C2 - Experimental Techniques

Question 1

Paper Chromatography - Method

Answer 1

1. A pencil line is drawn on chromatography paper and spots of the sample are placed on it. Pencil is used for this as ink would run into the chromatogram along with the samples
2. The paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent so the samples don’t wash into the solvent container
3. The solvent travels up the paper by the mobile phase, taking some of the coloured substances with it
4. Different substances have different solubilities so will travel at different rates, causing the substances to spread apart and onto paper in stationary phase. Those substances with higher solubility will travel further than the others
   This will show the different components of the ink / dye

Question 2

Copper sulphate crystallisation - method

Answer 2

1. Add 50 cm3 dilute acid into a beaker and warm gently using a Bunsen burner
2. Add the copper(II) oxide slowly to the hot dilute acid and stir until the base is in excess (i.e. until the base stops dissolving and a suspension of the base forms in the bottom - saturated solution)
3. Filter the mixture into an evaporating basin to remove the excess base; larger solids don’t fit through filter pores
4. Gently heat the solution and boil off as much water as possible
5. Carefully pour the copper sulphate into a petri dish and let it dry and crystallise
6. Using distilled water, wash dried crystals

Question 3

Fractional distillation - process (example)

Answer 3

Separates two or more substances from a mixture in liquid state
- relies on each substance having a different boiling point and uses a special piece of equipment called a fractioning column
- in a water-ethanol mixture, as the temperature increases, both ethanol and water vapour are in the fractioning column, only ethanol will move into the condenser since its boiling point (78.4 C) has been reached – it will then be condensed back into liquid form and drip down into a container
- however, until the boiling point of water (100 C) is reached, the water vapour will keep condensing in the fractioning column and drip back into the flask

Question 4

Explain the properties of a fractionating column (2)

Answer 4

• little glass rods inside it: increase surface area
• cooler at the top: substance vapour that’s boiling point is similar to the one’s that we want to separate will cool and condense back into liquid form (does not move to condenser)

Question 5

Explain the process of fractional distillation for crude oil

Answer 5

• Crude oil is made up of several components that have varying boiling points
• crude oil enters gas chamber, in which the liquid substances are turned into gas
• they then move to the fractionating column which moves from high temperatures to low temperatures at the top:
• the substances with the higher boiling points will, almost immediately, find an area of the column with a lower boiling point to theirs and will be condensed back to a liquid and collected
• the other gases continue to move upward until they find a temperature lower than their boiling points

Question 6

Order of products of crude oil fractional distillation

Answer 6

• (top) 20C - Refinery Gas
• 70C - Gasoline / Petrol (used in cars)
• 120C - Naptha (make chemicals)
• 180C - Kerosene (jet fuel, paraffin for heating and lighting)
• 260C - Diesel (for diesel engines)
• 300C - Lubricating Oil
• 350C - Fuel Oil
• Residue (make bitumen, asphalt)

Question 7

Define Rf value

Answer 7

distance sample traveled/distance of solvent front
- must give a DECIMAL
- it is a ratio

Question 8

Name and suggest appropriate apparatus for the measurement of time, temperature, mass and volume

Answer 8

time: stopwatch
temperature: thermometer
mass: scale
volume of water or a solid object: burette, pipette or measuring cylinder
volume of gas: gas syringe or inverted measuring cylinder

Question 9

Solution

Answer 9

The mixture formed when a solute dissolves in a solvent

Question 10

Solvent

Answer 10

A liquid substance capable of dissolving other substances; in which the solute dissolves - greater amount

Question 11

Solute

Answer 11

The substance that dissolves in a liquid (solvent) to form a solution - smaller amount

Question 12

Saturated solution

Answer 12

A solution with the maximum concentration of the solute dissolved in the solvent

Question 13

Soluble

Answer 13

A substance capable of dissolving

Question 14

Insoluble

Answer 14

A substance incapable of dissolving

Question 15

Pure substances

Answer 15

Pure substances are single elements or compounds that contain only one substance
- they have a fixed boiling and melting point (if that changes, they are no longer pure)

Question 16

Impure substances

Answer 16

Impure substances contain more than one kind of substance
- they lower melting points and increase the range of temperatures at which the sample will melt
- they increase boiling point and increase the range of temperatures at which the sample will boil

Question 17

Dissolved

Answer 17

Describes the process by which solute components are dispersed in a solvent

Question 18

Chromatography

Answer 18

A separation technique for the components of a mixture

Question 19

Mixture

Answer 19

A combination of two or more substances that are not chemically combined

Question 20

Compound

Answer 20

A substance made up of atoms of two or more different elements joined by chemical bonds

Question 21

Paper chromatography

Answer 21

Method of separating a mixture of different colours as inks

Question 22

Stationary phase of chromatography

Answer 22

Filter paper

Question 23

Mobile phase of chromatography

Answer 23

Solvent

Question 24

Locating agents - Chromatography (examples)

Answer 24

A locating agent is used to analyse colourless substances in paper chromatography
* Generally a chemical that reacts with the colourless substances to produce a result that shows colour.
* Examples of locating agents include, ninhydrin spray for amino acids, U.V light and Iodine.

Question 25

Crystallisation

Answer 25

The formation of crystals by cooling a saturated solution

Question 26

Copper sulphate equation (worded)

Answer 26

copper oxide + sulphuric acid –› copper sulphate + water

Question 27

Copper sulphate equation (symbols)

Answer 27

CuO(s) + H2SO4(aq) –› CuSO4(aq) + H2O(l)

Question 28

Simple distillation - process

Answer 28

Used to separate a liquid and soluble solid from a solution or a pure liquid from a mixture of liquids
- relies on the solvent having a much lower boiling point than the solute: when heated the solvent boils but the solute doesn’t
- the solvent escapes from the solution in its gas state: it is then cooled and condensed to its liquid state by a condenser