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Substances: The Identification of Pure Substances

In this worksheet, students will learn about what constitutes a pure substance and how to distinguish between pure substances and mixtures. This worksheet should be followed by Separation Techniques to find out how mixtures can be separated into the pure substances they are made of.

'Substances: The Identification of Pure Substances' worksheet

Key stage:  KS 3

Curriculum topic:  Chemistry: Pure and Impure Substances

Curriculum subtopic:  Identifying Pure Substances

Difficulty level:  

Worksheet Overview

QUESTION 1 of 10

The diagram below show the molecular models of various elements and compounds.

 

Look for the oxygen model at the bottom; it contains only oxygen atoms: two of them make an oxygen molecule. A substance that is made of only one type of molecule is a pure substance. In the case of oxygen there is only one type of atom, so this is actually a pure element

 

 

Molecular models

 

Look for the molecule of water at the top; it is made of oxygen and hydrogen atoms that have bonded together, so it is a compound. A substance made only of water molecules is pure water.

 

Seawater, on the other hand, is not a pure substance; it contains salt molecules dissolved in it, sand and seaweed. Seawater is a mixture. This means the water and salt molecules have mixed with each other, but they have not reacted (like in compounds), so they can easily be separated, if we let the water evaporate. Furthermore, we are not usually able to see the sand in seawater, but if it settles a bit, we can see the sand at the bottom of the container. This is because sand does not dissolve (it's insoluble) in water.

 

A pure substance in chemistry is different to a pure substance in everyday life. For example, orange juice may be advertised as pure, but for a chemist, orange juice is a mixture of water, sugar, citric acid, vitamin C and other nutrients.

 

In order to identify a pure substance you need to look at how many different elements and/or compounds it is made of. If it only contains one type of element or one type of compound it is a pure substance.

Is the substance modelled in the image an element, a compound or a mixture?

 

Write one word.

 

Molecules

Is the substance modelled in the image a pure substance?

 

Molecules

yes

no

Is the substance modelled in the image an element, a compound or a mixture?

 

 

Molecules

Is the substance modelled in the image a pure substance?

 

 

Molecules

yes

no

Is the substance modelled in the image an element, a compound or a mixture?

Beaker of different substances

In a container you can find the two types of molecules below; is this an element, a compound or a mixture?

 

Molecules 

Molecules

Air is made of oxygen, nitrogen, hydrogen, argon, carbon dioxide and other substances. Is air a pure substance?

yes

no

The image shows a mixture of silver chloride (AgCl) and water. Is silver chloride dissolved in water?

 

Solution of cilver chloride

yes

no

Try the following experiment at home with the help of an adult:

 

Dissolve one teaspoon of sugar in one cup of tap water.

Then boil some water in the kettle and pour the same amount of water in the same cup. Be careful not to burn your skin.

Try to dissolve the same amount of sugar again.

 

With which cup of water were you able to dissolve the sugar faster?

with tap water

with boiled water

How can you separate the salt from seawater?

 

Choose two answers.

Let the water and air react. Salt will be produced.

Let the water evaporate and collect the salt that will remain after all the water evaporates.

Pour the seawater through filter paper. Salt will remain in the paper, whereas water will go through.

Boil the seawater to make it evaporate faster and collect the salt that will remain after all the water evaporates.

  • Question 1

Is the substance modelled in the image an element, a compound or a mixture?

 

Write one word.

 

Molecules

CORRECT ANSWER
compound
EDDIE SAYS
This substance is made only of carbon dioxide molecules; it is a compound. CO2 is a compound of carbon and oxygen atoms, but the image shows only molecules of that compound.
  • Question 2

Is the substance modelled in the image a pure substance?

 

Molecules

CORRECT ANSWER
yes
EDDIE SAYS
This substance is made only of carbon dioxide molecules; it is a compound and a pure substance.
  • Question 3

Is the substance modelled in the image an element, a compound or a mixture?

 

 

Molecules

CORRECT ANSWER
element
EDDIE SAYS
This substance is made only of oxygen atoms bonded together in oxygen molecules; it is an element and a pure substance.
  • Question 4

Is the substance modelled in the image a pure substance?

 

 

Molecules

CORRECT ANSWER
yes
EDDIE SAYS
This substance is made only of oxygen atoms bonded together in oxygen molecules; it is an element and a pure substance.
  • Question 5

Is the substance modelled in the image an element, a compound or a mixture?

Beaker of different substances

CORRECT ANSWER
mixture
EDDIE SAYS
This substance is made of water, oil and sand; it is a mixture.
  • Question 6

In a container you can find the two types of molecules below; is this an element, a compound or a mixture?

 

Molecules 

Molecules

CORRECT ANSWER
mixture
EDDIE SAYS
This substance is a mixture. It is made of two different types of molecules mixed together but not chemically bonded.
  • Question 7

Air is made of oxygen, nitrogen, hydrogen, argon, carbon dioxide and other substances. Is air a pure substance?

CORRECT ANSWER
no
EDDIE SAYS
Air is not a pure substance; it is a mixture of all those elements and compounds.
  • Question 8

The image shows a mixture of silver chloride (AgCl) and water. Is silver chloride dissolved in water?

 

Solution of cilver chloride

CORRECT ANSWER
no
EDDIE SAYS
No, clearly silver chloride is deposited at the bottom of the container; it has a very low solubility in water.
  • Question 9

Try the following experiment at home with the help of an adult:

 

Dissolve one teaspoon of sugar in one cup of tap water.

Then boil some water in the kettle and pour the same amount of water in the same cup. Be careful not to burn your skin.

Try to dissolve the same amount of sugar again.

 

With which cup of water were you able to dissolve the sugar faster?

CORRECT ANSWER
with boiled water
EDDIE SAYS
Provided that the same amount of water and sugar were used, it is faster to dissolve the sugar in hot water. There's more energy in hot water, so it encourages the sugar and water molecules to mix more quickly.
  • Question 10

How can you separate the salt from seawater?

 

Choose two answers.

CORRECT ANSWER
Let the water evaporate and collect the salt that will remain after all the water evaporates.
Boil the seawater to make it evaporate faster and collect the salt that will remain after all the water evaporates.
EDDIE SAYS
You can separate salt from seawater by letting the water evaporate or boiling it to make it evaporate faster. The salt will remain after all the water evaporates. Salt particles are very small, so they would pass through the filter paper.
---- OR ----

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