Sour and slippery
Long before anyone could explain them, people sorted certain substances by taste and touch. Lemon juice, vinegar and yoghurt share one sharp taste, and the name of the group records it: acid comes from the Latin acidus, meaning sour.
A second group tasted bitter, felt slippery between the fingers, and could undo what acids did. Chemists now call this family bases. Soap makers knew one kind well. They burned plants such as saltwort, which grows on salty ground, and soaked the ash in water to draw out a caustic liquid called lye. Arabic-speaking chemists called that plant ash al-qalī, and European chemistry kept the word: an alkali is a base that dissolves in water. Nobody tests by tasting any more, because the strongest acids and alkalis burn.
What makes an acid an acid
Why do things as different as vinegar and battery acid behave alike? The answer came in the 1880s from the Swedish chemist Svante Arrhenius: dissolve an acid in water and it releases hydrogen ions. A hydrogen atom is one proton with one electron. Strip the electron away and a bare, positively charged proton is left, and that loose particle is what makes a liquid acidic. The more of them it holds, the more acidic it is.
Bases work the other way round: they take hydrogen ions up. Many alkalis do it by releasing hydroxide ions, each an oxygen atom bonded to a hydrogen atom, carrying a negative charge. Many drain cleaners are sodium hydroxide, and the hydroxide ions it sets free seize hydrogen ions wherever they meet them.
That answers the question you started with: Lemon juice is safe to drink and battery acid burns skin, yet chemists call both acids. What do they have in common?
Strong acids and weak ones
Vinegar and the juices in your stomach are both acids, but they are far from equal. The difference is how many hydrogen ions each one lets go of. The hydrochloric acid made by your stomach lining is a strong acid: in water, practically every molecule breaks apart and releases its hydrogen ion. Acetic acid, the acid in vinegar, is weak. At any moment fewer than one of its molecules in a hundred has let its hydrogen ion go; the rest stay whole.
Strength is not the same as concentration. Concentration is how much acid is dissolved in the water, and adding water makes any acid more dilute. A little strong acid in plenty of water can still be more acidic than neat vinegar, because a weak acid keeps most of its hydrogen ions to itself.
One number for acidity
By 1909 chemists could measure hydrogen ions, but the amounts came out as awkward decimals trailing long strings of zeros. The Danish chemist Søren Sørensen, who ran the chemistry department of the Carlsberg Laboratory in Copenhagen, was studying how acidity affects proteins and wanted something simpler. He turned each amount into one short number, the pH.
On his scale, the lower the number, the more hydrogen ions and the more acidic the liquid. Most everyday liquids fall between 0 and 14. Pure water sits at 7, because water molecules split into hydrogen ions and hydroxide ions in exactly equal, tiny numbers, so neither side wins. A substance at that midpoint is called neutral. Anything below 7 is acidic, and anything above 7 is alkaline.
Each step is ten times
The pH scale hides a surprise: its steps are not equal. Each whole number down the scale means ten times as many hydrogen ions. A liquid at pH 5 is ten times as acidic as one at pH 6, and pH 3 is a hundred times as acidic as pH 5. A scale that counts in multiplications like this is called logarithmic, the same kind used to measure earthquakes and loudness.
That is why small-looking differences matter. Ordinary rain is already slightly acidic, at about pH 5.6, because carbon dioxide from the air dissolves in it. Rain at pH 4.6 looks barely different on paper, yet it carries ten times as many hydrogen ions. Move two whole steps down the scale and the acidity has multiplied by ten twice.
Dangerous at both ends
The middle of the pH scale is gentle; both ends are not. Lemon juice sits near 2 and pure water at 7. Household ammonia, the sharp-smelling window cleaner, is near 11, and oven cleaner, made with sodium hydroxide, near 13. Strong acids and strong alkalis at the two extremes are both corrosive: they eat into metal, stone and living skin, and carry the same hazard symbol on the bottle.
They attack skin in different ways. A strong acid wrecks the proteins it touches. A strong alkali does that and something stranger: it splits the fats in your skin and turns them into soap, the same reaction soap makers use on olive oil. That is why a drop of lye feels slippery between the fingers. The slipperiness is your own skin being turned to soap.
Colours that tell
Tasting is dangerous, so chemists learned to let a colour do the testing. An indicator is a dye that changes colour with acidity. One of the oldest is litmus, a mixture of dyes extracted from lichens, its use as an acid test traditionally traced to about 1300 and the Catalan physician Arnaldus de Villa Nova. Blue litmus paper turns red in acid; red litmus turns blue in alkali.
Your kitchen holds another. Chop red cabbage and steep it in hot water, and the purple juice carries pigments called anthocyanins, which change shape with acidity. Add a little vinegar and it turns pink; stir in baking soda and it goes blue, and more alkali turns it green. Universal indicator, used in school labs, mixes several dyes so that each pH shows its own colour.
