Sodium Bicarbonate: What Baking Soda Actually Is, and What It Actually Does

Almost everything baking soda does — in a cake, in a saucepan, on a tooth, in a cyclist's bloodstream — comes from the same two properties. It is a weak base, and it falls apart into carbon dioxide when it meets an acid or enough heat. Understand those two things and most of the claims sort themselves into "obviously true", "plausible but small", and "someone is selling something".

What it is

Baking soda is sodium bicarbonate, NaHCO₃ — sodium, hydrogen, carbon and three oxygens, in a white crystalline powder. It occurs naturally as the mineral nahcolite and is manufactured at industrial scale, mostly by the Solvay process. It is one of the oldest food additives still in everyday use, and it is recognised as safe in food at normal levels by regulators worldwide.

Dissolved in water it makes a mildly alkaline solution, around pH 8.3. Not caustic, not aggressive — roughly the alkalinity of seawater, and far gentler than household ammonia or bleach.

Mechanism one: it neutralises acid

Add an acid and the bicarbonate ion mops up a hydrogen ion, producing water and carbon dioxide. That is why a spoonful in water settles heartburn — it is chemically neutralising stomach acid — and it is the same reason a paste takes the edge off an acidic stain. It is also, in a much more controlled way, what happens when athletes take it before a hard effort: raising bicarbonate in the blood gives the body more capacity to buffer the acidity that builds during intense exercise.

Mechanism two: it makes gas

Heat sodium bicarbonate past about 80 °C, or mix it with an acid, and it decomposes and releases carbon dioxide. In a batter, those bubbles are the entire point: they expand in the oven and the crumb sets around them. This is also the mechanism behind the sink-and-vinegar theatre — impressive foam, very little cleaning (more on that here).

Baking soda, baking powder, washing soda: not the same thing

• Baking soda is pure sodium bicarbonate. It needs an acid in the recipe — buttermilk, yoghurt, lemon, cocoa, honey, brown sugar — to do anything.
• Baking powder is baking soda plus a powdered acid plus a starch to keep them apart. It brings its own acid, so it works in recipes that have none. Most is "double acting": a little gas on mixing, the rest in the oven's heat.
• Washing soda is sodium carbonate, Na₂CO₃ — a different compound, far more alkaline, genuinely irritating to skin, and not food.
• Caustic soda is sodium hydroxide. Not a cousin. Do not confuse them.

A rough kitchen substitution: one teaspoon of baking powder ≈ a quarter teaspoon of baking soda plus half a teaspoon of cream of tartar. Going the other way rarely works well.

Is yours still any good?

Sodium bicarbonate is stable but slowly absorbs moisture and carbon dioxide from the air, especially in an open box. Drop a quarter teaspoon into a splash of vinegar: an immediate, vigorous fizz means it is fine, a lazy one means replace it. An open box in the fridge is generally considered done after around six months — though whether it was doing much in there is a separate question.

What it does not do

It does not "alkalise your body". Blood pH is held between about 7.35 and 7.45 by the kidneys and lungs with a precision that no food or powder meaningfully disturbs — and if something did shift it, that would be a medical emergency, not a wellness outcome. Urine pH changes readily and tells you almost nothing about blood. This matters because a large family of claims, some of them aimed at people with serious illness, is built on that misunderstanding. We go through them in the household piece and in our editorial policy, which explains why we will not print them.

The sodium, which nobody mentions

Half a teaspoon of baking soda contains roughly 600 mg of sodium — about a quarter of the daily limit most health authorities recommend, in one spoonful with no food attached. For occasional use in a healthy adult that is unremarkable. As a daily habit, or for anyone managing blood pressure, heart failure or kidney disease, it is a real consideration and the reason so much of the guidance below ends with "ask first".

Where to go next

The three uses with the most evidence behind them, in descending order of how strong that evidence is: exercise buffering, as a dentifrice abrasive, and as a short-term antacid. Everything else is either housekeeping or marketing.