(a)(i) Define the term hygroscopic. (ii) Give two difference: between a physical change and a chemical change. (iii) Using the kinetic theory of gases, expl...

(a)(i) Hygroscopic describes a substance that absorbs moisture (water vapour) from the atmosphere without dissolving in it (it becomes damp but does not form a solution).

(a)(ii) Two differences between physical and chemical change

(a)(iii) Charles' law by the kinetic theory

As the temperature of a fixed mass of gas rises, its molecules gain kinetic energy and move faster, striking the walls more frequently and more forcefully. At constant pressure the gas must expand so that the molecules hit a larger area; the volume therefore increases in direct proportion to the absolute temperature.

(b)(i) Increasing order of boiling point

\(CO_2 < CS_2 < NaH\).

\(CO_2\) and \(CS_2\) are both simple molecular substances held by weak van der Waals forces, but \(CS_2\) has larger, heavier molecules and therefore stronger forces than \(CO_2\) (which is a gas). \(NaH\) is an ionic (giant) solid held by strong electrostatic forces, so it has by far the highest boiling point.

(b)(ii) Chlorine with cold dilute sodium hydroxide:

(c) Impure magnesium with excess HCl

(i) \[Mg + 2HCl \to MgCl_2 + H_2\]

(ii) Amount of hydrogen: \[n(H_2) = \frac{8.6}{22.4} = 0.384\ \text{mol}\]

I. From the equation \(1\ \text{mol } Mg\) gives \(1\ \text{mol } H_2\), so \(n(Mg) = 0.384\ \text{mol}\): \[\text{mass of pure Mg} = 0.384 \times 24 = 9.21\ \text{g}\]

II. \[\text{percentage purity} = \frac{9.21}{15.0} \times 100 = 61.4\%\]

III. The reaction forms \(0.384\ \text{mol } MgCl_2\), which contains \(0.384\ \text{mol } Mg^{2+}\) and \(0.768\ \text{mol } Cl^-\), a total of \(1.152\ \text{mol}\) of ions: \[\text{number of ions} = 1.152 \times 6.02\times10^{23} = 6.94\times10^{23}\ \text{ions}\]