When acid meets base
Mix a strong acid and a strong alkali in just the right amounts and the result is neither: its pH settles at 7. This cancelling out is called neutralisation. The mechanism is simple. Each hydroxide ion from the alkali grabs a hydrogen ion from the acid, and the two join into one molecule of water. The warmth you can feel through the glass comes from those new bonds forming.
What happens to the rest? The other parts of the acid and the base stay in the water as ions, and together they make up a salt. Hydrochloric acid neutralised by sodium hydroxide leaves sodium chloride, ordinary table salt. To a chemist, though, a salt is any compound made this way, so every acid–alkali neutralisation follows one pattern: acid plus base gives a salt plus water.
Neutralising on purpose
Neutralisation is not only a laboratory trick; people rely on it every day. Heartburn is stomach acid rising into the gullet, whose lining has little protection against it. An antacid tablet carries a mild base, such as calcium carbonate or magnesium hydroxide, which neutralises some of that acid and eases the burn.
Farmers do the same thing on a larger scale. Rain, rotting plant matter and some fertilisers slowly make soil acidic, and in sour soil many crops grow poorly, partly because the acid frees aluminium that poisons their roots. So farmers spread lime, which is crushed limestone, mostly calcium carbonate. The carbonate takes up hydrogen ions from the water in the soil and nudges the pH back toward neutral, where most crops do best.
Acids that eat metal
Drop a strip of magnesium into dilute hydrochloric acid and it fizzes hard, shrinking until it vanishes. The bubbles are hydrogen gas. The metal hands electrons to the acid's hydrogen ions, which pair up into hydrogen molecules and escape, while the metal itself goes into the water as part of a salt, magnesium chloride. Hold a lit splint to a tube of the gas and it burns with a squeaky pop.
Not every metal joins in. Zinc fizzes steadily, iron slowly, and copper not at all. The order follows each metal's reactivity: how readily it gives up electrons. Metals more reactive than hydrogen push it out of an acid. Copper, silver and gold are less reactive, which is one reason they have lasted so well in coins and jewellery.
Acid and your teeth
Tooth enamel is the hardest substance in the human body, yet a sip of lemonade can soften it. Enamel is made mostly of hydroxyapatite, a crystal of calcium and phosphate, and like limestone it dissolves in acid. Below about pH 5.5, the liquid in your mouth starts pulling minerals out of it.
Acid reaches teeth in two ways. Acidic drinks such as cola and fruit juice bring it directly. Sugar brings it indirectly: bacteria living on the teeth feed on sugar and give off lactic acid right against the enamel. Saliva fights back. It carries bicarbonate, a mild base that neutralises acid, and minerals that rebuild the surface. Trouble comes when sugar arrives so often that saliva never gets the upper hand.
Holding blood steady
Your blood is slightly alkaline, and the body holds it in a remarkably narrow band, between pH 7.35 and 7.45. Stray much outside it and enzymes and other proteins stop working properly. Yet the body makes acid all the time: every cell gives off carbon dioxide, which forms carbonic acid in water.
What keeps the number steady is a buffer, a mixture that soaks up added acid or added base so the pH barely moves. Blood's main buffer is bicarbonate. It takes up spare hydrogen ions and becomes carbonic acid, which breaks down into water and carbon dioxide. The lungs finish the job. Breathe faster and you blow off more carbon dioxide, which pulls the blood back toward alkaline.
Rain that eats stone
Marble statues in old industrial cities have lost their carved detail, and the cause fell from the sky. Burning coal releases sulfur dioxide, a gas made from the sulfur the coal contains. In the clouds it turns into sulfuric acid, and the rain falls at around pH 4.3 instead of the 5.6 of clean rain.
Marble and limestone are both calcium carbonate, and acid dissolves carbonate, giving off carbon dioxide as it goes. So a statue fizzes away, a little with every shower. The same reaction works in your kitchen. Heating hard water, which carries dissolved calcium, leaves a chalky crust of calcium carbonate inside a kettle, called limescale. Boil diluted vinegar in the kettle and its acid fizzes the limescale away, just as the rain does to stone.
Cleaning without harm
Many household cleaners are acids or bases, and their strength decides how carefully you handle them. Baking soda is sodium bicarbonate, a weak alkali: a spoonful in water reaches only about pH 8, gentle enough to scrub a sink with bare hands. Oven cleaner is sodium hydroxide, a strong alkali near pH 13 that can burn skin and eyes, which is why its label calls for gloves.
The other rule is never to mix bleach with other cleaners. Bleach mixed with an acid, such as a vinegar-based or limescale cleaner, releases chlorine gas. Bleach mixed with ammonia releases chloramine vapours. Both are toxic and damage the lungs, so bleach is used on its own and rinsed away before anything else goes on.