How an antacid calms heartburn
Neutralization of stomach acid by calcium carbonate · Acid–base · Gas-forming
A chewable antacid is essentially chalk, and it fights stomach acid by turning it into water and a burp.
Reactants approach
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ΔH = -15 kJ · exothermic, releases heat
ΔH per mol CaCO3.
Meet the molecules
- Reactantsolid
CaCO3
calcium carbonate
- Reactantdissolved in water
H3O+
hydronium ion
- Productdissolved in water
Ca2+
calcium ion
- Productliquid
H2O
water
- Productgas
CO2
carbon dioxide
What you notice
Your stomach lining secretes hydrochloric acid, keeping stomach contents at around pH 1.5 to 3.5 so that enzymes can digest protein and swallowed microbes are killed. Heartburn happens when some of that acid escapes upward into the oesophagus, which lacks the stomach's protective mucus layer, causing a burning feeling behind the breastbone. Chewable antacids such as Tums contain calcium carbonate, the same compound as chalk and limestone. It reacts with the acid and raises the pH within minutes. The carbon dioxide it releases is one reason people sometimes burp afterwards.
Try it
Drop a crushed antacid tablet into a glass of vinegar coloured with a little red cabbage juice and watch it fizz as the colour shifts away from pink.
Safety: Taking more than the labelled dose of calcium carbonate antacids for long periods can raise blood calcium and harm the kidneys.
Step by step
- 1
Solid meets acid
Hydronium ions, H3O+, collide with carbonate ions on the surface of the chewed tablet.
- 2
Two proton transfers
Each carbonate ion accepts two protons, one at a time, becoming hydrogen carbonate and then carbonic acid. Each H3O+ that gives up its proton becomes a water molecule.
- 3
Carbonic acid breaks down
H2CO3 splits into H2O and CO2 gas.
- 4
Calcium dissolves
With its carbonate gone, the Ca2+ ion is released into solution. The stomach's chloride ions are spectators and are left out of the equation.