C5 Purity And Mixtures

Question 1

What does it mean when you refer to a substance as being pure in chemistry?

Answer 1

A pure substance is something that only contains one compound or element throughout (not mixed with anything)

Question 2

What does melting point tell us about a substance?

Answer 2

How pure it is.

Question 3

How can you test purity using melting point?

Answer 3

By sample measuring a substances melting point and comparing it with the reference melting point of the pure substance ( found in a data book)

The closer your measured value is to the actual melting point, the purer your sample is.

Question 4

What effect will impurities in the sample have on the melting point?

Answer 4

They will lower the melting point and increase the melting range of the substance.

Question 5

What is a substance that isn’t pure called?

Answer 5

A mixture

Question 6

What are mixtures?

Answer 6

Substances that contain more than one element or compound which aren’t chemically bonded together.

Question 7

The elements or compounds in a mixture can be separated out by what?

Answer 7

Physical methods such as
Filtration
Crystallisation
Simple distillation 
Fractional distillation
Chromatography

Question 8

What is crude oil a mixture of?

Answer 8

Different length hydrocarbon molecules.

Question 9

How are the chemical properties of a substance affected by being part of a mixture?

Answer 9

The chemical properties aren’t affected.

Question 10

What are the properties of a mixture?

Answer 10

A mixture of the properties of the separate parts.

Question 11

What is a physical method?

Answer 11

One that doesn’t involve a chemical reaction.

Question 12

What are formulations?

Answer 12

Mixtures with exact amounts of components.

Question 13

How are formulations made?

Answer 13

By following a formula.

Question 14

Complete the sentence.

Each component in a formulation is present in a ……………, and contributes to the properties of the formulation so that it meets its required function.

Answer 14

Measured quantity

Question 15

Paint is a formulation. Name the 4 parts.

Answer 15

Pigment (gives colour)
Solvent (dissolves the other components)
Binder (holds the pigment in place after it’s been painted on)
Additives (to change the physical and chemical properties of the paint)

Question 16

Why are the chemicals used and the amounts used changed in different types of paint?

Answer 16

So the paint is right for its job.

Question 17

Examples of everyday products that are formulations 9

Answer 17

Medicines
Cleaning products
Fuels
Cosmetics
Hygiene products
Fertilisers
Alloys
Food
Drink

Question 18

Why is it important that the substances used to make a formulation, such as drugs and cosmetics, is pure?

Answer 18

If impurities get into the formulation, people could be harmed when they use them.

Question 19

Ratios can be used to describe the composition of formulations. See p59 for an example

Answer 19

.

Question 20

The melting point of pure aspirin is 136degree C. The melting point of a sample of aspirin is measured as being between 128-132 degree C.

Give two reasons why this suggest that the sample is not pure aspirin (2 marks)

Answer 20

The sample melts over a range of temperatures (1 mark).

The melting point is lower than that of pure aspirin (1mark)