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  • Liquid Litmus exhibit

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.


Where:   Fisher Bay Observatory Gallery 6, Mezzanine
Exhibit Developers:   Charles Carlson and Richard Gagnon, 1985

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.

 
Phenomena: Atoms and Particles, Electromagnetic Forces
Keywords: electrolysis, water

More to explore

Science activity demonstrating electrolysis of water
Indicating Electrolysis

Break water into hydrogen and oxygen with a simple device.

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Bacteriopolis
Bacteriopolis

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Native Oyster Colony
Native Oyster Colony

Get a closer look at live, Olympia oysters, native to San Francisco.

When the Water Turns Red it Means the Salt Is Forming
When the Water Turns Red it Means the Salt Is Forming

Some tiny aquatic organisms thrive in very salty water.


IMLS acknowledgment

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

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