C2. Experimental techniques

Question 1

Name the apparatus for measuring time.

Answer 1

Stopwatch

Question 2

Name the apparatus for measuring temperature + its unit

Answer 2

Thermometer

Unit: Celcius

Question 3

Name the apparatus for measuring volumes of liquids + its unit.

Answer 3

Measuring cylinder
Volumetric pipette
Burette
Volumetric flask

Unit: cm3 or dm3

Question 4

Describe paper chromatography.

Answer 4

• A method for separating and purifying coloured compounds using filter paper and a solvent.
• Can be used to identify compounds.

Method:

1. First draw a pencil baseline across a piece of chromatography paper.
2. Then place a spot of the concentrated dye mixture, M, on the baseline using a very fine pipette.
3. Then put some spots of some pure dyes that you think the mixture might contain. A, B and C, on the line as well. The chromatography paper is put in a jar with the solvent. Make sure that the solvent level is below the level of the spots otherwise the dye will wash off into the solvent. As the solvent moves up the paper, the dyes in the mixture separate from each other.

Question 5

What is ‘purity’? Why is it important?

Answer 5

Purity: when there is only one substance present.

• Impurities in food additives or medical drugs may have harmful effects on health.
• The computer industry need pure silicon to produce a chips. If there are other substances in the silicon, the chip might not be able to work properly.

Question 6

How do we know if a substance is pure?

Answer 6

• Most pure substances have distinct melting and boiling points. They can be used to identify a substance.
• The boiling point is increased by impurities.
• The melting point is decreased by impurities.

Question 7

Name the methods of purification.

Answer 7

• Filtration
• Crystallisation
• Simple distillation
• Fractional distillation
• Chromatography

Question 8

Describe ‘filtration’. What does it separate and how?

Answer 8

Separate: an insoluble solid from a liquid.

• The solid remains on the paper. You can rinse it with distilled water.
• The liquid or filtrate passes through the paper.

Question 9

Describe ‘crystallisation’. What does it separate and how?

Answer 9

Separate: the soluble from a solution
- Works because a solvent can dissolve less and less solute, as its temperature falls.

1. Heat the solution, to evaporate some solvent.
2. Test the hot solution using a glass rod. If crystals form on the rod, you know the solution is saturated.
3. If the solution is saturated, stop heating, and leave it to cool. As it cools, crystals of the solute form.
4. Separate the crystals by filtering. Wash them with a little distilled water. (Take care, since they are soluble)

Question 10

Describe ‘simple distillation’. What does it separate and how?

Answer 10

Separate: the solvent from a solution

• The solvent boils off as a gas, as its boiling point.
• The gas condenses back to a liquid in the cool condenser.
• The solute remains in the flask.

Question 11

Describe ‘fractional distillation’. What does it separate and how?

Answer 11

Separate: two or more miscible liquids (liquids that mix completely)

• Suppose A has the lower boiling.
• Gas A passes into the condenser first, at A’s boiling point. It condenses there.
• Collect liquid A as above. Stop collecting when the temperature on the thermometer rises.
• Keep heating. Gas B passes into the condenser, at B’s boiling point. Collect liquid B in a separate beaker.

Question 12

Describe ‘chromatography’. What does it separate and how?

Answer 12

Separate: substances from a mixture.

Paper chromatography is used to:

• Find out how many substances there are in a mixture.
• To test for impurities
• To identify the substances in the mixture by Rf value.

Different speeds due to:

• Different solubility: more soluble travel further.
• Different attraction to stationary phase: less attractions travel farther.