Elements, compounds and mixtures 1.8-1.13

Question 1

Define element

Answer 1

Consist of one type of atom only

Question 2

Define compound

Answer 2

A substance that is made of two or more different elements which are chemically bonded together e.g. a chemical reaction to form a molecule of carbon dioxide
Difficult to separate the original elements out again
Properties of a compound are often different from the original elements

Question 3

Define mixtures

Answer 3

No chemical bond between the different parts of a mixture - can be separated out physically
Properties of a mixture are mixture of the properties of the separate parts
Isn’t a pure substance - not made up of a single element or compound
Therefore will melt or boil gradually over a range of temperatures

Question 4

What is the characteristic of a pure substance melting and boiling point?

Answer 4

Fixed melting and boiling point

Question 5

Do mixtures have fixed boiling and melting points?

Answer 5

NO

A mixture is not pure so it will melt or boil gradually over a range of temperatures

Question 6

What is the purpose of filtration?

Answer 6

• Separate an insoluble solid from a liquid
  E.g. if the product of a reaction is an insoluble solid you can filter it out from the liquid reaction mixture
• Purification –> solid impurities

Question 7

Describe the process of filtration

Answer 7

Put a filter paper in a cone shape into a funnel and pour your mixture into it
Liquid part runs through and leaves the solid residue behind

Question 8

What is the purpose of crystallisation?

Answer 8

Separate a soluble solid from a solution

Question 9

Describe the process of crystallisation

Answer 9

Pour the solution into an evaporating dish and gently heat the solution –> some water evaporate = solution more concentrated
Remove from heat when some of the water has evaporated and leave to cool
Salt should start to form crystals
Filter the crystals out of the solution and leave them in a warm place to dry

Question 10

Describe the process of chromatography

Answer 10

1. Draw a pencil line near the bottom of a sheet of filter paper
2. Add spots of different inks on the line at regular intervals
3. Loosely roll the sheet up and put it in a beaker of solvent (depending on what is being tested - water/ethanol) above the pencil line
4. Place a lid to stop the solvent evaporating
5. The solvent seeps up the paper with the inks
6. Each dye will move up the paper at a different rate and form a spot in a different place
7. Take out the paper when the solvent has nearly reached the top of the paper
8. End result is a chromatogram

Question 11

How does chromatography separate mixtures?

Answer 11

• Different dyes will move up the paper at different rates
• Some stick to the paper and others will dissolve more readily in the solvent (travel more quickly)
• The distance the dyes travel depends on the solvent and the paper used

Question 12

What is the Rf value in chromatography?

Answer 12

The ratio between the distance travelled by the dissolved substance (solute) and the distance travelled by the solvent

Question 13

What is the formula to find the Rf value?

Answer 13

Rf = distance travelled by solute/distance travelled by solvent

Question 14

How do you find the Rf value using the formula?

Answer 14

Distance travelled - measure from the baseline to the centre of the spot

Question 15

What is the purpose of chromatography?

Answer 15

• Separate mixtures
• Identify the substances present in a mixture –> if 2 inks have spots at the same distance they both contain that substance

Question 16

What is the purpose of simple distillation?

Answer 16

Separate out solutions

Question 17

Describe the process of simple distillation

Answer 17

1. Solution is heated - part of the solution that has the lowest boiling point evaporates
2. Vapour is then cooled, condenses and collected
3. The rest of the solution is left in the flask

Question 18

What is a disadvantage to simple distillation?

Answer 18

Can only separate things with very different boiling points

Question 19

What is the purpose of fractional distillation?

Answer 19

Separate a mixture of liquids

Question 20

Describe the process of fractional distillation

Answer 20

1. Put the mixture in the flask with a fractionating column on top and heat it
2. The different liquids have different boiling points so evaporate at different temperatures
3. Liquid with the lowest boiling point evaporates first –> temperature at its boiling point it reaches the top of the column (cooler at the top of the column)
4. Higher boiling points liquids condense before reaching the top therefore
5. Once the first liquid has been collected raise the temperature