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Chemists use symbols and formulae to represent elements, ions and compounds. Word equations and balanced chemical equations model the changes that happen in chemical reactions.

The Earth, its oceans and atmosphere are made of elements and compounds in different states. The particle model may be used to explain the different properties of the solid, liquid and gas states.

Exothermic reactions in solution give out energy and the temperature increases. Endothermic reactions take in energy and the temperature decreases.

Human activity is causing an increase in greenhouse gases. The Earth’s climate is changing and most scientists agree that the two are connected.

As the Earth’s population increases the demand for potable water increases. Climate change could make finding water sources harder. The treatment needed to make the water safe depends upon the source.