b) Elements, Compounds and Mixtures

What is an element?

Substance made from only one type of atom.

What is a compound?

A pure substance made up of two or more elements chemically combined.

What is a mixture?

A combination of two or more substances that aren’t chemically combined.

Type of melting and boiling point for pure substances and mixtures?

Pure substance: fixed temperatures.

Mixture: range of temperatures.

Describe how simple distillation is used?

Used to seperate a liquid and soluble solid from a solution.

1. Solution is heated, pure water evaporates and produces vapour which rises.

2. Vapour passes through condenser, where it cools and condenses, turning into pure liquid that’s collected by beaker.

3. After all water is evaporated only solid solute will remain.

Describe how fractional distillation is used?

Used to separate two or more liquids that are miscible with one another.

1. Solution is heated to temperature of the substance with lowest boiling point.

2. This substance will rise and evaporate first, vapours will pass through condenser and liquid will be collected.

3. All substance is evaporated and collected leaving behind other components of mixture.

Describe how filtration is used?

Used to separate an undissolved solid from a mixture of the solid and liquid.

1. Filter paper is placed in a filter funnel above a beaker.

2. A mixture of insoluble solid and liquid is poured into funnel.

3. Filter paper will only allow small liquid particles though.

4. Solid particles are too large so they will stay behind as residue.

Describe how crystallisation is used?

Used to separate a dissolved solid from a solution.

1. Solution is heated, solvent evaporates leaving saturated solution behind.

2. Saturated solution cools slowly.

3. Crystals being to grow since solid comes out of solution due to decreased solubility.

4. Crystals are collected by filtering solution.

5. They’re washed with cold distilled water to remove impurities and are then allowed to dry.

Describe how paper chromatography is used?

Used to separate substances that have different solubilities in a given solvent.

1. A pencil line is drawn on chronographs paper and spots of sample are placed on it.

2. Paper is lowered into solvent container.

3. Solvent travels up paper by capillary action taking some of coloured substances with it.

4. Different substances have different solubilities so will travel at different rates.

5. Substances with high solubility will travel further.

How to calculate Rf in chromatography?

Distance travelled by substance/distance travelled by solvent

how does chromatograms provide information about composition of structure?

One spot = pure substance

More than one spot = impure substance

How to investigate paper chromatography using inks and food colouring?

1. 1. Use ruler to draw horizontal pencil line 2cm from end of paper.

2. Use different capillary tube to put spot of each colouring on line.

3. Use fifth tube to put spot of unknown mixture on line.

4. Label each spot in pencil.

5. Pour water into beaker and clip top of paper to wooden spill.

6. Rest spill on top of beaker so bottom edge of paper is dipped into solvent.

7. Allow solvent to travel ¾ up paper.

8. Draw pencil line on dry part of paper as close to wet part as possible(solvent from line).

9. Measure distance between two pencil lines.(distance travelled by solvent).

10. For each spot measure distance from start line to middle of spot.