A pure substance in chemistry is a single element or compound that is not mixed with any other substance.

A property is a feature or characteristic of a substance that can be used to classify it, or describe how it behaves.

Pure substance

Combined science

In the Separating Substances unit, pupils will build on their knowledge of states of matter to learn about different techniques for separating mixtures.

In the Solid, Liquid and Gas States and Changes of State unit, pupils learned about the properties of different states of matter and how particles move during changes of state.

This unit covers the particle model, states of matter, and changes between them, distinguishing physical from chemical changes. It explains the concept of purity in scientific terms, using melting point data to identify pure and impure substances, and explores the limitations of the particle model.

States of matter

Physical activity and equipment requiring safe use

A pure substance in chemistry is a single element or compound that is not mixed with any other substance. Which of the following are pure substances in chemistry?

it contains at least two types of carbohydrate

it contains 200 mg of potassium per 100 ml

The image shows the label of a pack of orange juice. What information on the label tells us it is not chemically pure?

it has a melting range from 35 to 40$$^o$$C

it is made up of more than one type of compound

Honey has a melting range of 35 to 40$$^o$$C and it decomposes before it boils. Using this information and what you know about pure substances, what makes honey an impure substance?

How is the melting point of a chemical substance represented on a graph?

Set up a beaker with water, heating up above a Bunsen burner to a gentle boil.

Clamp a boiling tube, with solid substance and thermometer below water level.

Record temperature at suitable time intervals until it reaches 70$$^o$$C.

Note the point at which the solid starts to melt.

Lift the clamp with the tube from the hot water.

Record the temperature at suitable time intervals until it reaches 50$$^o$$C.

Note the point at which the liquid starts to solidify.

A melting and freezing experiment is done with stearic acid. Put the stages of the experiment in order:

Heat water in a pan until 60$$^o$$C and turn off the heat.

Wait until all the honey has just melted and remove from the pan.

Enjoy! You can repeat this if it crystallises again.

Honey can crystallise at room temperature, and starts to decompose (caramelise and smoke) at around 60$$^o$$C. How could you melt crystallised honey so this doesn't happen? Put the steps in order.

A {{}} material is composed of only one type of chemical.

What is the name given to a substance found in a material in very small amounts, but which can alter the material’s properties?

Which diagram correctly represents a mixture of two metals in the solid state?

 Mixtures boil at a single, specific temperature.

Mixtures contain a maximum of three types of chemicals.

Mixtures boil over a range of temperatures.

Mixtures must contain at least two types of chemicals.

Which of these statements about mixtures are true?

What is the highest temperature at which all of these substances are in the solid state?

Starting from a substance in the gas state, can you put the states and the temperatures at which changes of state happen in order?

Content guidance

Risk assessment required - equipment

Adult supervision required

Introduction

What will you need for this lesson?

Are you ready to learn?

Equipment

Content guidance

Supervision

Worksheet