Neutralisation

Neutralisation is the chemical reaction that happens when an acid meets a base. The two cancel each other out, leaving behind water and a salt. If you start with a strong acid and the right amount of strong base, the result is exactly neutral, with a pH of 7. Neutralisation is happening all around you: in your stomach when you take an indigestion tablet, in a beehive sting being treated with vinegar, in farmers fields being limed, and in factories cleaning up acid waste.

  • What it doesCancels acid + baseProduces water + salt
  • Word equationAcid + Base = Water + SaltAlways these four players
  • Final pH7 (if exact)Exactly neutral
  • Famous exampleHCl + NaOH = H2O + NaClAcid + base = water + table salt
  • In medicineAntacids vs stomach acidCalms heartburn
  • In farmingLime on acid soilHelps crops grow

The basic idea

Acids release hydrogen ions (H+). Bases release hydroxide ions (OH-). When these two ions meet, they combine to form water:

  • H+ + OH- -> H2O

The leftover ions (the bit of the acid that was not H+, and the bit of the base that was not OH-) combine to form a salt. The word "salt" in chemistry does not just mean the white stuff on your chips. It is any compound made when an acid and base react.

The simplest neutralisation

Mix hydrochloric acid (HCl) with sodium hydroxide solution (NaOH):

  • HCl + NaOH -> H2O + NaCl
  • The H+ from the acid joins the OH- from the base to make water.
  • The Na+ and Cl- left behind combine to form sodium chloride: ordinary table salt.

If you boil away the water, you are left with crystals of pure table salt. People sometimes make salt this way as a chemistry demonstration.

Fact Stomach acid is hydrochloric acid (HCl). When you take an antacid (like a chewable tablet for heartburn), the active ingredient is usually a base like calcium carbonate or magnesium hydroxide. It reacts with the stomach acid in exactly the same way as our example above: acid + base = water + salt, plus carbon dioxide gas if a carbonate is involved (the slight burp you might get).

What makes the salt

Different combinations of acid and base produce different salts. Some examples:

  • Hydrochloric acid + sodium hydroxide -> sodium chloride (table salt)
  • Hydrochloric acid + potassium hydroxide -> potassium chloride
  • Sulfuric acid + sodium hydroxide -> sodium sulfate
  • Sulfuric acid + potassium hydroxide -> potassium sulfate (used as fertiliser)
  • Nitric acid + ammonia -> ammonium nitrate (used as fertiliser and explosive)
  • Carbonic acid + calcium hydroxide -> calcium carbonate (limescale, chalk)

Neutralisation around you

Neutralisation reactions happen all the time, often without us noticing.

  • Indigestion tablets: the base in the tablet neutralises some of the excess acid in your stomach.
  • Toothpaste: mildly basic, it neutralises the acid produced by bacteria on your teeth, protecting the enamel.
  • Bee stings: bee venom is acidic. Some people use a paste of baking soda (a base) to neutralise it.
  • Wasp stings: wasp venom is basic, so a dab of vinegar (acid) can help.
  • Liming farmland: farmers add powdered lime (calcium hydroxide) to soil that has become too acidic from acid rain or fertilisers. Crops grow better at near-neutral pH.
  • Treating acid spills: factories that handle strong acids keep large amounts of sodium bicarbonate or lime nearby. If acid spills, they pour the base on top to neutralise it before clean-up.
  • Treating waste water: water from factories often needs its pH adjusted before being released back into rivers. Bases neutralise acid waste; acids neutralise basic waste.
Did you know? Some volcanoes pour out very acidic crater lakes. The El Chichon volcano in Mexico, after a big eruption in 1982, had a crater lake with a pH below 1. Engineers piped in lime slurry to neutralise it. Without the treatment, downstream rivers and farms would have been ruined when the lake eventually overflowed.

Energy in and out

Neutralisation reactions usually release energy as heat. They are exothermic. Try mixing baking soda and vinegar in a cup: the mixture warms up slightly as it fizzes. Strong acid plus strong base can give off so much heat that the solution boils.

How to know when neutralisation is complete

Chemists use a special chemical called an indicator that changes colour at a particular pH. By adding a few drops of indicator and slowly adding one solution to another until the colour changes, they can find the exact moment when neutralisation is complete. This is called titration.

Titration is one of the most useful techniques in a chemistry lab. It lets scientists work out exactly how much acid or base is in a sample, by carefully measuring how much of a known solution is needed to neutralise it.

Try this Crush an indigestion tablet (the chewable kind) and add a drop of red cabbage indicator. Stir it into a glass of water. The colour will be purple or blueish, showing the tablet is basic. Now slowly add lemon juice (acidic), counting drops as you go. After a few drops the colour will shift towards pink. Try with different brands of indigestion tablet and see which neutralises the most acid drops. You have just performed a simple titration at home.
Deeper dive: how neutralisation cleans up an industrial disaster

In October 2010, a giant red mud pond at an aluminium plant in Ajka, Hungary, burst its walls. About 1 million cubic metres of toxic mud, full of sodium hydroxide (a strong base with pH around 13), poured into nearby villages and rivers. The mud killed 10 people, burned the skin of many others and wiped out fish and other life in the local rivers.

Emergency teams faced an enormous challenge: neutralise hundreds of thousands of tonnes of strongly basic mud before it reached the Danube, the second-longest river in Europe. They worked around the clock, dumping huge amounts of acid (gypsum and acetic acid) into the rivers to bring the pH back down. Where rivers met the Danube, they used barriers and pumps to dilute the spill further. The effort worked: although the local rivers were severely damaged, the disaster was kept out of the main Danube.

The clean-up showed both the power of acid-base chemistry and its limits. Neutralisation can save rivers, fields and lives in an emergency, but it does not undo the damage to ecosystems or the people who were burned. The Hungarian government later imposed stricter rules on industrial sludge ponds across Europe, partly to make sure such large-scale neutralisation jobs would not be needed again.

For more, see the pH scale and what is a base.