Acids, Bases & Salts

The word acid comes from the Latin acidus, meaning sour. Lemon, tamarind, curd and vinegar taste sour because they contain acids; soap, baking soda and lime water feel slippery and taste bitter because they are bases. But taste is a dangerous test in a laboratory, so chemists asked a better question: what is it that acids share at the atomic level? The answer came from Svante Arrhenius (1884): when dissolved in water, an acid gives hydrogen ions (H⁺) and a base gives hydroxide ions (OH⁻). In reality H⁺ never floats alone; it sticks to a water molecule to form the hydronium ion (H₃O⁺).

Arrhenius's idea works only in water, and it cannot explain why ammonia (NH₃), which has no OH⁻ in it, behaves as a base. Brønsted and Lowry solved this by looking at protons: in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, ammonia accepts a proton so it is the base. The acid that loses H⁺ becomes its conjugate base; the base that gains H⁺ becomes its conjugate acid. These two always come as a pair, differing by exactly one H⁺: HCl / Cl⁻, NH₄⁺ / NH₃, H₃O⁺ / H₂O.

How do you know whether the solution in a bottle is an acid or a base without tasting it? You use an indicator: a substance that changes its colour in an acid or a base. Think of it as a traffic signal for chemistry. Some indicators come from plants (natural indicators), and some are made in factories (synthetic indicators). A third type works by smell instead of colour (olfactory indicators).

A single indicator only tells 'acid or base'. The Universal indicator is a mixture of several dyes and gives a whole range of colours, so it also tells how strong (the pH value). Red means strongly acidic, orange-yellow weakly acidic, green neutral (pH 7), blue weakly basic, and violet strongly basic. pH paper is paper soaked in universal indicator.

Saying 'strong acid' or 'weak acid' is vague. Chemists wanted a number, and in 1909 the Danish biochemist S. P. L. Sørensen gave one: the pH scale. The 'p' stands for the German word potenz (power) and 'H' for hydrogen, so pH means 'power of hydrogen'. It measures how many hydrogen ions a solution has, in a neat, short number. The usual scale runs from 0 to 14: pH 7 is neutral (pure water), below 7 is acidic, and above 7 is basic (alkaline). The further from 7, the stronger the acid or base.

The pH scale is logarithmic: pH = −log₁₀[H⁺]. Each step of one pH unit means ten times change in hydrogen ion concentration. So pH 3 is ten times more acidic than pH 4 and a hundred times more acidic than pH 5. Lower pH = more H⁺ = more acidic. This is why a drop of lemon juice (pH about 2) in water drops its pH sharply.

A buffer solution resists a change in pH when a small amount of acid or base is added. Blood is a buffer: its pH stays near 7.4 (a fall below about 6.8 or a rise above about 7.8 is life-threatening). A buffer made of a weak acid and its salt has a pH equal to the acid's pKa when both are in equal amounts; this is why a mixture of 0.1 M acetic acid and 0.1 M sodium acetate has pH 4.76.

What happens when an acid meets a base? They cancel each other's character. The H⁺ of the acid joins the OH⁻ of the base to make water, and what is left of the acid and base joins to make a salt. This reaction is called neutralisation: Acid + Base → Salt + Water + heat. Example: HCl + NaOH → NaCl + H₂O. In ionic form, the whole reaction is just H⁺ + OH⁻ → H₂O.

Nature is full of acids. Most of the sour things in your kitchen are weak organic acids, safe to eat, and each food has its own signature acid. Remember them as 'food – acid' pairs; they are a regular pick for one-mark questions.

Curd and the sour taste of fermented foods: milk bacteria (Lactobacillus) turn the milk sugar lactose into lactic acid. This is a chemical change because a new substance forms. Similarly, vinegar is made when Acetobacter bacteria ferment ethanol into acetic acid in the presence of air. Pickles are preserved by vinegar or by acid because the low pH stops bacteria from growing.

Mineral acids (also called inorganic acids) are made from minerals, not from living things. They are strong, corrosive and the workhorses of industry. The big three are hydrochloric acid (HCl), sulphuric acid (H₂SO₄) and nitric acid (HNO₃). Organic acids from plants and animals (acetic, citric, lactic) are weak and safe to eat; mineral acids can burn skin and must be handled with care. Their partners, the strong bases such as caustic soda and slaked lime, are just as important in daily life.

Concentrated sulphuric acid has three special powers beyond being an acid. (1) Hygroscopic: it absorbs moisture from air, so it is stored in airtight containers and is used to dry gases. (2) Dehydrating agent: it removes the water elements from sugar and leaves black carbon; it also turns blue copper sulphate crystals white. (3) Oxidising agent: when hot, it oxidises metals and non-metals. Hydrochloric acid, in contrast, mainly behaves as an acid.

A salt is the ionic compound that forms when the H⁺ of an acid is replaced by a metal (or ammonium) ion, usually in neutralisation. Common salt (NaCl) is the best-known, but chemists know thousands. Salts can be neutral, acidic or basic in water, depending on the parent acid and base that made them: strong acid + strong base gives a neutral salt (pH 7), strong acid + weak base gives an acidic salt (pH below 7), and weak acid + strong base gives a basic salt (pH above 7).

Now the household stars. Four calcium and sodium compounds appear in the exam again and again. Learn the story of each: where it comes from, what it does, and what its formula hides.

Water of crystallisation is the fixed number of water molecules that are part of the structure of a crystal, even though the crystal looks dry. Heat the crystal and the water leaves, so the colour and shape change. Copper sulphate is the classic demonstration: blue crystals of CuSO₄·5H₂O turn into a white powder (anhydrous CuSO₄) on heating, and turn blue again when water is added. This is why anhydrous copper sulphate is used as a test for water. Washing soda, gypsum and Plaster of Paris are also hydrated salts.

Baking powder is not the same as baking soda. Baking soda is only NaHCO₃; on heating it gives sodium carbonate, water and carbon dioxide (2NaHCO₃ → Na₂CO₃ + H₂O + CO₂), but the leftover sodium carbonate tastes bitter. Baking powder = baking soda + a mild edible acid (tartaric acid). When water is added, the acid and soda react and give CO₂ at once, which makes cakes and bhature soft and spongy, and no bitter taste is left because both are used up. Elements in baking powder: sodium, hydrogen, carbon and oxygen.