Baking Soda

What it is and how it is made. Baking soda is sodium hydrogencarbonate, $\text{NaHCO}_3$, a white crystalline powder that dissolves in water to give a mildly basic (alkaline) solution — basic enough to neutralise acid, but far too gentle to burn skin or harm if swallowed in small amounts. Industrially it is made from brine (concentrated $\text{NaCl}$ solution) saturated with ammonia and carbon dioxide gas, a reaction that is essentially one step of the larger Solvay-type process also used to manufacture washing soda:

where: the sodium and hydrogencarbonate ions combine and crystallise out of the brine because sodium hydrogencarbonate is only sparingly soluble under these conditions, while ammonium chloride stays dissolved. At this level you only need the reactants and the resulting compound, not the process engineering.

The reaction that matters most. Because $\text{NaHCO}_3$ is a base, it reacts with any dilute acid in the standard acid–carbonate/bicarbonate pattern already familiar from the rest of the chapter — salt, water and carbon dioxide gas, every time:

where: the up-arrow marks $\text{CO}_2$ escaping as a gas, which is exactly what you see as fizzing when the acid is added. This single equation, with the acid swapped for whatever acid is present, is the key to every use of baking soda discussed below.

Uses, all traced to the same reaction.

• Antacid. Being mildly basic, $\text{NaHCO}_3$ neutralises excess hydrochloric acid in the stomach, relieving indigestion — gentle enough to be swallowed, unlike a strong base such as $\text{NaOH}$.
• Raising agent in baking. Baking powder is a mixture of $\text{NaHCO}_3$ with a mild edible acid such as tartaric acid. When wetted or heated in the batter, the acid reacts with $\text{NaHCO}_3$ to release $\text{CO}_2$, which forms bubbles that make bread and cakes light and spongy.
• Fire extinguisher. In a soda-acid extinguisher, sulphuric acid and sodium hydrogencarbonate solution are kept apart in separate compartments and mixed on activation, producing a rapid jet of $\text{CO}_2$ gas that smothers a fire.
• Household cleaning agent. A mild alkali, safe for general cleaning where a stronger base would be too corrosive.

Baking soda and washing soda ($\text{Na}_2\text{CO}_3\cdot10\text{H}_2\text{O}$) share the same carbonate chemistry but differ in basic strength: $\text{NaHCO}_3$ is only mildly basic, so it is safe for food and the body, while the stronger, more concentrated basicity of washing soda suits it to industrial and heavy-duty cleaning rather than consumption.

Every one of baking soda's uses comes back to one physical fact: mixing $\text{NaHCO}_3$ with any dilute acid always releases $\text{CO}_2$ gas, and that gas either lifts something, neutralises something, or smothers something depending on where the reaction happens.

In baking, the $\text{CO}_2$ has nowhere to escape quickly through thick batter, so it forms trapped bubbles. As the batter sets during baking, those bubbles are locked in place, leaving a light, porous crumb — this is why baking powder, which supplies its own acid, works even without added lemon juice or buttermilk, while baking soda alone (with no acid partner) produces almost no gas unless heated strongly enough to decompose it directly.

In the stomach, the acid is already present (excess hydrochloric acid causing indigestion), so swallowed $\text{NaHCO}_3$ reacts with it directly. Because $\text{NaHCO}_3$ is only mildly basic, it neutralises just enough acid to relieve discomfort without being caustic or dangerous to consume — a property strong bases like $\text{NaOH}$ do not share.

In a fire extinguisher, the two reactants — dilute sulphuric acid and sodium hydrogencarbonate solution — are stored in separate compartments precisely so the reaction happens only on demand. Inverting the extinguisher mixes them, and the reaction is fast enough to generate a large volume of $\text{CO}_2$ gas under pressure almost instantly. Because $\text{CO}_2$ is denser than air and does not support combustion, the jet blankets the fire and cuts off its oxygen supply, extinguishing it. The same underlying chemistry, engineered into three different contexts, is why baking soda earns a place in the kitchen, the medicine cabinet and the fire safety cupboard all at once.

Question: A soda-acid fire extinguisher contains dilute sulphuric acid in an inner container and sodium hydrogencarbonate solution in the outer container. When the extinguisher is inverted, the acid mixes with the bicarbonate solution. Write the balanced equation for this reaction and explain how the gas produced helps put out a fire.

Step 1. Identify the reactants and expected products. Sulphuric acid is a diprotic acid ($\text{H}_2\text{SO}_4$, supplying two acidic hydrogens), so it needs two formula units of $\text{NaHCO}_3$ to react completely, following the standard bicarbonate–acid pattern: salt, water and $\text{CO}_2$ gas.

Step 2. Write and balance the equation.

Step 3. Check the balance. Sodium: 2 on each side. Sulphur: 1 on each side (as $\text{SO}_4$). Hydrogen: 2 (from the two $\text{HCO}_3^-$) plus 2 (from $\text{H}_2\text{SO}_4$) equals 4 on the left, matching $2\times2 = 4$ hydrogens in two water molecules on the right. Carbon: 2 on each side. Oxygen also balances once you count all species, confirming the 2:1 ratio of $\text{NaHCO}_3$ to $\text{H}_2\text{SO}_4$ is correct.

Step 4. Explain the fire-fighting action. The reaction is fast and produces a large volume of $\text{CO}_2$ gas quickly, under pressure, driving it out as a jet. Since $\text{CO}_2$ is denser than air and does not support combustion, the gas blankets the fire and cuts off its oxygen supply. Answer: the balanced equation is $2\text{NaHCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O} + 2\text{CO}_2\uparrow$, and the extinguisher works because the CO2 jet smothers the fire's oxygen supply.

Competitive stretch

If instead dilute hydrochloric acid ($\text{HCl}$, a monoprotic acid) were used, only a 1:1 ratio would be needed: $\text{NaHCO}_3 + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2\uparrow$. The lesson generalises: always check whether the acid supplies one or two acidic hydrogens before guessing the ratio of $\text{NaHCO}_3$ needed.

• Confusing baking soda ($\text{NaHCO}_3$, sodium hydrogencarbonate) with washing soda ($\text{Na}_2\text{CO}_3\cdot10\text{H}_2\text{O}$, sodium carbonate) — different formulas, different strengths as a base, and different primary uses.
• Thinking baking soda alone (without an acid) makes cakes rise — it is the reaction between $\text{NaHCO}_3$ and an added mild acid in baking powder that releases $\text{CO}_2$; baking soda by itself needs an acidic partner (or strong heat) to release gas.
• Guessing a 1:1 ratio for the soda-acid extinguisher equation with sulphuric acid — because $\text{H}_2\text{SO}_4$ is diprotic, the correct ratio is 2:1 ($\text{NaHCO}_3$ to acid).
• Assuming baking soda is a strong base like $\text{NaOH}$ — it is only mildly basic, which is exactly why it is safe as an antacid and in food.

Ask the tutor: want to see the full Solvay-type sequence that links baking soda to washing soda, or walk through the soda-acid extinguisher balancing step by step?