Q4. Name a physical method to separate: (a) sand from water; (b) salt from salt solution; (c) water from ink.
(a) Filtration. (b) Evaporation, or crystallisation. (c) Distillation: the water boils off and is condensed, leaving the dye behind.
Q5. One sugar solution is made from 20 g of sugar and 80 g of water, another from 10 g of sugar and 90 g of water. Find the mass per cent of sugar in each. What does this show about mixtures?
20/100 × 100 = 20% and 10/100 × 100 = 10%. Both are sugar solutions, yet their compositions differ: a mixture has a variable composition.
Q6. In a CO2 molecule, what fraction of the atoms are oxygen atoms, and what percentage of the mass is oxygen (C = 12.01, O = 16.00 u)?
Two of the three atoms, 2/3, are oxygen. By mass: 32.00/(12.01 + 32.00) × 100 = 32.00/44.01 × 100 ≈ 72.71%. The two fractions differ because an oxygen atom is heavier than a carbon atom.