Exploring Substances: Acidic, Basic, and Neutral

Open any kitchen cupboard and you'll find a mix of very different liquids: lemon juice, vinegar, a bar of soap, baking soda, plain water. Chemists sort substances like these into three broad groups — acidic, basic, and neutral — based on a chemical property, not on how they look.

Lemon juice, tamarind water, curd, and vinegar all belong to the acidic group. Soap solution, baking soda solution, lime water, and washing powder solution belong to the basic group. Plain tap water, a sugar solution, and a salt solution are neutral — neither acidic nor basic.

You might be tempted to simply taste each one and decide, since acids are famous for tasting sour. But tasting an unknown liquid is risky — some acidic or basic substances used in labs and factories can burn your mouth or throat. Scientists solve this problem with indicators: substances that change colour, or sometimes smell, depending on whether they touch something acidic or something basic, without you ever having to taste anything.

The indicator chemists reach for first is litmus. It isn't manufactured in a factory — it's a natural dye obtained from lichens, which are tiny living crusts formed when a fungus and an alga grow together on rocks, walls, and tree bark. Litmus is sold either as a liquid solution or soaked into paper strips, and the paper comes in two colours: blue and red.

The rule is simple and worth memorising: an acidic substance turns blue litmus paper red. A basic substance turns red litmus paper blue. A neutral substance changes neither — dip blue litmus in salt water and it stays blue; dip red litmus in it and it stays red.

So if you had unlabelled samples of lemon juice, baking soda solution, and sugar solution, you could identify each one with two strips of litmus paper. Lemon juice turns blue litmus red (acidic). Baking soda solution turns red litmus blue (basic). Sugar solution leaves both strips exactly as they were (neutral). Because litmus behaves differently in acidic and basic conditions, it earns the title acid-base indicator.

You can make your own indicator at home. Crush some fallen red rose petals, soak them in hot water for several minutes, and filter the coloured liquid — this red rose extract is itself an indicator. Add it to an acidic liquid and the extract turns a shade of red; add it to a basic liquid and it turns green. A sugar or salt solution leaves its colour unchanged.

Turmeric gives you a different kind of indicator. Paint turmeric paste onto strips of filter paper and let them dry into bright yellow turmeric paper. Dip this into a basic substance like soap solution and it turns reddish-brown; dip it into an acidic or a neutral liquid, and nothing happens — it stays yellow either way. That is an important limitation: turmeric paper can tell you this is basic with confidence, but it cannot tell an acid apart from something neutral, because both leave it unchanged.

A few indicators don't rely on colour at all. Finely chopped onion sealed in a jar develops a strong smell; that smell changes noticeably when a few drops of an acidic or a basic liquid are added to a cloth soaked in the onion's vapour. Indicators that work through smell rather than colour are called olfactory indicators. Extracts of purple cabbage, beetroot, and red hibiscus can also act as natural indicators, each with its own pattern of colour change.

Before indicators existed, people still noticed patterns. Edible acidic substances share a sour taste: lemon and amla owe their tang mainly to citric acid, tamarind owes its tartness to tartaric acid, vinegar gets its sharp taste from acetic acid, and curd turns tangy because of lactic acid produced as it sets. Whenever something tastes distinctly sour, there's a good chance it contains an acid.

Basic substances show a different everyday clue: touch, not taste. Rub a little baking soda solution between your fingers and it feels soapy or slippery — a texture typical of basic substances, which is exactly why soap itself feels the way it does. Bases are also often described as bitter-tasting.

That bitter-taste clue needs a careful warning, though. Bitter gourd (karela) is intensely bitter, yet it is not a basic substance at all — it simply contains naturally bitter compounds that have nothing to do with acid-base chemistry. So while sour usually meaning acidic is a fairly safe rule of thumb, bitter usually meaning basic is not reliable by itself. Taste and touch can point you in the right direction, but an indicator is the only way to be certain — and you should never taste an unfamiliar substance just to check.

What happens if you mix an acid and a base together? If you add just enough base to exactly cancel out an acid (or the other way around), the resulting mixture is no longer acidic or basic — it becomes neutral. This reaction is called neutralisation, and it always produces a salt and water, releasing heat in the process:

Acid + Base → Salt + Water + Heat

This single reaction solves several everyday problems. When a red ant bites, it injects formic acid under the skin, causing a stinging pain; dabbing moist baking soda (a base) onto the spot neutralises the acid and eases the sting. When farm soil becomes too acidic from overusing chemical fertilisers, farmers treat it with lime (a base) to bring it back toward neutral; if soil instead turns too basic, adding organic matter such as manure or composted leaves helps, since decomposing organic matter releases acids that neutralise the excess base. Factories whose waste is acidic are expected to neutralise it with basic substances before releasing it into rivers or lakes, protecting fish and other aquatic life that cannot survive in water that is too acidic.