Solutions, acids and bases, pH, neutralisation
Domain 5 - lahused, happed ja alused, pH, neutralisatsioon - covers what dissolving means, how concentration is expressed, and how acidity is measured and removed.
Solutions
- A solution is a homogeneous mixture of solute (lahustunud aine) and solvent (lahusti); water is the usual solvent.
- Solubility depends on temperature: most solids dissolve better in hot water, gases worse, and it is read from a solubility curve.
- Saturated means no more solute dissolves at that temperature; cooling a saturated solution crystallises the excess.
- Mass fraction w = m(solute)/m(solution) × 100 %, and m(solution) = solute + solvent.
Acids, bases, pH
- Acids release H⁺ (protons) in water; bases release OH⁻ or accept H⁺. Strong acids (HCl, HNO₃) ionise completely, weak ones (CH₃COOH, H₂CO₃) only partly, so equal concentration does not mean equal pH.
- pH < 7 acidic, 7 neutral, > 7 alkaline at 25 °C. The scale is logarithmic: pH 3 is ten times more acidic than pH 4.
- Indicators: litmus red in acid, blue in base; phenolphthalein colourless in acid and neutral, pink in base; universal indicator gives a colour chart.
- Neutralisation: acid + base → salt + water, H⁺ + OH⁻ → H₂O. Acid + carbonate → salt + water + CO₂.
Worked example
80 g of water dissolves 20 g of salt. Find w and the mass of salt in 250 g of the same solution. m(solution) = 80 + 20 = 100 g, so w = 20/100 × 100 % = 20 %. In 250 g: 0.20 × 250 g = 50 g of salt. A frequent error is dividing by 80 g (solvent only), which gives 25 % - wrong, because the definition uses the mass of the whole solution.
Traps that cost marks
- Using solvent mass instead of solution mass in w.
- Treating pH as linear rather than logarithmic.
- Assuming a strong acid must be concentrated - strength and concentration are different things.
- Saying phenolphthalein is blue in acid (it is colourless there).
- Forgetting to convert cm³ to dm³ before c = n/V.