Chemical Changes

Introduction

A shiny strip of magnesium ribbon, touched by a flame, burns with a dazzling white light and leaves behind a crumbly white powder. The powder is not magnesium: it does not shine, it does not bend and it does not conduct electricity. Something new has been made. This is a chemical change , and chemical changes are what chemistry is really about — making new substances from old ones.

Core explanation

Definition. A chemical change, or chemical reaction , is a change in which one or more new substances are formed. The new substances have different properties from the substances you started with.

Reactants and products. The starting substances are the reactants . The new substances made are the products . We show a reaction with an arrow, read as "reacts to form" or "gives":

magnesium + oxygen → magnesium oxide

Here magnesium and oxygen are reactants, and magnesium oxide is the product.

New properties. Compare the reactants and product in this example:

Substance Appearance Other properties --- --- --- Magnesium shiny grey metal conducts electricity, bends Oxygen colourless gas needed for burning Magnesium oxide white powder does not conduct as a solid, brittle

The product is not just a mixture of the reactants. Its atoms of magnesium and oxygen are joined together in a compound with its own properties.

What happens to the atoms? Atoms themselves are not destroyed or made in a chemical reaction. Instead, the bonds holding atoms together break, and new bonds form, so the atoms end up joined in different combinations. That is why the products are new substances but still contain the same elements as the reactants.

Everyday chemical changes. Cooking an egg, baking bread, burning gas in a cooker, milk going sour, fruit rotting, a firework exploding, iron rusting and plants making sugar in photosynthesis are all chemical changes. So are digestion and respiration in your own body.

Often hard to reverse. Most chemical changes are not easily reversed: you cannot unburn toast or uncook an egg. Some reactions can be reversed, and later pages look at these, but as a rule chemical changes are more difficult to undo than physical ones.

Energy changes. Chemical reactions usually give out or take in energy, often as heat or light. Burning releases a great deal of heat; cooking takes heat in. This energy change is linked to bonds breaking and forming.

Step-by-step reasoning

To show that a change is chemical:

1. List the substances present at the start (reactants). 2. Describe what is present at the end. 3. Compare their properties: colour, state, smell, hardness, how they behave. 4. If a substance with new properties has appeared, a chemical change has occurred.

Visual explanation

Imagine atoms as coloured beads joined by short sticks. Before a reaction, grey beads are packed together (magnesium) and pairs of red beads float nearby (oxygen molecules). After the reaction, each grey bead is joined to a red bead in a new pattern. The same beads are there, but their partners have changed.

Real-world analogy

A chemical change is like the dancers at a barn dance changing partners. The same people are on the floor at the end of the evening, but they are now paired up differently, and each new pair moves in its own way.

Real-world example

Baking powder contains sodium hydrogencarbonate and an acid. When wet batter is heated, they react to produce carbon dioxide gas, which makes bubbles in a cake. The gas is a new substance, which is why the cake rises and why the change cannot be undone.

Why?

Why do products have different properties from reactants? Properties depend on which atoms are joined to which, and how strongly. A chemical reaction changes these connections, so it makes a substance with a different set of properties.

Common misconception

"In a reaction, the reactants simply disappear." They do not vanish; their atoms are rearranged into products. Sometimes the product is a gas that escapes into the air, which can make it look as though matter has been lost.

Worked example

Question: When hydrogen burns in oxygen, water is formed. Name the reactants and product, and explain why this is a chemical change.

Reasoning: The starting substances are hydrogen and oxygen, both gases. The substance made is water, a liquid at room temperature with completely different properties.

Answer: Reactants: hydrogen and oxygen; product: water. It is chemical because a new substance is formed.

Quick check

1. In the reaction carbon + oxygen → carbon dioxide, which substance is the product? Answer: Carbon dioxide.

Exam focus

Learn the terms reactant, product and chemical reaction. Examiners often give a word equation and ask you to name the reactants and products, or ask you to explain a chemical change using the phrase "a new substance is formed".

Advanced insight

The number of possible chemical changes is enormous. Around 118 elements combine to make many millions of known compounds, and chemists create new ones every day. All of them arise from the same basic process: breaking some chemical bonds and forming others, while the total number of each type of atom stays constant.

Summary

A chemical change, or reaction, forms one or more new substances. Reactants are the starting substances and products are the new substances. Atoms are rearranged, not created or destroyed, so the products contain the same elements but have different properties. Chemical changes usually involve an energy change and are often difficult to reverse.

Practice questions

1. Define a chemical reaction. Answer: A change in which one or more new substances are formed. 2. In the reaction iron + sulfur → iron sulfide, name the reactants and the product. Answer: Reactants: iron and sulfur; product: iron sulfide. 3. Iron is magnetic but iron sulfide is not. What does this tell you about the change? Answer: The product has different properties from the reactant, so a new substance has formed and the change is chemical. 4. Give two everyday examples of chemical changes. Answer: For example, cooking an egg and wood burning. 5. Explain why atoms are not lost in a chemical reaction. Answer: Atoms are only rearranged into new combinations; bonds break and form, but the atoms themselves are neither created nor destroyed.