Acids, Bases and Salts

Acids are substances that produce H⁺ ions when dissolved in water - the more H⁺ ions present, the stronger the acid becomes. You'll recognise acids by their pH less than 7, sour taste, and ability to turn litmus paper red.

Strong acids like hydrochloric acid (HCl) completely break apart in water, creating loads of H⁺ ions and resulting in a very low pH $0-2$. Weak acids only partially ionise, giving pH values of 3-6. Remember that concentration is different from strength - a concentrated solution simply has more acid molecules per unit volume.

Bases neutralise acids to produce salt and water, whilst alkalis are simply soluble bases that produce OH⁻ ions in solution. Alkalis have a pH greater than 7 and turn litmus paper blue. Strong alkalis $pH 12-14$ completely ionise, weak ones $pH 8-11$ only partially ionise.

Top Tip: Use pH meters for accuracy (to 2 decimal places) or universal indicator for quick visual checks - but remember colours can look different to different people!

Neutralisation occurs when H⁺ ions from acids react with OH⁻ ions from alkalis: H⁺(aq) + OH⁻(aq) → H₂O$l$. This reaction produces salts - compounds where hydrogen ions are replaced by metal or ammonium ions. Different acids produce different salts: sulfuric acid makes sulfates, hydrochloric acid makes chlorides, and nitric acid makes nitrates.