Chemistry

(d) $\mathrm{4Na + O_2 \longrightarrow 2Na_2O}$

In a combination reaction two or more substances combine to form a single product. (a) is decomposition, (b) neutralisation, and (c) double displacement.

(b) King of chemical

Sulphuric acid is called the "king of chemicals" because of its very wide use in industry (fertilisers, dyes, detergents, batteries, etc.). Blue vitriol is $\mathrm{CuSO_4\cdot5H_2O}$.

(a) $\mathrm{H_2}$

Hydrogen has molar mass 2 g/mol, compared with 32 for $\mathrm{O_2}$, 44 for $\mathrm{CO_2}$ and 28 for $\mathrm{N_2}$, so it is the lightest gas.

(c) $\mathrm{Fe_2O_3\cdot 3H_2O}$

Limonite is hydrated iron(III) oxide, an ore of iron. $\mathrm{FeCO_3}$ is siderite, $\mathrm{Cu_2O}$ is cuprite and $\mathrm{Ag_2S}$ is silver glance.

(b) Alkylbenzene sulphonate

It is a synthetic detergent that works even in hard water. Sodium stearate is a soap, DDT is an insecticide, and potash alum is used for water purification.

(a) Aldrin

Aldrin is an organochlorine compound, an organic insecticide. Lead arsenate, lime sulphur and fluorides are inorganic insecticides.

(b) 23

Fe has atomic number 26, so a neutral atom has 26 electrons. $\mathrm{Fe^{3+}}$ has lost 3 electrons: $26-3=23$.

(c) $\mathrm{SO_3^{2-}}$

The sulphite ion is $\mathrm{SO_3^{2-}}$. $\mathrm{SO_4^{2-}}$ is sulphate, $\mathrm{S^{2-}}$ is sulphide and $\mathrm{HSO_4^-}$ is hydrogensulphate.

(c) $\mathrm{C_nH_{2n+1}}$

An alkyl group is an alkane ($\mathrm{C_nH_{2n+2}}$) minus one hydrogen atom, giving $\mathrm{C_nH_{2n+1}}$.

(c) $1s^2 2s^2 2p^6$

Mg ($Z=12$) is $1s^2 2s^2 2p^6 3s^2$. Mg$^{2+}$ forms by losing the two $3s$ electrons, leaving $1s^2 2s^2 2p^6$ (10 electrons, like neon).

(b) dissolution of sodium metal in water

Sodium reacts with water to form new substances: $2Na + 2H_2O \to 2NaOH + H_2$. Heating platinum wire, heating water and dissolving salt only change physical state or form, so they are physical changes.

(c) tritium

Tritium ($^3_1H$) has an unstable nucleus (1 proton, 2 neutrons) and emits beta particles. Protium and deuterium are stable.

(b) $\mathrm{(NH_4)_2SO_4}$

$2NH_4OH + H_2SO_4 \to (NH_4)_2SO_4 + 2H_2O$. Two $NH_4^+$ ions balance one $SO_4^{2-}$ ion.

(d) NaOH

NaOH is an alkali, soluble in water. $Fe(OH)_2$, $Cu(OH)_2$ and $Zn(OH)_2$ are insoluble hydroxides.

(c) $\mathrm{CO_2}$ in air

Milk of lime, $Ca(OH)_2$, absorbs $CO_2$ from air and forms a hard white layer of calcium carbonate: $Ca(OH)_2 + CO_2 \to CaCO_3 + H_2O$.

(c) C

The elements with Z = 12 (A), 17 (B), 18 (C) and 11 (F) are in the 3rd period (Mg, Cl, Ar, Na). Ionization potential generally increases across a period, so argon (C, Z = 18), a noble gas with a complete octet, has the highest value.

(a) $\mathrm{H_2O + CO_2 + O_2}$

Moist air contains water vapour, carbon dioxide and oxygen, which together cause rusting of iron.

(a) They contain free electrons

Metals have a sea of free (delocalized) valence electrons that move easily under a potential difference and carry charge and heat.

(b) $\mathrm{CO_2}$

Carbon dioxide has one carbon atom and two oxygen atoms. $\mathrm{H_2O}$ is water and NaCl is common salt.

(a) Vinyl Chloride

PVC (polyvinyl chloride) is formed by addition polymerisation of vinyl chloride, $\mathrm{CH_2{=}CHCl}$.

(a) $\mathrm{X_2Y_3}$

Group VA elements have valency 3 (they gain 3 electrons) and group VIA elements have valency 2. Criss-crossing the valencies gives $\mathrm{X_2Y_3}$.

(b) 2

Oxygen has 6 valence electrons (configuration 2,6) and gains 2 electrons to complete its octet, so its valency is 2.