Acids & alkalis, but bitesize.

Part 1Sour, slippery, or nothing at all

Lemon juice, vinegar and fizzy drinks all taste sharp and sour. Soap and bleach feel slippery. Pure water tastes of almost nothing at all. These are not random differences — chemists sort every substance using one single scale that runs from 0 to 14.

An ACID is a family of chemicals with a pH below 7. Many of them taste sour (though you must never taste a lab chemical to find out). An ALKALI is, roughly, the chemical opposite of an acid — it has a pH above 7. A NEUTRAL substance is neither acidic nor alkaline: it sits at exactly pH 7. Pure water is the classic example.

Part 2Reading the pH scale

The pH scale runs from 0 to 14 and measures how acidic or alkaline a substance is. The rule is short and worth memorising: pH less than 7 means acid, pH exactly 7 means neutral, pH greater than 7 means alkali.

The scale also tells you how strong something is. The further from 7 a substance sits, the more corrosive it is. So a pH of 1 is a much stronger, more dangerous acid than a pH of 5. A pH of 14 is a far more corrosive alkali than a pH of 8.

Part 3Where the everyday substances sit

It helps to know which substances live at which end of the scale. The strong acids (pH 0–2) are the laboratory acids: hydrochloric acid (HCl), nitric acid (HNO₃) and sulfuric acid (H₂SO₄). The weak acids (pH 4–6) are the household and food acids: vinegar, lemon juice, fizzy drinks. Water sits at exactly pH 7. The weak alkalis (pH 8–10) include toothpaste, soap and some bleach. The one strong alkali you meet in the lab is sodium hydroxide (NaOH), at pH 12–14.

Remember that pH 7 is a single point, not a range. There is only one value that is truly neutral.

Part 1The chemicals that cancel out acids

Indigestion tablets contain a chemical that "cancels out" the acid in your stomach. Some of these acid-cancelling chemicals dissolve in water; some do not. Either way, they all do the same job — they neutralise the acid and raise its pH back towards 7.

The general word for any chemical that does this is a BASE. A base is any chemical that cancels out (neutralises) an acid, raising its pH towards 7. Metal oxides and metal carbonates are common bases.

Part 2An alkali is a soluble base

Here is the key idea of the whole topic. An ALKALI is simply a base that is SOLUBLE in water. That single difference matters because dissolving lets a substance do more.

Because an alkali dissolves and has a high pH, it can raise a solution's pH above 7 — it can overshoot neutral and make the solution alkaline. An insoluble base does not dissolve, so it can only cancel an acid up to pH 7 and no further. It cannot overshoot.

So the rule is: ALL alkalis are bases — but NOT all bases are alkalis. The alkalis are the special, soluble members of the larger base family.

Part 3Quick checks

Part 1Telling acids and alkalis apart by colour

Imagine two clear, colourless liquids in beakers. One is a strong acid, one a strong alkali — and you must not taste or touch either, because both are corrosive. Yet a few drops of one special chemical instantly tells you which is which. That chemical is an indicator.

An INDICATOR is a chemical that changes colour depending on the pH of a solution. Digital pH probes also exist and give an exact number, but they are expensive — an indicator is quick and cheap, which is why you meet it first.

Part 2The universal indicator colours

Some indicators only give two colours — litmus, for example, is just red in acid and blue in alkali. Universal indicator is more useful because it shows the whole pH range, telling you not just whether a substance is an acid or an alkali but how strong it is.

Learn the colour order. At the acid end: red (strong acid) → orange → yellow (weak acid). At pH 7: green (neutral). At the alkali end: blue (weak alkali) → purple (strong alkali). The colours go red → orange → yellow → green → blue → purple as pH climbs from 0 to 14.

Part 3Reading the colour

The whole skill is to look at the colour and work backwards to the pH and the type of substance. Green means pH 7 and neutral. Red means a strong acid. Purple means a strong alkali. The green band is very narrow, because neutral is a single point (pH 7), not a range.

Part 1What happens when an acid meets an alkali

Add an alkali to an acid drop by drop and the pH climbs steadily towards 7. The two cancel each other out. This is a neutralisation reaction, and the products are not dangerous at all: one is just water, and the other is a chemical called a salt.

The general word equation is one you must know by heart:

acid + alkali → salt + water

If the right amounts react, the final solution ends up neutral, at pH 7. And "salt" does not mean table salt — it is a whole family of compounds. Table salt (sodium chloride) is just the most famous one.

Part 2Naming the salt

Naming the salt looks tricky but follows two simple rules. The acid sets the ending, and the alkali (or base) sets the metal — and the metal name always comes first.

The three acids give three endings: hydrochloric acid makes a chloride, sulfuric acid makes a sulfate, and nitric acid makes a nitrate. The metal part comes from the metal in the alkali or base.

Part 1A different reaction — and a different second product

Drop a piece of magnesium into acid and it fizzes vigorously, giving off a gas. Hold a lit splint to the gas and it goes "squeaky POP!" This time, water is NOT one of the products. So what is being made?

When a metal reacts with an acid, the products are a salt and hydrogen gas. The general word equation is:

metal + acid → salt + hydrogen

An easy way to remember it is MASH: Metal + Acid → Salt + Hydrogen. The salt is named with the same rules as before — the acid sets the ending, the metal sets the metal.

Part 2Testing the gas — the squeaky pop

Fizzing tells you a gas is being made, but it does not tell you which gas — you need a test. To test for hydrogen, hold a lit splint to the gas: a high-pitched "squeaky pop" is the positive result. The pop happens because the hydrogen burns very quickly with the oxygen in the air, releasing energy in a small explosion.

Compare this with carbon dioxide, which is made when a carbonate reacts with an acid. To test for carbon dioxide, bubble the gas through limewater: it turns cloudy white. So a fizzing metal gives hydrogen (pop), while a fizzing carbonate gives carbon dioxide (cloudy limewater).