Chemistry Questions and Answers

Answer: D

The hottest part of a gas flame is the non-luminous zone.

Where the air supply is plentiful, combustion is complete and the flame burns pale blue at about 1,500 °C. The luminous yellow zone owes its light to glowing unburnt carbon particles and is cooler, because combustion there is incomplete; the dark inner zone is unburnt gas and coldest of all.

Answer: A

The process is smelting.

The roasted or calcined ore is heated with a flux and a reducing agent above the melting point of the metal, so that the metal collects as a molten layer and the gangue floats off as slag. Roasting and calcination, by contrast, are carried out below the melting point and only convert the ore to its oxide.

Answer: A

\(\mathrm{H_2}\) at −73 °C has the highest molecular speed.

The root-mean-square speed is \[u_{rms}=\sqrt{\frac{3RT}{M}}\] so the ranking depends only on \(T/M\):

\(\mathrm{H_2}\): \(200/2=100\); \(\mathrm{CH_4}\): \(300/16=18.8\); \(\mathrm{N_2}\): \(1300/28=46.4\); \(\mathrm{O_2}\): \(273/32=8.5\).

Hydrogen's ratio is the largest by a wide margin, so its molecules are the fastest despite the low temperature.

Answer: B

Such a material is a thermosetting plastic.

Heat and pressure drive cross-linking between the polymer chains, producing a rigid three-dimensional network that cannot be softened and remoulded - bakelite and melamine are examples. A thermoplastic such as polythene has no cross-links and can be melted and reshaped repeatedly.

Answer: A

Fluorine's reactivity follows from its very high electronegativity.

At 4.0 on the Pauling scale it is the most electronegative element, so it pulls electrons from almost anything and forms strong bonds with the resulting energy release. The weakness of the F–F bond - only about \(155\ \mathrm{kJ\,mol^{-1}}\), far less than Cl–Cl - reinforces this, since little energy is needed to break the molecule apart. Fluorine has no d-orbitals available.

Answer: D

Metallic lustre is due to the free electrons in the metal.

The delocalised electrons of the metallic bond absorb light of all visible wavelengths and re-emit it immediately, so the surface reflects nearly all the light that falls on it. The same sea of electrons accounts for metals' electrical and thermal conductivity.

Answer: D

Boiling cannot remove permanent hardness.

Boiling decomposes the bicarbonates of calcium and magnesium and so removes only temporary hardness. Permanent hardness comes from their chlorides and sulphates, which are unaffected by heat; it is removed by adding washing soda or caustic soda, by ion exchange, or by distillation.

Answer: D

One Avogadro number of helium atoms weighs 4.00 g.

By definition, \(N_A=6.022\times10^{23}\) atoms is one mole, and one mole of an element weighs its gram atomic mass. Helium's atomic mass is 4, so \[m = 1\ \text{mol}\times 4\ \mathrm{g\,mol^{-1}} = 4.00\ \mathrm{g}\] Multiplying 4 by \(6.02\times10^{23}\) would give the mass of that many moles, not of that many atoms.

Answer: C

Diamond is the hardest form of carbon, and rates 10 on the Mohs scale.

Each carbon atom is \(sp^3\) hybridised and covalently bonded to four others in a rigid three-dimensional lattice. In graphite the atoms are \(sp^2\) hybridised in sheets held together only by weak forces, so the sheets slide over one another and graphite is soft enough to write with.

Answer: B

Zone refining is used to purify germanium.

The method exploits the fact that impurities are more soluble in the molten than in the solid metal: a narrow molten zone is moved slowly along the rod, sweeping impurities to one end, which is then cut off. It gives the extreme purity that semiconductors such as germanium and silicon require.