How does the SAME purple juice turn pink in lemon and green in soap?

Squeeze some red cabbage and you get purple juice. Drip it in lemon — it flashes pink. Drip it in soapy water — it flashes green. Same juice, two colors. Did the juice get dyed… or is it telling us something? Let's find out.

1The two things you need to know

Sour vs soapy — and a juice that can tell them apart

You only need two ideas. Watch each one:

Liquids come in two opposite kinds

Acids are the sour ones — lemon, vinegar — full of loose grabby hydrogen bits. Bases are the soapy, slippery ones — soap, baking-soda water — they soak those bits up. Plain water sits right in between.

Cabbage juice is a tiny detector

Its color molecule bends into a different shape depending on how many grabby bits are around — and each shape reflects a different color. So the juice doesn't have one fixed color. It reads the liquid it lands in.

2Two ways the detector can bend

The sour side vs the soapy side

The detector molecule is built from tiny grabby-hydrogen-bit holders. Meet it with a sour or a soapy liquid and it rearranges into a different shape — and a different shape reflects a different color:

In an acid

The sour side bends it one way

Lots of loose grabby hydrogen bits latch on. The molecule folds one way — so it reflects a new color.

In a base

The soapy side bends it the other way

The grabby bits get pulled away. The molecule folds the opposite way — so it reflects a different color.

3Your turn — change how many grabby bits are around

Drag the dial and watch one cup change color

Here is a single cup of cabbage juice. Slide to add more loose grabby hydrogen bits on one side or pull them away on the other, and watch the detector molecule fold into shape after shape — the color keeps shifting as you go.

Grabby bits around the molecule: about middle
MORE grabby bitsmiddleFEWER grabby bits

4Now the real test

Two cups of the SAME juice. Drip something in each 🧪

Both cups below hold the exact same purple cabbage juice. You'll tip lemon into the left one and soapy water into the right one. Guess first — then drip and watch.

Guess before you drip

Same purple juice in both cups. You drip lemon (sour) in the left and soapy water (soapy) in the right. Will the two cups end the SAME color or DIFFERENT colors?