5.1 Purity & Formulations

Question 1

What is a ‘pure substance’ in chemistry?

Answer 1

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

Question 2

Describe how the melting point can tell you how pure a substance is (3)

Answer 2

• A chemically pure substance will melt at a specific temperature.
• You can test the purity of a sample by measuring its melting point and comparing it with the reference melting point of the pure substance.
• Impurities in your sample will lower the melting point and increase the melting range of your substances.

Question 3

What temperature does pure water boil at?

Answer 3

100 degrees celsius

Question 4

What are ‘mixtures’?

Answer 4

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

Question 5

The parts of a mixture can be either elements or compounds. How can they be separated? (5)

Answer 5

By physical methods such as:
- Filtration
- Crystallisation
- Simple distillation
- Fractional distillation
- Chromatography

Question 6

What are ‘formulations’?

Answer 6

Formulations are useful mixtures with a precise purpose that are made by following a ‘formula’. Each component in a formulation is present in a measured quantity, and contributes to the properties of the formulation so that it meets its required function.

Question 7

What is the definition of a ‘physical method’ of separating elements/compounds?

Answer 7

A physical method is one that doesn’t involve a chemical reaction

Question 8

What is crude oil a mixture of?

Answer 8

Crude oil is a mixture of different length hydrocarbon molecules

Question 9

Paints are formulations. What components do they contain? (4)

Answer 9

• They contain a pigment to give the paint colour
• They contain a solvent to dissolve the other components
• They contain a binder which holds the pigment in place after it’s been painted on
• They contain other additives to change the physical and chemical properties of the paint.

Question 10

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

Answer 10

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

Question 11

Q: In a certain formulation made from water and ethanol, the ratio of water to ethanol is 2:1. What volume of ethanol is present in 15cm cubed of the formulation?

Answer 11

Volume of ethanol = proportion of ethanol x total volume = 1/3 x 15cm = 5cm

Question 12

In general, what effect does having impurities in your samples have on melting and boiling point?

Answer 12

• It will lower the melting point
• It will increase the boiling point

Question 13

Chemical analysis (also known as analytical chemistry) is about:

Answer 13

The instruments and methods we use to separate, identify, and quantify different substances.

Question 14

What are three things that would require a formulation?

Answer 14

• Paints
• Medicines
• Cleaning agents

Question 15

If we test a substance for purity by measuring its boiling point, is that a physical test or a chemical test?

Answer 15

Physical