Capstone: The Kitchen Lab

Three real experiments with supermarket chemicals: brew a pH indicator, size a CO₂ balloon by stoichiometry, and grow crystals.

lesson 3 of 3 in this unit

Builds on: 4.1 Acids & Bases: The pH Scale 2.3 Stoichiometry: Recipe Math 3.3 Solutions & Concentration

Everything you need is in a supermarket

This capstone needs no lab: red cabbage, vinegar, baking soda (Natron), salt, a lemon, glasses, a bottle and a balloon. Total cost: a few euros. What it proves: everything from Units 0–4, running on your own table.

Experiment 1 — the red-cabbage pH rainbow

Chop a few red-cabbage leaves, pour boiling water over them, wait ten minutes, and strain. The purple liquid is an anthocyanin indicator — the same trick as phenolphthalein, courtesy of a vegetable. Line up glasses of household liquids, add a splash of juice to each, and photograph your rainbow: red in lemon juice, purple in tap water, green in soap water. You are literally seeing H⁺ concentrations.

Experiment 2 — the stoichiometric balloon

NaHCO₃ + CH₃COOH → CO₂↑ + H₂O + CH₃COONabaking soda + vinegar: 1 mol soda (84 g) releases 1 mol CO₂ (~24 L at room conditions)

Put vinegar in a bottle, baking soda in a balloon, stretch the balloon over the neck, and tip. The fizz is CO₂ inflating the balloon. The capstone twist: predict first. Weigh your baking soda, compute the moles, and estimate the litres of CO₂ before you tip. 4 g of soda ≈ 0.048 mol ≈ 1.1 L — a nicely fist-sized balloon. Chemistry that forecasts is chemistry you understand.

Experiment 3 — crystals on a string

Stir salt into hot water until no more dissolves (saturation, Unit 3). Hang a thread from a pencil into the solution and park the glass somewhere quiet. Over days, as water evaporates and the solution cools, the excess ions must rejoin a lattice — and they prefer to join your thread’s growing crystals. You are watching the ionic bonding of Unit 1 build architecture at visible scale.

Kitchen safety — the honest version

Everything here is food-grade, but treat it like a lab anyway: no glass near the stove’s edge, label your glasses, don’t drink the experiments (the cabbage juice is technically edible; the soap water is not), and cap the vinegar bottle after the balloon is on — sealed pressure has no balloon-shaped escape route.

⚗️ Lab — Digital Twin: The Cabbage Rainbow

A preview of your kitchen counter: seven liquids plus cabbage juice, and a mixer glass to explore the full color range before you brew the real thing.

Match each glass color to its pH — this is the cheat sheet for your photo.
Slide your own mix from 1 to 14 and memorize the red → purple → green → yellow arc.

pH of your own mixpH 7.0

Experiment checklist — 0 / 7

Brew the indicator: pour boiling water over chopped red cabbage, steep 10 minutes, strain into a jar.Set up 5+ glasses (lemon juice, vinegar, tap water, baking-soda water, soap water), add a splash of indicator to each.Sort the glasses by color and photograph your pH rainbow — red to green in order.Weigh your baking soda (kitchen scale), compute moles (M = 84 g/mol) and predict the CO₂ volume (~24 L/mol).Balloon over the bottle neck, tip the soda into the vinegar — compare the balloon to your prediction.Make a saturated salt solution in hot water and hang a thread in it.After 3–7 days: inspect your crystals with a magnifier — look for the cubic lattice shape of NaCl.