A chemical reaction forms new substances with new properties. Atoms cannot be seen rearranging, but four observations point to a reaction: a change of state, a change of colour, a gas given off, and a change of temperature.
Milk left out in summer turns sour. An iron nail left in damp air gets a reddish-brown coating. Food changes when it is cooked. In each case, the substance at the end is not the substance at the start.
A chemical reaction is a change in which one or more substances, the reactants, turn into one or more different substances, the products. The atoms are not created or destroyed. They are rearranged into new combinations, and the new combinations have different properties.
Rust is a compound of iron, oxygen and water. It is brown, brittle and not attracted by a magnet the way iron is. Those differences in properties are how we know it is a new substance.
A physical change is different. Melting ice gives liquid water, which is still water. Tearing paper gives smaller pieces of paper. The substance keeps its identity.
The rearrangement happens between atoms, which are about 10−10 m across, far too small to see. So a chemist at a bench cannot watch it directly.
What can be seen are the effects. A new substance has new properties, so something visible usually changes. Four kinds of change appear again and again.
Magnesium ribbon is held in a flame. It burns with a bright white light and leaves a white powder. What shows that this was a chemical reaction, not just the metal getting hot?
The white powder. Magnesium is a shiny grey metal. The powder is magnesium oxide, formed when magnesium combines with oxygen from the air. It has different properties: it is white, it does not conduct electricity, and it does not shine.
Heating a metal on its own makes it glow, but after cooling it is the same metal. Here, a different substance is left after cooling. The change is to the substance, not just to its temperature.
Each observation is a consequence of new substances forming:
Four observations. None of them shows the atoms. Each is a visible result of the atoms having formed new substances.
Add a few granules of zinc to dilute hydrochloric acid. Bubbles form on the zinc: a gas is given off. The gas is hydrogen. The test tube becomes warm: a change of temperature.
Add potassium iodide solution to lead nitrate solution. Both solutions are colourless. When they mix, a bright yellow solid forms. This is a change of colour and also a change of state, because a solid has formed from two solutions. The yellow solid is lead iodide, which does not dissolve in water.
An ice cube melts into water, which freezes again overnight. Its state and temperature both changed. Has a chemical reaction taken place?
No. At every stage, the substance is water. Ice and liquid water are the same substance, H₂O, in different states.
The four observations suggest that a reaction may have happened. They do not prove it, because some physical changes also show them. The deciding question is: has a new substance, with different properties, been formed? For melting and freezing, the answer is no, so they are physical changes.
The rest of this unit covers how reactions are written, how equations are balanced, and the main types of reaction. Each of these starts from recognising that a reaction has happened.
When you see one of the four observations, check for a new substance. If there is one, a reaction has occurred.
1What defines a chemical reaction?
2Is each change chemical or physical?
3Magnesium ribbon burns and leaves a white powder. Which observation shows most clearly that a reaction occurred?
4Zinc granules are added to dilute hydrochloric acid. Which observations show a reaction?
More than one answer is correct.
5Potassium iodide solution is added to lead nitrate solution, and a yellow solid forms. Besides a change of colour, this shows a change of ____.
6Why are the four observations evidence for a reaction rather than proof of one?
7Which is an example of a temperature change caused by a reaction?
Next: how a reaction is written, first in words and then in formulae.
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