Element, compounds and mixtures

Question 1

what is an element?

Answer 1

a substance made from only one type of atom

Question 2

what is a compound?

Answer 2

substance made from two or more elements that have reacted chemically with each other

Question 3

what is a mixture?

Answer 3

• consists of 2 or more elements or compounds not chemically combined together
• chemical properties of each substance in the mixture are unchanged

Question 4

what is a pure substance?

Answer 4

a single element or compound, not mixed with any other substance

Question 5

what is a pure substance (in everyday language)?

Answer 5

substance that has had nothing added to it, its in its natural state

Question 6

when do pure substances melt and boil?

Answer 6

in specific temperatures

Question 7

what are five ways for separating mixtures?

Answer 7

• simple distillation
• fractional distillation
• filtration
• crystallisation
• paper chromatography

Question 8

how do you do simple distillation

Answer 8

• it works when the liquid has different boiling points
• commonly used to separate ethanol from water
• sequence of distillation is as follows: heating -> evaporating -> cooling -> condensing

Question 9

what is simple distillation used for?

Answer 9

separating a pure liquid from a mixture of liquids

Question 10

how do you do fractional distillation

Answer 10

• Solution is heated to the temperature of the substance with the lowest boiling point and vapors of this substance will rise and evaporate.
• Vapors pass through a condenser where it cools and condenses, turning into a liquid that is collected in a beaker.
• After all of the substance is evaporated and collected, a mixture of substance will be left behind.

Question 11

why do we use Filtration?

Answer 11

To separate an undissolved solid from a mixture of the solid and a liquid / solution ( E.g Sand from a mixture of Sand and Water)

Question 12

how to do Filtration?

Answer 12

• Filter paper is placed in a filter tunnel, which is placed above another beaker
• A mixture of insoluble solids and liquids is poured into the filter tunnel, which only allows small liquids particles to pass through as filtrate
• Solid particles are too large to pass through as filtrate, so this particles will stay behind as residue

Question 13

why do we use Crystallisation?

Answer 13

to separate a dissolved solid from a solution (solids are more soluble in hot solved than in cold ones) (e.g copper sulphate from a solution of copper sulphate in water)

Question 14

how to do Crystallisation?

Answer 14

• The solution is heated, allowing solvent to evaporate and leave a saturated solution
• The saturated solution is left to cool down, and solids will start coming out of the solution to grow into crystals
• Crystal are collected and allowed to dry

Question 15

why do we use Paper chromatography?

Answer 15

to separate substances that have different solubilities in a given solvent (e.g different coloured inks mixed to form black ink)

Question 16

how to do Paper chromatography?

Answer 16

• Draw a line with a pencil on chromatography paper and the concentrated spot of ink/dye is placed on it.
• Lower the paper into a bucket of solvent, which allows the solvent to travel up the paper taking the particles of coloured substances with it.
• Different substances have different solubilities so the solvent will travel at different rates, causing the substance to be spread along the vertical length of paper.
• This will show the different components of the ink/ dye.

Question 17

Practical: investigate paper chromatography by using inks/food colourings
(there are 6 points you need to make for this question)

Answer 17

1. Draw a line 1 cm (baseline) from the bottom of the paper (use pencil not ink. Ink dissolves into the solvent and a pencil doesn’t).
2. One drop of dye of each sample must be dropped at different points along the baseline.
3. The paper must be then suspended in a beaker which contains a small amount of solvent. The bottom of the paper must be touching the solvent , but the baseline with the dyes should be above the level of the solvent (this is so that the dryer doesn’t simply dissolve into the solvent in the beaker).
4. Put a lid to cover the beaker so that the atmosphere becomes saturated with the solvent (and so that the solvent does not evaporate from the surface of the beaker).
5. When the solvent is near the top, the paper should be removed from the solvent and a pencil line drawn (and labelled) should reach up the paper. This is called the solvent font.
6. The chromatography is then left to dry so that all the solvent evaporates.

Question 18

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

Answer 18

• Chromatogram shows the composition of a mixture as different coloured substances. It will spread apart as they have different solubilities so it will travel at different rates.
• Pure substances will only produce one spot on a chromatogram during paper chromatography.

Question 19

how to calculate Rf values?

Answer 19

Rf = distance of dye from baseline/ distance of solvent front from baseline