Middle school chemistry asks students to understand acids, bases and pH. In this experiment you brew a natural indicator from ordinary red cabbage, then use it to test everyday substances — white vinegar, baking soda solution, soapy water — so students see the difference between acidic and basic with their own eyes.

It needs no hazardous reagents, every material can be found at home, and it makes an ideal first experiment for the acids-and-bases unit.

Safety first (read this before you start)

  • Making the indicator means steeping red cabbage in hot water or heating it briefly. Work with an adult and watch out for scalds; if you simmer it in a pot, don't let it boil dry.
  • Only test food-grade or everyday household liquids — white vinegar, lemon juice, baking soda, soapy water. Never use drain cleaner, toilet-bowl cleaner, concentrated hydrochloric acid or any other strongly corrosive chemical.
  • Every sample is for observing color only — do not taste any mixed liquid. Wash the containers and your hands when you're done.

Materials and equipment

  • A few leaves of red cabbage (purple cabbage);
  • Hot water, a heat-resistant bowl or small pot, a strainer or cheesecloth;
  • Several small clear cups (disposable clear cups work best — easy to compare colors);
  • Test samples: white vinegar, lemon juice, baking soda solution, soapy water (or a washing-soda solution), and plain water as a control.

Step-by-step procedure

Part 1: Brew the indicator

  1. Tear the cabbage leaves into small pieces and put them in the heat-resistant bowl.
  2. Pour in freshly boiled water (let it cool slightly first) until the leaves are covered, and steep for 10–15 minutes, until the water turns a clear purple-blue. (With an adult's help you can also simmer the leaves over low heat for a few minutes for a stronger brew.)
  3. Strain out the leaves with the strainer or cheesecloth. The purple liquid that remains is your red cabbage indicator.

Part 2: Test each sample

  1. Pour an equal amount of each test liquid into its own clear cup and label it.
  2. Add an equal amount of cabbage indicator (a few milliliters) to every cup and swirl gently.
  3. Put the plain-water cup in the middle as the control, line the cups up left to right in the students' predicted order from acid to neutral to base, then observe and compare the colors.
Controlled-variables tip: keep the amount of indicator in every cup as equal as possible, or the color comparison won't be fair. Keep the order of addition and the amount of swirling consistent too — this is the basic discipline of a control experiment in chemistry.

What you should observe

Red cabbage indicator shows different colors at different acidities. The general pattern looks like this (exact shades depend on concentration):

  • White vinegar, lemon juice (acidic) → turns red or pink;
  • Plain water (close to neutral) → stays purple, almost unchanged;
  • Baking soda solution (weakly basic) → turns blue or blue-green;
  • Soapy water (more strongly basic) → turns green, even yellow-green.

Record each color and its liquid in a table, and you get a full "acid-base color strip" running from red to green.

Interactive simulation: this experiment has a drag-and-play simulator in our Interactive Lab. The lab interface is currently Chinese-only — the controls are simple sliders and switches, so it is still easy to explore.

The science: why the colors change

Red cabbage contains a family of pigments called anthocyanins. Their molecular structure changes with the acidity (pH) of the solution around them, and each structure reflects a different color. That is the whole idea of an acid-base indicator: use a color change to "indicate" whether a solution is acidic or basic.

White vinegar contains acetic acid and lemon juice contains citric acid — both acidic, so they turn the anthocyanins red. Baking soda (sodium bicarbonate) solution and soapy water are basic, turning the pigment blue or green. Plain water is close to neutral, so the color barely changes. The redder the color, the stronger the acid; the more yellow-green, the stronger the base — exactly the same logic as the pH paper used in a school laboratory.

The indicators in a typical middle school textbook, litmus and phenolphthalein, work the same way: litmus turns red in acid and blue in base; phenolphthalein turns pink in base but stays colorless in acid and neutral solutions. Red cabbage juice is essentially a "natural, multi-color litmus" that can distinguish several levels at once.

