Elements, Compounds and Mixtures

Question 1

Element

Answer 1

A substance made of atoms that all contain the same number of protons and cannot be split into anything simpler.

Question 2

Compound

Answer 2

A pure substance made up of two or more elements chemically combined.

Question 3

Mixture

Answer 3

A combination of two or more substances (elements and/or compounds) that are not chemically combined.

Question 4

What is a pure substance?

Answer 4

• A pure substance may consist of a single element or compound which contains no other substances.

Question 5

Describe the boiling points of pure and impure substances.

Answer 5

• Pure substances melt and boil at specific and sharp temperatures
• Impure substances have a range of melting and boiling points as they consist of different substance

Question 6

Process of Simple Distillation

Answer 6

• Simple distillation separates a liquid from a soluble solid or a pure liquid from a mixture.
• The solution is heated, the liquid evaporates and turns to vapour, which passes through a condenser, cools, and condenses into a pure liquid.
• The solid remains behind after all the liquid has evaporated.

Question 7

Fractional Distillation

Answer 7

• Fractional distillation separates miscible liquids by their boiling points.
• The mixture is heated; the liquid with the lowest boiling point evaporates first, passes through a condenser, cools, and is collected.
• This leaves the other components behind.

Question 8

Filtration

Answer 8

Filtration separates an undissolved solid from a liquid. The mixture is poured into a filter funnel with filter paper. Liquid passes through as filtrate, while solid particles stay behind as residue. Centrifugation can also separate similar mixtures.

Question 9

Crystallisation

Answer 9

• Crystallisation separates a dissolved solid from a solution.
• The solution is heated to evaporate some solvent, creating a saturated solution.
• It is tested with a glass rod — if crystals form, it is saturated.
• The solution cools slowly, forming crystals as solubility decreases.
• Crystals are filtered, washed with cold distilled water, and dried.

Question 10

Paper Chromatography

Answer 10

• Paper chromatography separates substances by their solubility in a solvent.
• Spots of sample are placed on a pencil line on chromatography paper, which is then placed in solvent.
• The solvent rises by capillary action, carrying substances at different rates.
• More soluble substances travel further, separating the mixture into its components.

Question 11

Why is pencil used in chromatography?

Answer 11

• Pencil is used for this as ink would run into the chromatogram along with the samples.
• The pencil line must stay above the solvent.

Question 12

Solubility Equation

Answer 12

(Mass of solid / Mass of water removed) x 100

Question 13

Pure and Impure Substances in Chromatography

Answer 13

• Pure substances will produce only one spot on the chromatogram
• Impure substances will produce more than one spot on the chromatogram

Question 14

RF Formula

Answer 14

Distance moved by substance / Distance moved by solvent