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