Technology Class

Shoestring Science

STEM education on a shoestring budget

  • Hannah

60 Second STEM: pH Indicator


  • Red/purple grape juice (100% works best)

  • Dish

  • Baking soda

  • Lemon juice


  1. If using grape juice from concentrate, prepare grape juice according to the directions on the container.

  2. Pour grape juice into the dish, coming about halfway up the container. If desired, pour a second dish to use as comparison.

  3. Add baking soda into the grape juice until you notice a color change. The juice should change from a deep red/purple to a dark blue.

  4. Add lemon juice to the mixture. The mixture should bubble up and change color to a red/pink where the vinegar or lemon juice touches.

Make it an experiment:

There are two ways that you might be able to take this investigation and transform it into an experiment:

  1. Find the range of colors that the grape juice can turn. pH indicators often turn different colors with different pH's. Test different household items to see if they cause a different color change and either look up their pH or determine it using a commercially available pH indicator. Can you create a pH scale using grape juice?

  2. Find other household items that can be used as a pH indicator. Are there other juices or foods that turn color? Can you make predictions about foods that might work and then test them?

What’s happening:

With this investigation, we are exploring pH. pH, the way scientists describe it, refers to the negative log of the hydrogen ion concentration which is a very fancy way of saying how acidic or basic something is. The pH scale runs from 0 to 14 with 7 being neutral like plain tap water in most places. If something has a pH lower than 7, we say it is acidic. Acids, when we can eat them, taste sour like lemon juice. If something has a pH higher than 7, we say it is a base. We don't eat very many bases but the ones we do taste bitter. Baking soda is a good example of a base.

In this investigation we are using grape juice as a pH indicator. A pH indicator is something that changes color in reaction to different pH's. Grape juice does this because of a specific chemical in red grapes: anthocyanins. Anthocyanins are a group of pigments that change color in different pH's. In acids they are purple to red (very hard to se when comparing to grape juice) and in bases they are blue to green. Different foods have different amounts or types of these pigments and so some are better indicators then others.

The Video:

Click here to watch Hannah from Shoestring Science explain and complete this investigation


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