Chemistry Questions and Answers

Answer: C

The pH of pure water is 7 at 25 °C.

Water self-ionises to the extent that \[[\mathrm{H^+}] = [\mathrm{OH^-}] = 10^{-7}\ \mathrm{mol\,L^{-1}}\] so \(\mathrm{pH} = -\log(10^{-7}) = 7\). The value drifts below 7 at higher temperatures, but the water stays neutral because the two concentrations remain equal.

Answer: C

Formaldehyde is used to preserve specimens.

Formalin, a 40 per cent aqueous solution, cross-links the amino groups of proteins and so hardens the tissue and kills any bacteria or fungi that would decompose it. Its own pungency and toxicity are the drawback.

Answer: A

PVC stands for polyvinyl chloride.

It is the addition polymer of vinyl chloride, \(\mathrm{CH_2{=}CHCl}\). Rigid PVC is used for pipes and window frames; adding a plasticiser makes it flexible enough for cable insulation and flooring.

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: C

John Dalton proposed the first atomic theory, in 1808.

His postulates were that matter consists of indivisible atoms, that all atoms of an element are alike in mass and properties, and that compounds form when atoms combine in fixed small whole-number ratios. The theory accounted at once for the laws of conservation of mass and of definite and multiple proportions.

Answer: B

The mass number is the sum of the protons and neutrons in the nucleus, together called the nucleons.

It is always a whole number and is at least equal to the atomic number, which counts protons only. Atomic weight, by contrast, is an average over the natural isotopes and so is usually fractional.

Answer: B

Copper was the first metal used by man.

It occurs native and is soft enough to hammer cold, so it was worked long before smelting was understood; the Chalcolithic or Copper Age is named for it. Bronze is an alloy of copper and tin and came later, and iron later still because it needs much higher temperatures to smelt.

Answer: D

Hydrogen is called the fuel of the future.

It has the highest energy content per unit mass of any fuel, about \(142\ \mathrm{kJ\,g^{-1}}\), and its only combustion product is water: \[\mathrm{2H_2 + O_2 \rightarrow 2H_2O}\] The obstacles are producing it without fossil fuels and storing a gas of such low density safely.

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

The law of constant proportions was enunciated by Joseph Proust in 1799.

It states that a given compound always contains the same elements in the same proportion by mass, whatever its source or method of preparation - water is always 1 part hydrogen to 8 parts oxygen. Dalton's law of multiple proportions came a few years later.