Acids, Bases and Salts
Acids, Bases and Salts is examined through indicator colour changes, pH reasoning and the preparation, formula and uses of four named salts: bleaching powder, baking soda, washing soda and plaster of Paris.
Core rules
- Acids release H+ (as H3O+) in water; bases release OH-. Acid + metal gives salt + H2; acid + carbonate or bicarbonate gives salt + water + CO2; acid + base gives salt + water.
- pH below 7 is acidic, 7 neutral, above 7 basic. The scale is logarithmic, so pH 2 is ten times more acidic than pH 3.
- Chlor-alkali process: electrolysis of brine gives NaOH, Cl2 and H2.
Named salts
Bleaching powder CaOCl2; baking soda NaHCO3, used in antacids and baking powder; washing soda Na2CO3.10H2O, which contains water of crystallisation; plaster of Paris CaSO4.1/2H2O, made by heating gypsum to 373 K.
Worked example
What is the pH of 0.01 M HCl? HCl is a strong acid, so [H+] = 10 raised to -2 M and pH = 2. Dilute the solution ten times and [H+] becomes 10 raised to -3 M, so pH rises to 3. Dilution always moves an acid's pH towards 7 but never beyond it, because the acid is still present.
Traps
Writing Na2CO3 for baking soda; treating strong and concentrated as the same word; thinking pH 7 means pH 0; claiming soap lathers well in hard water, where calcium and magnesium ions form scum.