Colour Change as Evidence

Introduction

A cut apple turns brown, a copper roof slowly turns green, and red cabbage juice turns pink in vinegar. Colour changes are among the easiest signs of change to spot, and they often mean a chemical reaction has taken place. But not always: mixing paints, heating a metal until it glows, or watering down squash also change colour. This page shows how to judge when a new colour really means a new substance.

Core explanation

Why colour changes in reactions. Colour is a property of a substance. It depends on which wavelengths of light the substance absorbs and reflects, and that depends on its particles. When a reaction makes a new substance, the new particles often absorb light differently, so the colour changes.

Chemical colour changes.

- Heating copper in air. Pinkish-brown copper becomes coated with black copper(II) oxide. - Rusting. Grey iron becomes orange-brown hydrated iron(III) oxide. - Copper roofs weathering. Over years, copper forms a green layer called patina, made mainly of copper carbonate compounds. - Browning of food. Cut apples brown as substances in the fruit react with oxygen; toast browns as sugars and proteins react on heating. - Indicators. Litmus turns red in acid and blue in alkali; universal indicator shows a range of colours. The indicator molecules react with the acid or alkali and change structure. - Displacement. An iron nail placed in blue copper sulfate solution becomes coated with pink-brown copper, and the solution gradually turns pale green as iron sulfate forms.

Physical colour changes.

- Dissolving and diluting. Blue copper sulfate crystals give a blue solution; adding more water makes it paler. The same substance is present, just spread out more. - Mixing colours. Mixing yellow and blue paint gives green, but both pigments are still there and unchanged. - Glowing when hot. An iron poker glows red at high temperature and turns grey again on cooling. The heat makes the metal give out light; its identity is unchanged. - Crushing crystals. Large blue crystals of copper sulfate look paler when ground into powder, because light is scattered differently.

Judging the evidence. Ask:

1. Is the new colour just a stronger or weaker version of the old one (dilution) or a blend of colours (mixing)? If so, probably physical. 2. Does the colour return when conditions are restored, such as cooling a hot metal? If so, probably physical. 3. Is the new colour completely different, and accompanied by other signs such as gas, heat or a change in texture? If so, probably chemical.

Reversible chemical colour changes. Some chemical colour changes can be reversed. White anhydrous copper(II) sulfate turns blue when water is added, and blue hydrated crystals turn white again on heating. This is still a chemical change, because the compound gains or loses water chemically. Reversibility alone does not make it physical.

Step-by-step reasoning

To judge a colour change:

1. Describe the colour before and after. 2. Check whether the change is only dilution, mixing or glowing. 3. Look for other signs of reaction. 4. Decide whether a new substance with its own colour has formed.

Visual explanation

Imagine two rows of test tubes. The top row shows a blue solution getting paler as water is added: the same blue in weaker shades. The bottom row shows an iron nail in blue solution, with the liquid turning green and a brown coating forming. The first row is dilution; the second is a new substance appearing.

Real-world analogy

A colour change is like someone's voice sounding different. If they are just speaking more quietly, it is the same voice (dilution). If an entirely new voice is heard, someone new has joined the conversation (a new substance).

Real-world example

The Statue of Liberty in New York was originally the reddish-brown colour of new copper. Over about 30 years it turned green as the copper reacted with oxygen, carbon dioxide, water and pollutants in the air. The green patina is a new substance that now protects the copper beneath from further corrosion.

Why?

Why does diluting a coloured solution make it paler but not change its colour? The same coloured particles are present, only spread through a larger volume, so fewer of them are in the path of the light. They absorb the same wavelengths, just less strongly.

Common misconception

"Any change of colour proves a chemical reaction." Diluting squash, mixing paints and heating a metal until it glows all change colour without forming a new substance. A colour change is strong evidence only when the new colour belongs to a new substance.

Worked example

Question: A shiny copper coin is heated in a flame and turns black. After cooling it stays black. Is this physical or chemical? Explain.

Reasoning: A glowing hot metal returns to its original colour on cooling, but this coin stays black. The black layer is a new substance with a different colour: copper oxide, formed by reaction with oxygen.

Answer: Chemical change; a new substance, copper oxide, has formed.

Quick check

1. Adding water to a dark blue solution makes it light blue. Is this chemical? Answer: No; it is dilution, a physical change, because the same substance is present in a larger volume.

Exam focus

Examiners often ask you to explain whether a colour change shows a reaction. Name the new substance when you can, such as copper oxide or rust, and mention other supporting evidence. Remember that indicator colour changes are chemical.

Advanced insight

Many coloured compounds contain transition metal ions such as copper, iron, cobalt and chromium. Their colours depend on the ion's charge and on the particles surrounding it. Iron(II) compounds are often pale green, while iron(III) compounds are often orange-brown, so a colour shift can reveal a change in the ion itself — the chemical reason rust looks so different from iron.

Summary

A colour change is often a sign of a chemical reaction, because a new substance absorbs light differently. Examples include copper oxide forming, rusting, food browning, indicators and displacement reactions. Dilution, mixing, glowing and crushing change colour physically. Judge a colour change by asking whether a new substance with its own colour has formed, and look for other signs too.

Practice questions

1. Give two examples of colour changes caused by chemical reactions. Answer: For example, iron rusting to orange-brown and copper turning black when heated in air. 2. Give two examples of colour changes that are physical. Answer: For example, diluting a coloured solution and a metal glowing red when hot. 3. An iron nail in blue copper sulfate solution becomes coated in brown metal while the solution turns green. What evidence is there of a chemical change? Answer: Two new colours appear, the brown copper coating and the green iron sulfate solution, showing that new substances have formed. 4. Why is an indicator changing colour a chemical change? Answer: The indicator molecules react with the acid or alkali and change their structure, forming a substance with a different colour.