What Is an Acid?

An acid is a chemical that releases hydrogen ions (H+) when dissolved in water. Acids usually taste sour, can dissolve some metals and minerals, and turn special test papers (called litmus paper) red. The word "acid" comes from the Latin acidus, meaning sour. You meet acids every day: in the lemon juice on your fish, the vinegar on your chips, the cola in your glass and even the acid in your own stomach that helps digest food.

  • What it isReleases H+ ions in waterThe hydrogen ion is the key
  • TasteSourLemon, vinegar, sour sweets
  • pH rangeBelow 7The lower, the stronger
  • Litmus testTurns redBlue paper changes colour
  • Common everyday acidVinegar (acetic acid)Weak and safe to taste
  • Strong lab acidHydrochloric acidPowerful, also in your stomach

What makes a chemical an acid?

A chemical is an acid if, when it dissolves in water, it releases hydrogen ions (H+) into the solution. The more H+ ions released, the stronger the acid.

For example, when hydrochloric acid (HCl) dissolves in water:

  • HCl breaks apart completely into H+ and Cl- ions.
  • The free H+ ions are what give the solution its acidic properties (sour taste, reaction with metals, low pH).

The Cl- ion is not the part that makes it an acid: the H+ ion is the active ingredient in every acid.

How sour, how strong

The strength of an acid is measured on the pH scale, which runs from 0 to 14. The lower the number, the stronger the acid.

  • pH 0-2: very strong acids (stomach acid, battery acid)
  • pH 3-4: strong but safe acids (vinegar, lemon juice, cola)
  • pH 5-6: weak acids (rainwater, milk)
  • pH 7: neutral (pure water)
Fact Your stomach contains a very strong acid (hydrochloric acid, around pH 1.5) that would burn your skin if it got out. It is used to digest food and kill germs. Your stomach lining produces a thick layer of mucus every few hours to protect itself from being eaten alive. If that mucus layer is damaged, the acid can cause painful ulcers.

What acids do

All acids share certain properties:

  • Sour taste: this is why lemons and vinegar taste sharp. (Never taste an acid in a lab, only safe acids in food.)
  • Turn litmus paper red: a classic chemistry test.
  • React with reactive metals: producing hydrogen gas. You can see this fizzing when an acid touches a piece of zinc or iron.
  • React with bases: in a neutralisation reaction, an acid and a base cancel each other out to produce water and a salt.
  • React with carbonates: giving off carbon dioxide gas. This is why putting vinegar on baking soda fizzes (baking soda is a carbonate).

Strong acids and weak acids

Not all acids are the same. Strong acids break apart completely in water, releasing all their H+ ions. Weak acids only partly break apart, releasing only some of their H+ ions.

  • Strong acids: hydrochloric (HCl), sulfuric (H2SO4), nitric (HNO3). These are common in laboratories and industry.
  • Weak acids: acetic acid (in vinegar), citric acid (in citrus fruit), carbonic acid (in fizzy drinks), lactic acid (in muscles after exercise).
Did you know? When you run hard for a long time and your muscles start to burn and ache, it is partly because your cells are producing lactic acid. Your muscles run out of oxygen and switch to a less efficient way of releasing energy, leaving lactic acid behind. The burn fades when you stop and your blood carries the acid away.

Common acids around you

  • Stomach: hydrochloric acid (pH 1.5 to 3.5)
  • Lemons, limes, oranges: citric acid (pH 2 to 3)
  • Vinegar: acetic acid (pH around 2.5)
  • Cola: phosphoric acid + carbonic acid (pH around 2.5)
  • Tea: tannic acid (pH around 5)
  • Yoghurt: lactic acid (pH around 4.5)
  • Rainwater: carbonic acid from dissolved CO2 (pH around 5.5)
  • Car batteries: sulfuric acid (very strong, pH below 1)
  • Bee stings: formic acid
  • Ant bites: formic acid (named after ants, from the Latin formica)

Why some acids are dangerous

Weak acids in food are safe to eat, but strong acids can be very dangerous. They can burn skin, attack metal, dissolve clothes and damage eyes. Always handle strong acids with proper protection, in a properly equipped lab, with adult supervision.

The danger is not just about the type of acid but also the concentration. Concentrated vinegar (much stronger than the kitchen kind) can give you a chemical burn even though acetic acid is "weak" by chemistry standards.

Try this Make red cabbage indicator at home. Chop a red cabbage, boil the leaves in water for 10 minutes and strain the purple liquid. This is a natural pH indicator: it turns pink in acids, blue/green in bases and stays purple in neutral solutions. Test small splashes of lemon juice, vinegar, fizzy water, milk, soap solution and baking soda water. You will see clear colour changes that tell you what is acidic and what is not.
Deeper dive: how acid rain damaged Europes forests

Throughout the 1970s and 1980s, scientists noticed that vast areas of forest in Germany, the Czech Republic, Scandinavia and parts of the UK were dying. Whole hillsides of pine and spruce went bare and grey. Lakes in Norway and Sweden went so acidic that fish disappeared. The culprit: acid rain.

Coal-burning power stations and factories were releasing huge amounts of sulfur dioxide (SO2) and nitrogen oxides (NOx) into the air. High in the atmosphere these gases reacted with water vapour to form sulfuric and nitric acids. The acids then fell as rain, often hundreds of kilometres downwind of the original pollution source.

The damage was severe. Acid rain stripped nutrients from soils, damaged tree leaves and needles, killed fish in lakes and even ate away at limestone buildings and statues. The medieval cathedrals of Cologne and Krakow lost the sharp details on their carvings as the rain slowly dissolved them.

The solution was teamwork. Countries across Europe agreed to limit sulfur dioxide emissions from power stations through the Convention on Long-Range Transboundary Air Pollution (1979) and later protocols. Power stations were forced to add "scrubbers" that strip SO2 from their exhaust. Diesel and petrol were reformulated. Today, sulfur emissions in Europe are over 80 per cent lower than they were in 1980, and many forests and lakes are slowly recovering. Acid rain is one of the great environmental success stories, showing what international cooperation on chemistry can do.

For more, see the pH scale and common acids in everyday life.