Carbon dioxide is a must-know gas in middle school chemistry. The textbook prepares it from marble chips and dilute hydrochloric acid — reagents you don't have at home — but baking soda and white vinegar produce plenty of carbon dioxide, enough to verify its key properties one by one.

The point of this experiment is not "look, bubbles" — it is learning to use observations to prove what a gas is and what it can do.

Safety first (read this before you start)

  • Never seal the reactants inside a capped bottle or any closed container. Carbon dioxide is produced quickly, pressure builds fast, and a sealed container can burst and injure someone. Use open containers the entire time.
  • Vinegar and baking soda foam up vigorously and can overflow — work over a sink or a tray, and fill containers no more than one third full.
  • Any step with a candle needs an adult present: tie back long hair, clear away flammable items, keep a cup of water nearby. An adult does all the lighting and blowing out.
  • Work in a well-ventilated room and don't generate large amounts in a small closed space — carbon dioxide is denser than air and pools in low places.
  • White vinegar has a sharp smell; keep it out of your eyes. Wash your hands afterwards; the leftover liquids can go down the drain with plenty of water.

Materials and equipment

  • Baking soda (sodium bicarbonate), 2–3 tablespoons;
  • White vinegar, about 100 mL (the stronger the vinegar, the faster the fizz);
  • 2 wide-mouth glasses or small beakers (one to react, one to collect);
  • 2 short candles, matches or a lighter (adult use only);
  • Optional but recommended: a little food-grade slaked lime (to make your own limewater), or use red cabbage indicator instead;
  • A sheet of paper (as a pouring chute), a spoon, a tray.

Step 1: Make and collect the gas

  1. Put 2 tablespoons of baking soda in the reaction glass and spread it evenly over the bottom.
  2. Pour white vinegar slowly down the inside wall of the glass. It fizzes at once — those bubbles are carbon dioxide. Pour gently so the foam doesn't surge over the rim.
  3. Wait ten seconds or so for the fiercest fizzing to calm down. The upper part of the glass is now filled with carbon dioxide. Because it is denser than air, it sits in the glass like "invisible water" instead of escaping right away.
  4. To transfer the gas: tilt the collecting glass next to the reaction glass, lay the sheet of paper between them as a chute, then tip the reaction glass gently so the gas flows down the chute into the collector. This one move is itself a demonstration that the gas is denser than air.
Why not downward gas collection: when the textbook covers collecting gases it distinguishes upward displacement of air from downward displacement of air. Carbon dioxide is denser than air, so it is collected by upward displacement of air (tube reaching to the bottom of the bottle, pushing the air out the top). Hydrogen, being less dense than air, uses downward displacement. The deciding factor is always density.

Step 2: Three property tests

Test 1 · It neither burns nor supports burning

  1. Light a short candle (adult's job) and stand it on the tray.
  2. Slowly pour the collected carbon dioxide from above onto the flame (without touching the candle).
  3. The flame should go out almost immediately. Carbon dioxide does not burn itself, and it does not support burning.

Test 2 · Denser than air (staircase candles)

  1. Stand two candles on the tray, one tall and one short, and light both.
  2. From above, pour carbon dioxide slowly down along the side of the setup.
  3. The lower candle goes out first, the taller one after. The gas floods the low ground first, just like a liquid — proof that it is heavier than air.

Test 3 · It turns clear limewater milky

  1. If you have food-grade slaked lime, stir a little into water, let it settle, and pour off the clear liquid on top — that is clear limewater.
  2. Pour carbon dioxide into the limewater, or bubble your gas into it through a straw.
  3. The limewater turns from clear to milky white. This is the standard test for carbon dioxide.
  4. No limewater? Bubble the gas into water tinted with red cabbage indicator instead: the color shifts slightly toward red, showing that carbon dioxide dissolves in water to form acidic carbonic acid.

What you should observe

  • Baking soda + vinegar → vigorous bubbling, and the glass wall turns slightly cool;
  • Pouring the gas over a flame → the flame goes out;
  • Staircase candles → the lower one dies first, the higher one after;
  • Bubbled into clear limewater → the solution turns milky (a white precipitate appears);
  • Bubbled into cabbage-indicator water → the color shifts toward red, showing the solution is acidic.

The science

Baking soda is sodium bicarbonate (NaHCO₃); the active ingredient of white vinegar is acetic acid (CH₃COOH). They react to form sodium acetate, water and carbon dioxide:

NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂↑

The textbook's laboratory preparation uses marble chips (mainly calcium carbonate) with dilute hydrochloric acid — same principle: a carbonate meets an acid and releases carbon dioxide:

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑

Each test maps to one property. The dying flame shows carbon dioxide neither burns nor supports burning — which is exactly how a CO₂ fire extinguisher works: it cuts off oxygen and blankets the burning surface. The staircase candles going out bottom-first show its density is greater than air's (relative molecular mass of CO₂ is 44 versus about 29 for air on average).

Limewater turns milky because the calcium hydroxide in it reacts with carbon dioxide to form calcium carbonate, insoluble in water — the white precipitate:

Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O

And carbon dioxide dissolving in water forms carbonic acid (CO₂ + H₂O → H₂CO₃), which is acidic — that is why the indicator shifted red. A soda going flat after you open it, and tasting less sharp as it does, is the very same reaction running in reverse.

Notes for teachers and parents

  • Time and grouping: 30 minutes; groups of four; the teacher hands out the reagents.
  • Make clear before hands-on work: containers must never be sealed; limewater must never go in the mouth.

The step most often skipped. The "never seal the container" rule deserves its own emphasis — state it once, with a consequences sketch, before distributing any reagents. Run the three tests in the order "pour over one flame → staircase candles → limewater turns milky": the phenomena get more striking each time.

How to know they really got it. Students meet the bar when they can state the three properties in three sentences and write both chemical equations.

Take it further

  • Build a "balloon pump": wrap baking soda in a tissue and drop it into a balloon, stretch the balloon over the mouth of a bottle of vinegar, then let the packet fall in — the carbon dioxide inflates the balloon. The bottle must stay open under the balloon; never screw on a cap.
  • Race the reaction: with equal amounts of baking soda, add cold vinegar to one cup and warm vinegar to another and see which fizzes faster. A first taste of how temperature affects reaction rate.
  • Connect it to breathing: blow through a straw into clear limewater — it turns milky too, proving the air we exhale carries carbon dioxide. This links chemistry straight to energy metabolism in biology.

When a student can explain the three observations with the three sentences — doesn't burn or support burning, denser than air, turns clear limewater milky — and write those two equations, the carbon dioxide chapter is truly learned.