Secondary 1 Topic

Elements, Compounds and Mixtures – Distinguish the differences

This science revision note aims to help your child distinguish among elements, mixtures and compounds

 

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Learning outcomes:

Do distinguish among elements, mixtures and compounds by memorising these definitions.

Next, apply the definitions to the exam-based questions.

 

1) Examples of elements, compounds and mixtures.

 

Example of mixtures:

  • E.g. alloys which are mixtures of a metal and another element.
    • Steel is a mixture of iron and carbon.
  • a mixture of two elements, oxygen (O2) and helium (He).

 

Example of compounds:

  • carbon dioxide (CO2) consists of one carbon atom joined to two oxygen atoms.
  • water (H2O) consists of one oxygen atom joined to two hydrogen atoms.

 

Example of elements:

  • Sodium (Na) is an element found in the periodic table.

 

Fun fact!

Sodium is Highly Reactive with Water: Sodium is so reactive that it must be stored in oil to prevent it from reacting with the water vapour in the air. 😲

When sodium metal comes into contact with water, it reacts vigorously, producing sodium hydroxide and hydrogen gas.

This reaction is so intense that it can cause the hydrogen gas to ignite, sometimes resulting in an explosion. This characteristic makes sodium both a fascinating and hazardous element to work with in chemistry.

 

Watch this video on elements, compounds and mixtures :

Question

The table below shows the properties of 3 unknown substances, X, Y and Z.

State if X, Y and Z are an element, a compound or a mixture. [3]

X  ………………………………..

Y  ………………………………..

Z  ………………………………..

 

 STEP 1: Recall

What are the properties of elements, compounds and mixtures?

Use Your Knowledge:

  • Elements don’t break down when electricity is passed through them.
  • Compounds may break down under certain conditions.
  • Mixtures can often be separated by physical methods.

 

STEP 2: Analyse each substance

– X: Breaks down with electricity (indicating a compound) and cannot be separated by physical methods (indicating it’s not a mixture).

– Y: Does not break down with electricity (indicating an element) and cannot be separated by physical methods (indicating it’s not a mixture).

– Z: Breaks down with electricity (indicating a compound) and can be separated by physical methods (indicating it’s a mixture).

 

STEP 3: Piecing your answer together.

Answers

Answers

  • X: A compound (It breaks down when electricity passes through it, indicating a chemical change)
  • Y: An element (It does not break down when electricity passes through it, indicating it’s a pure substance)
  • Z: A mixture (It breaks down when electricity passes through it, indicating a chemical change in one of the substances that made up the mixture Z, and it can be separated by physical methods, suggesting it’s a mixture)

 

Common mistakes

 

Wrong answer: X is a mixture.

 Teachers’ comments: Students might confuse compounds with mixtures if they focus only on the electrical behaviour. It’s essential to consider both the electrical behaviour and the separability of the substances (can the substances be separated by physical methods?). That way, you can determine the nature of the substances accurately.

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