Elements, Mixtures And Compounds

Question 1

Elements

How are they shown in models and diagrams

Answer 1

• Elements are substances that can’t be split into anything simpler by chemical means.
• An element contains only one type of atom.
In models or diagrams, they are shown as atoms of a single colour or size

Question 2

Compounds

Examples

How are diagrams shown?

Answer 2

Compounds are formed when two or more elements chemically combine.
• Examples are carbon dioxide and methane.
Diagrams of compounds show more than one type of atom bonded together

Question 3

Mixtures

Can they be separated?

Answer 3

• Mixtures contain elements and/or compounds that are not chemically combined.
• No chemical reaction occurs.
• The various substances are mixed together
They can be separated by physical methods such as filtering or distilling

Question 4

Difference between mixtures and compounds

Answer 4

• The properties between compounds and mixtures are different.
• For example, a mixture of hydrogen and oxygen is a colourless gas which explodes when you heat it.
The compound, water, is a colourless liquid which puts the flame out

Question 5

Melting and boiling point

Answer 5

• Pure substances such as elements and pure compounds, melt and boil at fixed positions.
  
  • Pure substance means a fixed melting or boiling point for example the melting point of water is 0 degrees and the boiling point 100 degrees.
  • However, mixtures are different. They usually melt or boil over a range of temperatures.
  • If an impure substance comes, then it lowers the melting point of a substance and raises the boiling point.

Question 6

Naming Compounds

Answer 6

When naming a compound containing a metal and a non-metal, the ending of the non-metal changes. When a metal and a non-metal are combined, the ending of the non-metal changes to –ide.
For example calcium and fluorine = calcium fluoride.

If the compound also contains oxygen, then it ends in –ate.
For example, sodium, carbon and oxygen is mixed together, it makes sodium carbonate
Or copper sulphate contains copper, sulphur and oxygen.
However, an exception is hydrogen and oxygen which makes hydroxide.

Question 7

Filtration

Answer 7

• Filtration is used to separate a solid from a liquid.
  
  • For example, sand can be separated from water by filtration.
  • The substance left in the filter paper (sand) is called the residue and the liquid (water) that comes through is called the filtrate.

Question 8

Crystallisation

Answer 8

• Crystallisation can be used to separate a solute from a solution.
  
  • For example it could be used to separate sodium chloride from a sodium chloride solution.
  • The solution is heated in a evaporating basin to boil off some of the water until an almost saturated solution is formed.
  • This can be tested by dipping a glass rod into the solution and seeing if crystals form quickly on its surface when it is removed.
  • The Bunsen burner is then turned off and the crystals allow to form as more water evaporates and solution cools.
  • The crystals can now be removed from the mixture by filtration.

Question 9

Simple distillation

Answer 9

• Simple distillation can be used to separate the solvent from a solution.
  
  • For example, water from copper sulphate solution.
  • The solution is heated and the water boils.
  • The water vapour enters the condenser where it cools back to a liquid.

Question 10

Fractional distillation

Answer 10

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

Question 11

Paper chromatography

Answer 11

To separate a mixture of coloured substances
For example, dyes in ink

The different dyes in the mixture have different solubilities.
The more soluble the dye is, the further it travels up the paper

Question 12

Interpreting chromatagrams

Answer 12

A chromatagram can be used to separate and indentify the components of a mixture

Number of spots = number of dyes

If 2 spots travel the same distance up the paper using the same solvent, they are the same dye

Rf values (retardation factor) can be used to indentify the spots:
Rf value = distance moved by the spot/distance moved by the solvent
-(both measured from the pencil line)

Question 13

Practical points - paper chromatography

What should you use to draw the base line?
Lid?
Solvent level?
What happens if a dye doesnt move?

Answer 13

Use a pencil to draw the base line as ink may travel up the paper
Make sure the solvent level is below the base line or the dyes will just dissolve into the solvent
Use a id to stop the solvent evaporating away
If a dye does not move, it is insoluble in the solvent being used, so try a different solvent