Principles Of Chemistry - Elements, Compounds And Mixtures

Question 1

What is an element?

Answer 1

Substance made from one type of atom

Question 2

What is a compound?

Answer 2

Substance made from two or more elements that have chemically reacted with each other

Question 3

What is a mixture?

Answer 3

Consists of two or more elements or compounds that are not chemically combined together. The chemical properties of each substance in the mixture is unchanged

Question 4

What is a pure substance?

Answer 4

A single element or compound that is not mixed with any other substance

Question 5

What is simple distillation?

Answer 5

Used to separate a pure liquid from a mixture of liquids

• works when the liquids have different boiling points
• commonly used to separate ethanol and water
• with ethanol as the example, ethanol has a lower boiling point that water, so will evaporate first. The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid.
• sequence of events; heating -> evaporating -> cooling -> condensing

Question 6

What is fractional distillation?

Answer 6

• the oil is heated in the fractionating column and the oil evaporates and condenses at a number of different temperatures
• the many hydrocarbons in oil can be separated into fractions, each of which contains molecules with a similar number of carbon atoms
• the fractionating column works continuously, heated crude oil is piped in at the bottom. The vaporised oil rises up the column and the various fractions are constantly tapped off at the different levels where they condense.
• the fractions can be processed to produce fuels and feedstock for the petrochemical industry

Question 7

What is filtration?

Answer 7

If you have produced eg a precipitate (which is an insoluble salt), you would want to separate the salt/precipitate from the salt solution.
- you would do this by filtering the solution, leaving behind the precipitate

Question 8

What is crystallisation?

Answer 8

If you were to have produced a soluble salt and you wanted to separate this salt from the solution that it was dissolved in.

• you would first warm the solution in an open container, allowing the solvent to evaporate, leaving a saturated solution
• allow this solution to cool
• the solid would come out of the solution and crystals will start to grow, these will then be collected and allowed to dry

Question 9

What is paper chromatography?

Answer 9

• Used to separate mixtures and give information to help identify substances
• involves a stationary phase and a mobile phase
• separation depends on the distribution of substances between the phases.
• the more soluble a substance is, the further it will travel up the page

Question 10

What is the Rf value and how do we calculate it?

Answer 10

Used to identify different components in chromatography.

Rf value = distance moved by substance / distance moved by solvent