C2 + C4.1

Question 1

Name and suggest appropriate apparatus for..
1. time
2. temperature
3. mass
4. volume (burettes, pipettes and measuring cylinders)

Answer 1

Time: stopwatch or stopclock
Temperature: thermometer or digital thermometers
Mass: digital balance (must be tared before use)
Volume: can be determined using several types of apparatus. (ex. pipettes, burettes)

Question 2

Demonstrate knowledge and understanding of paper chromatography

Answer 2

• Chromatography used to separate mixtures + give information to help identify substances.
• Involves a stationary phase (paper) and mobile phase (solvent)
• Separation depends on distribution of substances between the phases

Question 3

how do you conduct chromatography (how to set up)

Answer 3

1. draw a baseline in pencil (around 1cm from bottom of the paper)
2. the mixtures should be spotted on the baseline
3. chromatography paper suspended vertically in solvent that doesn’t go above the baseline
4. the solvent takes the mixtures up
5. mark a solvent front in pencil to determine Rf value

Question 4

Interpret simple chromatograms

Answer 4

• If two or more substances are the same, they will produce identical chromatograms.
• Impure substance / mixture = show more than one spot
• Pure substance = show up with one spot

Question 5

Rf values?

Answer 5

• Rf values used to identify components of mixtures
• Rf value of a particular compound is always the same
• Allows chemists to identify unknown substances
• Rf value is a ratio, and has NO UNITS
• Rf = retention factor

Question 6

Formula for Rf value

Answer 6

Rf = distance moved by spot from baseline / distance moved by solvent front from baseline

Question 7

Understand the importance of purity in substances for use in everyday life

Answer 7

It is important to ensure that there are no impurities in medical drugs + food additives (it will have harmful effects on our body and health)

Question 8

Recognise that mixtures melt and boil over a range of temperatures

Answer 8

• Different mixtures melt and boil over a range of temps

Examples:
Pure water boils = 100⁰C, but the addition of 10g of sodium chloride will raise the boiling point to 100.2⁰C.

Pure water melts = 0⁰C, but the addition of 10g of sodium chloride will lower the melting point to -0.6⁰C.

Question 9

Identify substances and assess their purity from melting point and boiling point information

Answer 9

• Pure substances: fixed boiling and melting point
• Impure substances: no fixed melting and boiling points (has a range of temperatures)

The purity of a compound can be established using data from its melting point / boiling point or chromatography

Question 10

Describe and explain methods of separation and purification using paper chromatography

Answer 10

paper chromatography = the method of separating pigments using filter paper

solvent moves up the chromatography paper,
the dyes in the mixture separate themselves,
solvent moves up the paper and the point where it stops is called the solvent front

Question 11

Describe and explain methods of separation and purification using filtration
3. crystallisation
4. distillation
5. fractional distillation

Answer 11

filtration : passing a solution with undissolved/insoluble solids through a piece of filter paper in a filter funnel

solution = filtrate
solid (result of filtration) = residue

Residue should then be washed with distilled water to remove any solution between its solid particles

Question 12

Describe and explain methods of separation and purification using crystallisation
4. distillation
5. fractional distillation

Answer 12

Crystallisation: heating a solution to extract a crystalline solid from it

crystallisation point can be found by placing a drop of the solution on a cold surface from time to time to see if crystals form

Question 13

Describe and explain methods of separation and purification using fractional crystallisation
4. distillation
5. fractional distillation

Answer 13

Fractional Crystallisation: If there are two dissolved substances with different solubilities at different temperatures, then fractional crystallisation is used to separate them

Question 14

Describe and explain methods of separation and purification using distillation
5. fractional distillation

Answer 14

Distillation used for obtaining the solvent from a solution

Example using salt water:
The solution (salty water) is heated until it boils and evaporates as steam, leaving behind the salt in the flask

The salt doesn’t evaporate because it has a much higher boiling point than water.

Water is very volatile so it evaporates quickly and enters the condenser, which is cold. the steam condenses and turns back into water and pours into a container at the end of the condenser

Question 15

Describe and explain methods of separation and purification using fractional distillation

Answer 15

Fractional Distillation is used to separate a mixture of liquids with different boiling points

The more volatile solvent (lower boiling point) evaporates first and moves up the column, into the condenser where it turns into a liquid and is collected in a container.

When it reaches a certain height, the temperature in the column falls below the non-volatile solvent’s boiling point and it condenses and falls back into the flask as liquid.

Question 16

Suggest suitable separation and purification techniques, given information about the substances involved.

Answer 16

ex. mixture of salt and sand. You need to separate the mixture into pure dry crystals of salt and sand.

Salt is soluble in water but sand is not so the sand will settle at the bottom (because it is insoluble in water) while the water will turn into salt water (solvent) as salt dissolves in it.

separate the sand and solvent using filtration- the sand will be the residue and solvent will be the filtrate.

then use crystallisation to extract the salt from the solvent (evaporate the solvent, filter the crystals and dry it on filter paper).

Now you have dry, pure salt crystals and sand.