So what exactly are acids and bases? The middle school definition is compact: a compound whose positive ions in water are all hydrogen ions (H⁺) is an acid; one whose negative ions are all hydroxide ions (OH⁻) is a base. The acetic acid in vinegar supplies H⁺, while soapy water and baking soda solution carry extra OH⁻. The indicator can "recognize" them because anthocyanin molecules bond with H⁺ or OH⁻ — and once the structure changes, so does the color of light the molecule absorbs and reflects.

How pH is defined. pH expresses acidity on a scale from 0 to 14: pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic, and the farther from 7, the stronger the acid or base. One crucial detail: each step of 1 on the pH scale means a difference of about 10 times in acidity — a solution at pH 3 is a hundred times more acidic than one at pH 5, not just slightly more. Rough pH values for familiar liquids: lemon juice 2–3, white vinegar about 3, cola about 2.5, pure water 7, baking soda solution 8–9, soapy water 9–10.

Four things students most often get wrong

  • "An indicator can measure the exact pH value." It can't. Cabbage juice and litmus only tell you roughly whether something is acidic or basic. To read a number you need pH paper (compare the changed color against the standard color chart) or a pH meter. And when using pH paper, never dip the strip directly into the sample bottle — that contaminates the whole solution. The correct technique is to transfer one drop onto the paper with a glass rod.
  • "The stronger the acid, the more dangerous it must be." Danger also depends on concentration and corrosiveness. Lemon juice at pH 2–3 is "more acidic" than many dilute lab acids, yet you can drink it — while a drain cleaner at only pH 11 can badly burn skin. Safety depends on the specific substance, not just the pH number. That is exactly why this experiment uses food-grade materials only.
  • "A color change means a neutralization reaction happened." No. The indicator changing color is just its own molecules responding to the environment — it indicates; the tested solution has not turned into something else. A true neutralization is an acid and a base reacting to form a salt and water — like the bubbles when vinegar meets baking soda, which really are evidence of a new substance forming.
  • "Cabbage indicator keeps forever." It doesn't. Anthocyanins are unstable: after a day or two at room temperature the juice turns brown and stops working. Freshly made works best; leftovers keep two or three days at most, sealed in the fridge. Once the color goes dull, don't use it for comparisons anymore.

Notes for teachers and parents

  • Time and grouping: two class periods (including brewing); groups of four.
  • Make clear before hands-on work: hot-water steeping needs an adult; food-grade samples only, and absolutely no tasting.

The step most often skipped. Brew the indicator fresh — overnight it browns and fails. Have each group predict the acid-to-base order before adding the indicator; the cups they guessed wrong are the ones they'll remember. It's also a natural moment to add a quick vinegar-plus-baking-soda neutralization demo.

How to know they really got it. Students meet the bar when they can explain which color means acidic or basic, and match pH < 7, = 7 and > 7 to what they saw in the cups.

Take it further

  • Run a neutralization: into the cup of vinegar that turned red, add baking soda solution a little at a time. Watch the color climb back from red to purple and on toward blue — probably with bubbles (carbon dioxide) along the way. That is an acid and a base reacting before your eyes.
  • Make your own pH color chart: photograph or draw each result, label it with its substance, and assemble your personal acid-base reference card.
  • Make your own test strips: soak filter paper or a white napkin in cabbage juice, dry it, and cut it into strips — homemade "cabbage pH paper." Write on it with a cotton swab dipped in vinegar and the writing shows up red; write with baking soda solution and it comes out blue-green. That is the simplest possible acid-base test strip.
  • Test rainwater and tap water. Normal rain dissolves carbon dioxide from the air, giving it a pH of about 5.6 — slightly acidic by nature. If your sample reads clearly more acidic than that, it may be acid rain. Line up rainwater, tap water and pure water side by side and look for subtle differences in indicator color.
  • Try another material. Purple grape skins, purple sweet potato, rose petals and blueberries all contain anthocyanins and also work as indicators — while the pigments in carrots and spinach don't change color with acidity at all. Run the comparison, then ask: why can only certain pigments serve as indicators?

When a student can explain clearly why vinegar turns the indicator red and soapy water turns it green — and connect that to acidity, basicity and pH — the chemistry of this experiment has truly landed.