b - Elements, mixtures and compounds

1b (1.8-1.13)

element

consist of one type of atom only
e.g. oxygen, copper

mixture

• material composed of 2+ elements/compounds

• physically mixed together

• no chemical bond

• properties of mixture are mixture of properties of separate parts

• e.g. air (mixture of several gases), crude oil (mixture of hydrocarbons, mostly liquids)

compound

• made up of atoms of 2+ different elements joined by chemical bonds

• properties often totally different from properties of original elements

• e.g. carbon dioxide is compound formed from chemical reaction, one C atom reacts with two O atoms to form molecule of carbon dioxide

Pure substance

• Made of single element/compound

• Has specific melting + boiling point
  e.g. pure ice melts at 0ᵒC, pure water boils at 100ᵒC

• Mixture not pure - will melt/boil gradually over range of temperatures

filtration

used to separate insoluble solid from a liquid/solution

1. Put filter paper in funnel and pour in mixture

2. Liquid part runs through paper, leaving behind solid residue

crystallisation

used to separate soluble solid from solution

1. Pour solution into evaporating dish + gently heat solution
   Some water will evaporate, solution becomes more concentrated

2. Once some water has evaporated/when crystals start to form, remove dish from heat + leave solution to cool

3. Salt should start to form crystals as it becomes insoluble in cold, high conc. solution

4. Filter crystals out of solution + leave in warm place to dry

Paper chromatography

used to separate dyes

1. Draw line near bottom of filter paper (use pencil as pencil marks are insoluble so won’t dissolve in solvent)

2. Add spots of diff inks to the line at regular intervals

3. Loosely roll sheet up + put in beaker of solvent e.g. water

4. Ensure level of solvent is below baseline - don’t want inks to dissolve in solvent

5. Put lid on container to stop solvent evaporating

6. Solvent seeps up paper, carrying inks with it

7. Each dye in inks moves up paper at diff rate + forms spot in diff place

8. When solvent has nearly reached top of paper, take paper out of beaker + leave to dry

9. End result is called chromatogram

How chromatography separates mixtures

• Different dyes move up paper at different rates

• Some stick to paper, others dissolve more readily in solvent + travel quicker

• Distance travelled by dyes depends on solvent + paper used

Rf value

Rf = distance travelled by solute/distance travelled by solvent

1. To find distance travelled by solute, measure from baseline to centre of spot

2. Chromatography often used to see if certain substance is in mixture
   Run a pure sample of substance you think might be in mixture alongside sample of mixture itself
   If sample has same Rf values as one of the spots, they’re likely to be same

simple distillation

used to separate pure liquid from solution

1. Heat the solution
   Part of solution with lowest BP evaporates

2. Vapour is cooled, condenses + collected

3. Rest of solution is left behind in flask

4. Can use simple distillation to get pure water from seawater
   Water evaporates, condenses and is collected

Problem: can only be used to separate things with very different BPs

fractional distillation

used to separate mixture of liquids with different boiling points

1. Put mixture in flask + put fractionating column on top, then heat it

2. Different liquids have different BPs so evaporate at diff temps

3. Liquid with lowest BP evaporates first
   When temp on thermometer matches BP of liquid, it reaches top of column

4. Liquids with higher BPs also start to evaporate but column is cooler towards top, so they only get part of the way up before condensing + running back down towards flask

5. When first liquid has been collected, raise temp until next one reaches the top