Lemon juice, soap, and table salt seem to have nothing in common — yet they sit at three landmark points on a single scale that chemists use to describe almost every solution on Earth.
Bite into a lemon and your face puckers at the sourness; touch a bar of soap and your fingers feel slightly slippery. Those two everyday sensations sit at opposite ends of one of chemistry's most useful ideas: a single scale that measures whether a substance is an acid, a base, or perfectly neutral in between.
The first workable scientific definition came from Swedish chemist Svante Arrhenius in the 1880s. According to the Arrhenius definition, an acid is a substance that releases hydrogen ions (H⁺) when dissolved in water, while a base is a substance that releases hydroxide ions (OH⁻) when dissolved in water. This simple idea explained why acids and bases behave so differently, and why mixing them together tends to cancel out their effects.
| Property | Acids | Bases |
|---|---|---|
| Taste | Sour | Bitter |
| Feel | No distinct texture | Slippery, soapy |
| Litmus paper | Turns blue litmus red | Turns red litmus blue |
| Reaction with metals | Often produces hydrogen gas | Rarely reacts with metals |
The Arrhenius definition works well but is limited to water. In 1923, Danish chemist Johannes Brønsted and English chemist Thomas Lowry independently proposed a broader definition still used today: an acid is any substance that donates a proton (a hydrogen ion, H⁺), and a base is any substance that accepts a proton. This wording covers many more reactions than the original Arrhenius definition, including ones that don't happen in water at all.
When hydrochloric acid (HCl) reacts with ammonia (NH₃), HCl donates a proton to NH₃, forming the ammonium ion (NH₄⁺). By the Brønsted-Lowry definition, HCl is the acid and NH₃ is the base — even though no water is involved.
Chemists measure exactly how acidic or basic a solution is using the pH scale, which runs from 0 to 14. A pH below 7 is acidic, a pH above 7 is basic (also called alkaline), and a pH of exactly 7, the value for pure water, is considered neutral. The scale works in powers of ten, meaning a solution with a pH of 4 is ten times more acidic than one with a pH of 5.
The pH scale runs from 0 (strongly acidic) to 14 (strongly basic), with 7 marking neutral.
An indicator is a substance that changes colour depending on the pH of the solution it's placed in, giving chemists a quick visual way to test acidity without specialised equipment. The most common indicator, litmus, is extracted from lichen and turns red in acidic solutions and blue in basic ones. A universal indicator goes further, shifting through a full range of colours to estimate a more precise pH value, while a digital pH probe offers the most accurate readings of all.
When an acid and a base are combined in the right amounts, their opposing properties cancel each other out in a neutralisation reaction, producing water and a salt. This is simply a specific case of the double replacement reactions covered in the previous article: the hydrogen ion from the acid combines with the hydroxide ion from the base to form water, while the remaining ions pair up to form the salt.
Hydrochloric acid reacting with sodium hydroxide: HCl + NaOH → NaCl + H₂O. The acid and base neutralise each other, producing ordinary table salt and water.
In everyday language, "salt" usually means the table salt sprinkled on food. In chemistry, a salt is a far broader term: it is any ionic compound formed when the hydrogen ion of an acid is replaced by a metal or another positive ion, typically as the product of a neutralisation reaction. Salts are all around us, well beyond the kitchen shelf.
| Salt | Formula | Common Use |
|---|---|---|
| Sodium chloride | NaCl | Table salt |
| Calcium carbonate | CaCO₃ | Chalk, limestone, antacid tablets |
| Sodium bicarbonate | NaHCO₃ | Baking soda |
| Potassium nitrate | KNO₃ | Fertiliser |
Antacid tablets rely directly on neutralisation. Excess stomach acid, mostly hydrochloric acid, causes the burning discomfort of heartburn. Antacids contain mild bases, often calcium carbonate or magnesium hydroxide, which react with and neutralise that excess acid, forming water and a harmless salt, and easing the discomfort within minutes.
Acids and bases might seem like opposites doing battle, but they're better understood as two ends of the very same scale, endlessly capable of cancelling each other into something entirely new and useful — water and salt. From the antacid that settles a stomach to the soap that cleans a hand, this single reaction, hydrogen ions meeting hydroxide ions, quietly shapes an enormous share of everyday chemistry. The next article in this series turns to what happens when substances change state entirely, exploring states of matter and the changes between them.
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