C2.1 - Purity and Separating Mixtures

Question 1

What is an elements atomic number?

Answer 1

The amount of protons.

Question 2

What is relative atomic mass? What is the symbol? (atomic mass of an element)

Answer 2

The mean mass of an atom of an element compared to 1/12 of the mass of a carbon-12 atom. Symbol - Ar (subscript r)

Question 3

What is relative formula mass? What is the symbol?

Answer 3

The mean mass of a unit of a substance compared to 1/12 of the mass of a carbon-12 atom. Symbol - Mr (subscript r)

Question 4

How do you work out relative formula mass?

Answer 4

Using your periodic table, find the relative atomic mass (mass number) of the elements present. Multiply each element’s atomic mass by the amount of atoms of each element. Add each element’s atom’s atomic masses together. There are no units.

(e.g. H₂O. Hydrogen’s atomic mass is 1. There are 2 hydrogens so 1x2 = 2. Oxygen’s atomic mass is 16. There is 1 oyxgen atom so 16x1 = 16. 16+2 = 18.)

Question 5

What is the Mr of CaSO4? (Ca has an Ar of 40, S has an Ar of 32 and O has an Ar of 16.)

Answer 5

136.

Question 6

What is empirical formula?

Answer 6

An empirical formula shows the simplest whole-number ratio of the atoms of each element in a compound.

Question 7

What is a pure substance in Chemistry?

Answer 7

A pure substance consists of just one element or compound.

Question 8

What is an alloy? Are they pure?

Answer 8

Alloys are mixtures, that contain at least one metal. Mixtures have more than one element or compound and are impure. Thus, alloys are impure.

Question 9

How can you determine if a substance is pure?

Answer 9

• Pure substances melt at specific temperatures. Impure substances melt over a range of temperatures, not a specific temperature.
• Impure substances melt at a lower temperature, and boil at a higher temperature.

Question 10

What is a solution? Are solutions pure?

Answer 10

A mixture formed when a solute dissolves in a solvent. Mixtures are never pure.

Question 11

What is a solvent?

Answer 11

A substance that can dissolve a solute to form a solution.

Question 12

What is a solute?

Answer 12

A substance that dissolves in a solvent.

Question 13

What is solubility?

Answer 13

A measure of how much solute can dissolve in a given solution at a certain temperature.

Question 14

What is a saturated solution?

Answer 14

A solution that has dissolved the maximum mass of solute at a given temperature.

Question 15

What are the four main separation techniques in Chemistry?

Answer 15

Filtration, Crystallisation (Evaporation), distillation, chromatography

Question 16

What does filtration do?

Answer 16

Separates insoluble substances (substances which haven’t and can’t dissolve in the given solvent) from a solvent.

Question 17

What does crystallisation do?

Answer 17

Separates a soluble substance (a susbtance which has dissolved) from a solvent.

Question 18

What does distillation do?

Answer 18

Separates a solvent from a solution (separates liquids) (fractional - multiple with close boiling points but not too close.)

Question 19

What does chromatography do?

Answer 19

Separates pigments based on solubility in given the solvent.

Question 20

How does filtration work?

Answer 20

Filtration works because filter paper has tiny holes which the (undissolved) insoluble substances cannot pass through, and are left as residue. The solvent (filtrate) can pass through (it is a liquid) and drops into the conical flask), separating the substances.

Question 21

What happens in crystallisation?

Answer 21

You heat a solution (in a evaporating basin over boiling water over a bunsen burner) slowly to evaporate the solvent. When a few teaspoons of the solvent is left, you turn off the heat and leave it. You will see crystals of the solute form. You can then filter the crystals and small amount of solvent to get pure crystals.

Question 22

Why does crystallisation work?

Answer 22

Crystallisation works beacuse as you slowly heat the solution the solvent starts evaporating and the solution becomes saturated. A solution is saturated when no more solute can be dissolved at that temperature. Crystals start forming and when you turn off the heat and let it cool, the solubility of the solute decrease, so more crystals form.

Question 23

How does distillation work?

Answer 23

Simple distillation - We heat a solution until the solvent with the lower boiling point evaporates, goes through the condensor, condenses and is collected in a beaker. The other solvent (all are liquids) is let behind as residue. The boiling parts are far apart.

Fractional distillation - Same thing with closer boiling points and more liquids (solvents). The fractionating column makes sure only one solvent at its boiling temperature makes it to the condensor and is collected in a beaker. Any other liquids which evaporate travel through th fractionating column and don’t have the energy to make it to the condensor.

Question 24

What are the three types of chromatography?

Answer 24

Paper Chromatography, Thin Layer Chromatography (TLC) and Gas Chromatography

Question 25

What are the mobile and stationary phases in all three?

Answer 25

Paper Chromatography - paper (mobile), solvent
TLC - Silica or alumina powder on a glass plate, solvent
Gas - Silica or alumina power in metal columns, unreactive carrier gas (e.g. nitrogen)

Question 26

What is a retention factor?

Answer 26

Retention factors (Rf values) of substances are measured by:
distance travelled by substance / distance travelled by solvent.
They are individual to a substance in the given solvent. You can identify substances with it.

Question 27

Why is gas chromatography useful?

Answer 27

Gas chromatography separates the components of a mizture and also measures their amount. In a chromatogram (the results), the time each component comes out is the Rf, and is specific. The peaks (chromatograms look like a heart rate) tell us the amount.

Question 28

Why is TLC preffered over Paper chromatography?

Answer 28

• Quicker
• More sensitive, so smaller sampples can be used
• There is a larger range of solvents to choose from.

Question 29

How can you check your Rf value and equation is ‘correct’?

Answer 29

The answer is always 0.something (no units)