# pH & Indicators

Part of the Chemistry Merit Badge Digital Resource Guide: [Introduction & Overview](https://merit-badge.university/merit-badges/chemistry/guide/index.md)

*3. Chemistry & Cooking*

> **3b.** Prepare and use red cabbage indicator to measure the pH of five common household liquids, including water. Discuss your observations with your counselor.

Red cabbage might not look like a chemistry tool, but it contains a natural pigment that changes color depending on whether a solution is acidic, neutral, or basic. In this experiment, you will make your own **pH indicator** and use it to test common household liquids.

### What Is pH?

pH is a scale that measures how acidic or basic (alkaline) a solution is. The scale runs from 0 to 14:

- **0–6:** Acidic (the lower the number, the stronger the acid)
- **7:** Neutral (pure water)
- **8–14:** Basic/alkaline (the higher the number, the stronger the base)

The "p" in pH stands for the German word *Potenz* (power), and "H" stands for hydrogen. pH actually measures the concentration of hydrogen ions (H⁺) in a solution. More hydrogen ions = more acidic.

> **Did You Know?**
>
> The pH scale is logarithmic, meaning each number represents a tenfold change. A solution with a pH of 3 is ten times more acidic than one with a pH of 4, and a hundred times more acidic than one with a pH of 5. Lemon juice (pH 2) is 100,000 times more acidic than pure water (pH 7).

### Why Red Cabbage Works

Red cabbage gets its purple color from a class of pigments called **anthocyanins**. These molecules change their chemical structure — and therefore their color — depending on the pH of their environment:

| pH Range | Color | Type |
|---|---|---|
| 1–3 | Red/Pink | Strong acid |
| 4–6 | Purple/Violet | Weak acid |
| 7 | Blue/Violet | Neutral |
| 8–10 | Blue/Green | Weak base |
| 11–14 | Green/Yellow | Strong base |

This range of color changes makes red cabbage one of the best natural pH indicators available.

### Making Your Indicator

**Indicator Materials** — What you will need

- [ ] Red cabbage: About one quarter of a head, chopped into small pieces.
- [ ] Boiling water: About 2 cups.
- [ ] Pot and stove: To boil the cabbage.
- [ ] Strainer or cheesecloth: To separate the liquid from the cabbage.
- [ ] Clear cups or glasses: At least six (one for each liquid you test, plus extras).
- [ ] Household liquids to test: At least five, including water.
- [ ] White paper: Place behind the cups to see colors more clearly.
- [ ] Labels or tape: To mark each cup.

**Step 1:** Chop the red cabbage into small pieces and place them in a pot.

**Step 2:** Pour boiling water over the cabbage and let it steep for 15–20 minutes. The water should turn a deep purple-blue color.

**Step 3:** Strain out the cabbage pieces. The purple liquid is your pH indicator. Let it cool to room temperature.

**Step 4:** Pour equal amounts of indicator into each clear cup (about 2–3 tablespoons).

**Step 5:** Add a small amount of each test liquid to a separate cup and observe the color change.

![Six clear cups showing red cabbage indicator at different pH levels, ranging from red (acidic) through purple (neutral) to green (basic)](https://ik.imagekit.io/doulacoop/tr:w-800/mbu-assets/merit-badges/chemistry/guide/cabbage-indicator-colors?v=05b19409)

### Suggested Liquids to Test

Here are some common household liquids that give a good range of results:

- **Water** (pH ~7) — Should stay purple/blue (neutral)
- **Lemon juice or white vinegar** (pH ~2–3) — Should turn pink/red (acidic)
- **Baking soda dissolved in water** (pH ~8–9) — Should turn blue/green (basic)
- **Milk** (pH ~6.5) — Should stay purple with a slight pink tint (slightly acidic)
- **Laundry detergent dissolved in water** (pH ~10–11) — Should turn green (basic)

Other options: orange juice, soda, soapy water, antacid tablets dissolved in water, or apple juice.

> **Tip**
>
> Arrange your cups from most acidic to most basic after testing. This creates a beautiful rainbow that clearly shows the pH spectrum. Take a photo for your counselor meeting — it is a great visual aid.

### What to Discuss with Your Counselor

When you meet with your counselor, be ready to explain:

- **What pH is** and how the scale works
- **Why red cabbage changes color** (anthocyanins change structure with pH)
- **Your results** — Which liquids were acidic? Basic? Neutral?
- **Any surprises** — Did any liquid test differently than you expected?
- **Real-world applications** — Where is pH important? (Swimming pools, aquariums, soil for gardening, human blood, food preservation)

### pH in Everyday Life

pH is not just a lab concept — it matters in the real world:

- **Swimming pools** must maintain a pH of 7.2–7.8 for safety and comfort
- **Your blood** is kept at a very precise pH of 7.35–7.45; even small changes can be life-threatening
- **Soil pH** determines which plants can grow; blueberries need acidic soil, while most vegetables prefer slightly acidic to neutral
- **Acid rain** (pH below 5.6) damages buildings, kills fish, and harms forests
- **Your stomach** has a pH of 1.5–3.5 — acidic enough to dissolve metal

> **Safety First!**
>
> Some household chemicals are corrosive — especially drain cleaners, oven cleaners, and concentrated bleach. Do not test these without adult supervision. Wear gloves when handling strong acids or bases, and never mix unknown chemicals together.

Video: [Why Is Red Cabbage a pH Indicator?](https://youtu.be/4AElaDIhMyA)

[Compound Chemistry — Red Cabbage Indicator Infographic](https://www.compoundchem.com/2017/05/18/red-cabbage/): A beautiful infographic showing the color spectrum of red cabbage indicator across the full pH range.

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