Mixtures, Elements and Compounds

Introduction

In the Junior School CBC (KICD), Integrated Science studies matter. Grade 8 learners classify substances and learn how to separate mixtures in daily life.

Elements

An element is a pure substance made of only one kind of atom. Examples: hydrogen (H), oxygen (O), carbon (C), iron (Fe), gold (Au). They cannot be broken down into simpler substances by chemical means.

Compounds

A compound is formed when two or more elements chemically combine in a fixed ratio.

• Water (H2O): hydrogen and oxygen.
• Carbon dioxide (CO2): carbon and oxygen.
• Common salt (NaCl): sodium and chlorine.

A compound has properties different from its elements.

Mixtures

A mixture contains two or more substances physically combined, not chemically joined, in any proportion. Examples: air, sea water, soil, sand and sugar.

• The parts keep their own properties.
• They can be separated by physical methods.

Separating mixtures

Worked example

A mixture contains sand, salt and iron filings. Step 1: use a magnet to remove iron. Step 2: add water to dissolve the salt. Step 3: filter to remove sand. Step 4: evaporate the filtrate to obtain salt.

Try it

• Classify as element, compound or mixture: oxygen, sugar, milk, copper.
• Describe how to get salt from sea water.
• Name two ways farmers use separation, for example winnowing and sieving.