Acids, Bases and Salts
Acids, Bases and Salts examines indicators, the pH scale, neutralisation, and the preparation and uses of common salts. Questions often test pH reasoning and salt identities.
Core ideas
- Acids taste sour and turn blue litmus red; bases taste bitter, feel soapy and turn red litmus blue.
- Indicators include litmus, methyl orange, phenolphthalein and natural indicators such as turmeric.
- pH below 7 is acidic, 7 is neutral, above 7 is basic. Lower pH means more acidic; each unit is a tenfold change in hydrogen ion concentration.
- Acid + base -> salt + water is neutralisation. Acid + metal -> salt + hydrogen. Acid + carbonate -> salt + water + carbon dioxide.
- Common salts: sodium chloride, baking soda (NaHCO3), washing soda (Na2CO3.10H2O), plaster of Paris (CaSO4.1/2H2O), bleaching powder (CaOCl2).
- Water of crystallisation is fixed water in a salt crystal, as in blue copper sulphate pentahydrate.
Worked example
A solution has pH 3. It is diluted ten times with water. For a strong acid, concentration falls tenfold, so pH rises by about 1, to about pH 4. Reasoning: pH is logarithmic; tenfold dilution changes pH by one unit, not by ten units. The solution remains acidic because pH is still below 7.
Traps that cost marks
Do not say a stronger acid always has a higher pH; it has a lower pH. Neutralisation does not always give a neutral solution if the acid or base is in excess. Baking soda is sodium hydrogen carbonate, not sodium carbonate. Never taste laboratory chemicals.