Chapter 2-Elements, Compounds and Mixtures

Question 1

What is an element?

Answer 1

An element cannot be split into anything simpler by an chemical reaction and all the atoms are of the same type (have the same atomic number).

Question 2

What is a compound?

Answer 2

A compound contains two or more different atoms chemically joined together in a fixed ratio.

Question 3

What is a mixture?

Answer 3

A mixture contains elements and/or compounds that are not chemically combined. They can be mixed together in any proportion and can be separated by physical methods e.g. filtering.

Question 4

What is the difference between the melting and boiling points of a pure substance and a mixture?

Answer 4

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

Question 5

What are the five experimental techniques for the separation of mixtures?

Answer 5

Filtration, Crystallisation, Simple distillation, Fractional distillation, Paper chromatography

Question 6

What is filtration used to separate and how is it done?

Answer 6

Filtration is used to separate an insoluble solid from a liquid e.g. sand and water. The insoluble solid residue e.g. sand is collected on the filter paper.

Question 7

What is crystallisation used to separate and how is it done?

Answer 7

Crystallisation is used to separate a dissolved substance (solute) from a solution e.g. sodium chloride crystals from sodium chloride solution. The solution is heated to boil off some of the water. When a saturated solution is obtained, leave it to cool and crystals will form. Remove the crystals from the remaining solution by filtration and dry them. Safety-wear goggles in case solution spits into eye.

Question 8

What is simple distillation used to separate and how is it done?

Answer 8

Simple distillation is used to separate the solvent from a solution e.g. water from copper sulphate solution. The solution is heated and the water boils. The water vapour enters the Liebig condenser where it cools and condenses back into a liquid.

Question 9

What is fractional distillation used to separate and how is it done?

Answer 9

Fractional distillation is used to separate a mixture of liquids with different boiling points e.g. water and alcohol. The vapour of the liquid with the lowest boiling point will rise up the column first and enter the condenser where it cools and turns back into a liquid.

Question 10

What is paper chromatography used to separate and how is it done?

Answer 10

Paper chromatography is used to separate a mixture of coloured substances e.g. dyes in ink. The different dyes in the mixture have different solubilities. The more soluble the dye, the further it travels up the paper.

Question 11

How does a chromatogram provide information about the composition of a mixture?

Answer 11

A chromatogram can be used to separate and identify the components of a mixture e.g. dyes. In the mixture, the minimum number of dyes are the number of different spots that appear on the chromatography paper. Any single dyes which are contained in the mixture will have both the same colour and will have travelled the same distance up the paper as spots in the mixture (using the same solvent).

Question 12

What is the Rf value and how is it calculated?

Answer 12

The Rf value (retardation factor) can be used to identify the spots in the mixture. It is calculated using this formula: distance moved by the spot/distance moved by the solvent (both measured from the pencil line).

Question 13

How can the Rf value be used to identify the components of a mixture?

Answer 13

If the spots in a mixture have the same Rf values as single dyes, we can conclude that the mixture contains these dyes.

Question 14

Give a brief method for the paper chromatography practical.

Answer 14

1. Draw a line using pencil across a piece of chromatography paper about 1cm from the bottom of the paper.
2. Put a spot of the mixture of dyes and each of the single dyes on the pencil line and allow them to dry.
3. Suspend the chromatography paper in a beaker that contains a small amount of solvent so that the bottom of the paper goes into the solvent.
4. Put a lid on the beaker.
5. When the solvent has moved to about 1cm from the top of the paper, remove it from the beaker and draw a pencil line to show where the solvent got to.
6. Leave the paper to dry so that all the solvent evaporates.

Question 15

What is a safety point to be noted for the paper chromatography practical?

Answer 15

Avoid skin contact with the solvents and dyes, especially if you have sensitive skin.

Question 16

Why should you use a pencil to draw the base line?

Answer 16

If you use a pen, the colours in the ink may move up the chromatography paper with the solvent.

Question 17

Why should the solvent level be below the pencil line?

Answer 17

The solvent should be below the pencil line so that the inks/colourings don’t just dissolve in the solvent.

Question 18

Why should you use a lid?

Answer 18

You should use a lid to stop the solvent from evaporating from the surface of the paper.

Question 19

What is the solvent front?

Answer 19

The solvent front is the highest level on the solvent paper at any time.

Question 20

What two factors affect the distance that the dyes move up the paper?

Answer 20

The affinity they have the paper (how well they ‘stick’ to the paper) and how soluble they are in the solvent which moves up the paper.

Question 21

What does it mean if a dye has not moved from the base line and what should you do?

Answer 21

It means that the dye is insoluble in the solvent being used. You should try using a different solvent.