Chemistry Questions and Answers

Answer: C

The nucleus consists of protons and neutrons.

Together they are called nucleons and account for almost the whole mass of the atom. The electrons occupy the surrounding orbitals; the nucleus itself is about \(10^{-15}\) m across, some hundred thousand times smaller than the atom.

Answer: A

The \(\mathrm{H^+}\) ion contains no electrons.

A hydrogen atom has one proton and one electron; removing that electron leaves a bare proton. This is why \(\mathrm{H^+}\) never exists free in solution - it attaches at once to a water molecule to give the hydronium ion \(\mathrm{H_3O^+}\).

Answer: D

Potassium reacts most violently with cold water.

Reactivity increases down Group 1 as the outer electron becomes easier to lose. Sodium melts into a ball and fizzes; potassium reacts fast enough that the hydrogen ignites with a lilac flame. Calcium reacts steadily and magnesium hardly at all with cold water.

Answer: B

The sawdust catches fire because concentrated nitric acid is a powerful oxidising agent.

It oxidises the cellulose rapidly, and the heat evolved is enough to raise the wood above its ignition temperature. Concentrated sulphuric acid, by contrast, chars sugar and wood by dehydration, leaving black carbon rather than setting it alight.

Answer: C

Zirconium is essential in the construction of nuclear reactors.

Zircaloy is used to clad the fuel rods because zirconium has an exceptionally small neutron-capture cross-section - it lets neutrons pass rather than absorbing them - while resisting corrosion by hot water at high pressure. Cobalt and nickel absorb neutrons far more strongly.

Answer: B

This illustrates the law of definite proportions.

Proust's law says a given compound always contains the same elements combined in the same fixed ratio by mass, whatever its source. Water from any origin is 1 part hydrogen to 8 parts oxygen by weight.

Answer: D

Acid rain is caused by sulphur dioxide, carbon dioxide and oxides of nitrogen.

\(\mathrm{SO_2}\) and \(\mathrm{NO_x}\) from burning coal and from vehicle exhaust are oxidised and dissolve to give sulphuric and nitric acids, and \(\mathrm{CO_2}\) gives weak carbonic acid. Rain below about pH 5.6 is counted as acid rain.

Answer: B

For neutral atoms, the periodic trends in atomic size can be described like this
Atomic size decreases when going from left to right across a period,
Atomic size increases when going from top to bottom down a group.
Let's use these trends to try and figure out which atom would have the largest radius.
If you look at elements that are located in the same group, potassium will have a larger atomic radius when compared with sodium. Likewise, bromine will have a larger atomic radius when compared with chlorine.
Now focus on potassium and bromine. Since potassium is located at the start of period 3, and bromine at the end of the same period, potassium will have a larger atomic radius than bromine, and thus the largest atomic radius of the four given atoms.

Answer: A

Diamond is an element.

It is pure carbon, an allotrope in which every atom is \(sp^3\) hybridised and bonded tetrahedrally to four others. Graphite, fullerenes and graphene are other allotropes of the same element.

Answer: C

Acetic acid is the oldest known organic compound.

Vinegar, a dilute solution of it, has been made by fermenting wine since antiquity, and Jabir ibn Hayyan concentrated it by distillation in the eighth century. Urea is significant for a different reason - Wöhler's 1828 synthesis of it was the first laboratory preparation of an organic compound from inorganic material.