Principles of Chemistry - Elements, Compounds and Mixtures

Question 1

what is an element?

Answer 1

The simplest type of substances made up of only one type of atom

Question 2

what is a compound?

Answer 2

a substance that contains two or more elements chemically joined together in fixed proportions

Question 3

what is a mixture?

Answer 3

different substances in the same space, but not chemically combined

Question 4

what are the diatomic elements?

Answer 4

• Hydrogen (H₂)
• Nitrogen (N₂)
• Fluorine (F₂)
• Oxygen (O₂)
• Iodine (I₂)
• Chlorine (Cl₂)
• Bromine (Br₂)

Question 5

what are the boiling points of pure and mixed substances?

Answer 5

a pure substance has a fixed melting and boiling point, but a mixture may melt or boil over a range of temperatures

Question 6

what are the experimental techniques for the separation of mixtures?

Answer 6

• simple distillation
• fractional distillation
• filtration
• crystallisation
• paper chromatography

Question 7

what is simple distillation?

Answer 7

• used to separate a liquid from a solution (e.g. water from salt water)
• the saltwater is boiled
• the water vapour condenses back into a liquid when passed through the condenser
• the salt is left behind in the flask

Question 8

what is fractional distillation?

Answer 8

• used to separate a mixture of different liquids that have different boiling points (e.g separating alcohol from a mixture of alcohol and water)
• water boils at 100 and alcohol boils at 78
• by using the thermometer to carefully control the temperature of the column, keeping it at 78C, only the alcohol remains as vapour up to the top of the column and passes into the condenser.
• The alcohol vapours then condense back into a liquid.

Question 9

what is filtration?

Answer 9

• used to separate an insoluble solid from a liquid
• mixture is poured into the filter paper
• sand does not pass through and is left behind (residue) but the water passes through the filter paper and is collected in the conical flask (filtrate)

Question 10

what is crystallisation?

Answer 10

• used to obtain a salt which contains water of crystallisation from a salt solution
• gently heat the solution in an evaporating basin to evaporate some of the water until crystals form on a glass rod.
• leave to cool and crystallise.
• filter to remove the crystals.
• dry by leaving it in a warm place.

Question 11

what is paper chromatography?

Answer 11

used to separate the parts of a mixture into their components (e.g. the different dyes in ink can all be separated and identified)

Question 12

how do you carry out paper chromatography?

Answer 12

• a baseline is drawn on the paper in pencil
• the mixture is spotted onto the baseline alongside known or standard reference materials
• the end of the paper is then put into a solvent which runs up the paper and through the spots, taking some or all of the dyes with it
• different dyes will travel different heights up the paper

Question 13

what is a chromatogram?

Answer 13

the resulting pattern of dye from paper chromatography

Question 14

how do you calculate the Rғ value?

Answer 14

distance of a dye from baseline/distance of a solvent front from baseline