reaction, the product may be insoluble or a crystallised product,
• remove solid impurities from a solution, its used to remove some solids in water for our domestic supply,

Question 8

Evaporation

Answer 8

Evaporation means a liquid changing to a gas or vapour. In separation, its removing the liquid from a solution, usually to leave solid crystals.

It can be done quickly with gentle heating or left out to ‘dry up’ slowly in ‘open air’.

The solid will almost certainly be less volatile than the solvent and will remain as a crystalline residue.

Evaporation is often followed by crystallisation.

Question 9

Crystalisation

Answer 9

Crystallisation can mean a liquid substance changing to its solid form. However, the term usually means what happens when the liquid from a solution has evaporated to a point beyond the solubility limit. Then solid crystals will ‘grow’ out of the solution because the solution is too concentrated for all the solid to remain dissolved at that temperature.

Crystallisation is often done from a hot concentrated solution, because most substance are more soluble the hotter the liquid.

Consequently on cooling a hot concentrated solution, crystals form as the solubility gets less and less and the solubility limit is exceeded, so crystals must form.

Crystallisation is also an important purification method because trace impurities tend to stay in solution. This may due to their solubility limit not being exceeded or the impurity particles will not fit into the regular crystal lattice of the desired crystallising product.