Why Plant Pigments Change Colour

Introduction

Red cabbage juice can be red, purple, blue, green or yellow, yet it contains the same pigment throughout. How can one substance show so many colours? The answer lies in the pigment's molecules. Acids and bases do not add paint to the solution; they change the shape and arrangement of electrons in the pigment molecules, and that changes which light is absorbed. This page explains, in simple terms, why plant pigments change colour.

Core explanation

Why things have colour. White light, such as sunlight, is a mixture of all the colours of the rainbow. When light falls on a coloured substance, some colours are absorbed and the rest are reflected or pass through. We see only the colours that reach our eyes. A substance that absorbs green light, for example, looks red-purple, because red and blue light are left. The colour we see is the complementary colour of the light that is absorbed.

Light mainly absorbed Colour we see --- --- blue-green red green purple or magenta yellow-orange blue red blue-green or green violet-blue yellow

What decides which light is absorbed. Pigment molecules contain long chains and rings of atoms joined by alternating single and double bonds. Electrons can spread out along this system. The size and shape of this spread-out electron system decides which colour of light the molecule absorbs. A small change to the molecule shifts the absorption to a different colour.

How acids and bases change the molecule. Acids provide hydrogen ions (H⁺) in water; bases remove them. Anthocyanin molecules have several places where a hydrogen ion can be attached or removed:

- In strongly acidic solutions, the molecule gains a hydrogen ion and carries a positive charge. This form absorbs green-blue light and looks red . - Near neutral, the molecule loses that hydrogen ion and becomes uncharged. It absorbs yellow-green light and looks purple . - In basic solutions, it loses further hydrogen ions and becomes negatively charged. It absorbs orange-red light and looks blue ; mixtures of blue and yellow forms look green . - In very strongly basic solutions, one ring of the molecule opens up, giving a yellow form.

Each form is a slightly different substance. Gaining or losing a hydrogen ion changes the charge and the electron arrangement, so each form absorbs a different colour. The indicator is really a family of closely related forms, and the acidity decides which form is most common.

Reversibility. Because hydrogen ions can be added back as easily as they are removed, most of these changes are reversible . Add acid to green cabbage juice and it turns purple, then red again. The exception is prolonged strong base, which can break the molecule permanently.

Step-by-step reasoning

To explain a colour change from purple to red when acid is added:

1. The acid provides hydrogen ions. 2. The pigment molecules gain hydrogen ions. 3. Their charge and electron arrangement change. 4. They now absorb a different colour of light. 5. We see the complementary colour, which is red.

Visual explanation

Imagine a light-switch diagram. On the left, a pigment molecule holds an extra hydrogen ion and absorbs green light, so the flask appears red. An arrow labelled "base removes H⁺" points right to the same molecule without that hydrogen ion, now absorbing orange light, so the flask appears blue. A reverse arrow labelled "acid adds H⁺" completes the loop.

Real-world analogy

Think of a coat with a reversible lining. Turn it one way and it is red; turn it inside out and it is blue. It is the same coat, only rearranged. An indicator molecule is similarly the same molecule, rearranged by hydrogen ions so that it shows a different colour.

Real-world example

Blue cornflowers and red poppies both owe some of their colour to anthocyanin-type pigments, but the conditions inside their petal cells differ. In cornflowers, the pigment is bound together with metal ions and other molecules into a stable blue complex, while in red petals the pigment exists mainly in its red form. Plants tune flower colour to attract particular pollinators.

Why?

Why do different indicators change colour at different levels of acidity? Each pigment molecule holds its hydrogen ions with a different strength. A molecule that holds them loosely loses them in fairly acidic solutions; one that holds them tightly changes only in basic solutions.

Common misconception

"The acid reacts with the pigment and destroys it, making a new colour." Usually nothing is destroyed. The pigment simply gains or loses hydrogen ions, and the change reverses when the acidity is changed back.

Worked example

Question: A pigment absorbs mainly yellow-orange light in basic solution. What colour does the solution appear, and what happens to the colour if acid is added so that the pigment gains hydrogen ions and absorbs blue-green light instead?

Reasoning: Absorbing yellow-orange leaves blue as the colour seen. Absorbing blue-green leaves red as the colour seen.

Answer: Blue in the basic solution; it changes to red after adding acid.

Quick check

1. What do bases do to the hydrogen ions of an indicator molecule? Answer: Bases remove hydrogen ions from the indicator molecule.

Exam focus

At this level, explain indicator colour changes in terms of the molecule gaining or losing hydrogen ions, which changes the colour of light it absorbs. Avoid saying that the acid or base "has" the colour; the colour belongs to the indicator.

Advanced insight

Chemists label the fully acid form of an anthocyanin the flavylium cation and the blue forms quinoidal bases. The yellow form in strong base is called a chalcone, formed when the central ring opens. Measuring how strongly each form absorbs light at each wavelength is done with an instrument called a spectrophotometer.

Summary

We see the complementary colour of the light a pigment absorbs. Pigment molecules have spread-out electron systems whose shape sets that absorption. Acids add hydrogen ions and bases remove them, changing the molecule's charge and electron arrangement, and therefore its colour. These changes are usually reversible, which is why plant pigments can act as indicators.

Practice questions

1. Why does a substance that absorbs green light look purple or magenta? Answer: Green light is removed, so the remaining red and blue light reaches the eye, which we see as purple or magenta. 2. What happens to an anthocyanin molecule when acid is added? Answer: It gains hydrogen ions, changing its charge and electron arrangement so it absorbs a different colour and looks red. 3. Explain why the colour of red cabbage juice can be changed back after adding a base. Answer: The change is reversible; adding acid returns hydrogen ions to the pigment molecules, restoring the earlier form and colour. 4. Why do two different indicators not change colour at the same acidity? Answer: Their molecules hold hydrogen ions with different strengths, so they gain or lose them at different levels of acidity.