Topic 1 - Atomic structure and the periodic table

Definition of compound, mixture, element, molecule

Element = A substance containing only one type of atom

Compound = Two or more different elements chemically bonded together

Mixture = Different elements or compounds not chemically bonded together

Molecule = Any element chemically combined, e.g. methane containing carbon and hydrogen, water containing hydrogen and oxygen, ammonia, nitrogen and hydrogen

examples of physical separation techniques and when it can be used

Filtration

Distillation

Crystallisation

Chromatography

Can only be used to separate mixtures

Method of Filtration

Used to separate an insoluble solid from a liquid

Equipment:

• Filter paper

• Filter funnel

• Conical flask

Method:

1. Pour the mixture into the filter paper

2. This will cause the liquid to pass through the tiny pores in the filter paper

3. The liquid that passes through the filter paper is known as the filtrate

4. The solid material cannot pass through the filter paper, so it is trapped

5. Once all the liquid has passed through the tiny pores of the filter paper we have the liquid separated from the solid

Method of Crystallisation

Used to separate a soluble solid from a liquid but keeping the solid instead of the liquid

Method:

1. Dissolve the soluble solid into the liquid to form an aqueous solution

2. Pour the aqueous solution into an evaporating basin

3. Using a Bunsen burner gently heat the aqueous solution - allow some of the water to evaporate but not all otherwise you won’t form crystals

4. Place the evaporating basin by a windowsill and leave it overnight for crystals to form

Method of simple distillation

Used to separate a soluble solid from a liquid but keeping the liquid instead of the solid

Apparatus:

1. Place the solution with the liquid and dissolved solid into the flask

2. The flask is connected to a glass tube. This glass tube is surrounded by a condenser.

3. Cold water from the tap continuously runs through the condenser, keeping the internal glass tube cold.

4. A thermometer is also included

Method:

1. Heat solution using a Burner

2. As the solution is heated the liquid evaporates turning into a vapour

3. The vapour rises up the glass tube

4. As the vapour passes over the thermometer the thermometer reading increases

5. The solution is usually heated until it boils

6. The vapour then passes into the condenser

7. The vapour then condenses when it is in the glass tube surrounded by the condenser because cold tap water is circulating around the condenser

8. The vapour therefore turns into a liquid as it passes through the condenser

9. The solid is in the flask while the liquid is collected in the beaker

Pros and cons with simple distillation

Simple distillation can be used to produce drinking water from sea water. However a lot of energy is required for simple distillation so generally it is not used to produce drinking water

What is fractional distillation

In fractional distillation we separate a mixture of different liquids. However, these different liquids must have different boiling points.

Apparatus used in fractional distillation

Apparatus:

1. A mixture of two different liquids in a flask

2. The flask containing the mixture is attached to a fractionating column containing hundreds of glass beads

3. At the top of the fractionating column we have a thermometer

4. Then there is a condenser which cold tap water circulates around

Method for fractional distillation

Method:

1. Gently heat the mixture ensuring that in the mixture the different liquids have different boiling points

2. All of the liquids will start to evaporate but the one with the lower boiling point will evaporate more easily.

3. Now there is a mixture of two different vapours entering the fractionating column.

4. When the vapours reach the fractionating column they condense and drip back into the flask where the liquids evaporate again.

5. This repeated evaporation and condensation increases the amount of the lower boiling point chemical in the fractionating column.

6. The temperature on the thermometer will rise meaning that a mixture of the different vapours is passing over the thermometer. However, the mixture will contain more of the chemical with the lower boiling point.

7. These vapours now pass into the condenser and turn back to a liquid. However, this liquid is still a mixture of the different chemicals

8. Eventually the temperature on the thermometer stops rising this will be the reading of the lower of the two boiling points.

9. Now we have mainly one chemical passing into the condenser. As this chemical condenses we can collect it in a fresh beaker.

10. This is our first proper fraction in other words the chemical which has the lowest boiling.

11. Eventually the temperature on the thermometer begins to rise again meaning that a mixture of vapours are passing into the condenser. However, this mixture mainly contains the chemical with the higher boiling point.

12. When the thermometer reaches a constant temperature we are now collecting a relatively pure sample of the second chemical.

13. We have now separated the different chemicals based on their boiling points.

Pros and cons of fractional distillation

1. If the two liquids have very similar boiling points then it is much harder to separate them so we many need to carry out several rounds of fractional distillation.

2. The equipment is not really useful for separating a very large volume of liquid. e.g. refining crude oil

What is Paper chromatography

Paper chromatography allows us to separate substances based on their different solubilities.

Method for Paper chromatography

1. Take a piece of chromatography paper.

2. draw a pencil line near the bottom of the paper.

3. Then put a dot of our first colour onto the pencil line

4. Then put a dot of our second colour next to the first colour. It is possible to do this for several colours ensuring there is enough space on the paper

5. Then place the bottom of the paper into the solvent. Ensuring that the solvent doesn’t go above the pencil line to prevent the colour dissolving to the solvent

6. As the solvent moves up the paper it dissolves the ink in the two coloured dots which causes the ink to be carried up the paper dissolved in the solvent

What are the two phases in Paper chromatography

1. The paper is called the stationary phase because the paper does not move.

2. The solvent is called the mobile phase because the solvent moves.

paper chromatography results and what they mean

If only one spot of colour forms on the paper this means that the original spot placed on the pencil line is a single pure colour.

However, if there are two different spots of colour forming on the paper this tells us that the original colour placed on the pencil line was a mixture of two different colours.

This tells us that a pure compound will produce a single spot in all solvents. Whereas the compounds in a mixture may separate into different spots depending on the solvent.

Why does paper chromatography work

paper chromatography works because different substances have different solubilities.

A more soluble substance is more attracted to the mobile phase.

Why is the starting line drawn in pencil

If we drew the line in pen, the pen ink could dissolve in the solvent and move up the paper