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Liquid Litmus

Liquid Litmus
Ionized water swirls in pH-indicating dye.

Here you can see the chemical reactions taking place in an electrolytic cell, as electricity flowing through a tank of salt water and pH-indicating dye creates zones of acidic (yellow) and basic (blue) solutions.

What’s going on?

Electricity flows between the two submerged wires of this exhibit, causing chemical changes in the liquid that you see as a change in color.

The liquid in this tank is mostly water with a little salt and some special dyes. The dyes turn yellow where there are positive hydrogen ions and bluish-purple where there are negative hydroxide ions.

The dyes are pH indicators—they show whether the liquid is acidic or alkaline. By definition, acidic things like lemon juice are substances that contain an overabundance of positive hydrogen ions. Alkaline substances contain an overabundance of negative hydroxide ions. So the yellow in the tank is acidic, while the bluish-purple is alkaline.

Going further

At the positively charged wire, water breaks up into hydrogen ions (H+) and oxygen gas (O2), which you see as little bubbles rising from the wire.

2H2O --> O2 + 4H+ + 4e

The hydrogen ions (H+) that are left behind make the surrounding liquid acidic, so the dye turns yellow.

At the negatively charged wire, water breaks up into hydroxide ions (OH-) and hydrogen gas (H2), which also rise as little bubbles from the wire.

2H2O + 2e --> H2 + 2OH

The hydroxide ions (OH-) that are left behind make the surrounding liquid basic (or alkaline), so the dye turns bluish-purple.

litmus test color chart label

The dyes in this exhibit change color in response to changes in pH—also known as a litmus test.


IMLS acknowledgment

This web project was made possible in part by the Institute of Museum and Library Services [MA-30-16-0175-16].


IMLS acknowledgment

This web project was made possible in part by the Institute of Museum and Library Services [MA-30-16-0175-16].