Chemistry Questions and Answers

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.

Answer: A

Milk is a mixture.

It is an emulsion of fat droplets in an aqueous solution of lactose, proteins and mineral salts, and its composition varies with the animal and its diet. Because the components are not chemically combined, they can be separated physically - churning gives butter.

Answer: C

Ammonia is the compound.

\(\mathrm{NH_3}\) has nitrogen and hydrogen chemically combined in a fixed 1:3 ratio. Mercury is an element, ozone is an allotrope of the element oxygen, and air is a mixture whose composition varies.

Answer: D

Silicon is an element.

It consists of one kind of atom, of atomic number 14. Alumina is a compound, \(\mathrm{Al_2O_3}\); brass is a mixture of copper and zinc; and glass is a mixture of silicates.

Answer: B

Lithium is the lightest metal, with a density of \(0.53\ \mathrm{g\,cm^{-3}}\).

It is only about half as dense as water, so it floats on it - as do sodium (0.97) and potassium (0.86). Hydrogen is lighter still but is a non-metal under ordinary conditions.

Answer: C

Dalton's atomic theory explains the conservation of mass.

Its postulate is that atoms are indivisible and are neither created nor destroyed in a chemical reaction; a reaction only rearranges them. Since the same atoms are present before and after, the total mass cannot change.

Answer: D

Kinetic energy increases with increasing temperature.

For an ideal gas the average translational kinetic energy per molecule is \[\bar{E} = \frac{3}{2}k_BT\] so it is directly proportional to the absolute temperature and independent of the gas's identity. Temperature is, in effect, a measure of that molecular motion.

Answer: D

Graphite is used as a moderator.

Its light carbon nuclei slow fast fission neutrons to thermal energies through repeated elastic collisions, without absorbing many of them. The first reactor, Fermi's Chicago Pile-1 of 1942, was a graphite pile.

Answer: A

This is nuclear fission.

\[\mathrm{^{235}_{92}U + ^1_0n \rightarrow ^{141}_{56}Ba + ^{92}_{36}Kr + 3\,^1_0n + energy}\] The two fragments are of comparable mass, and the extra neutrons released can trigger further fissions, giving a chain reaction. Hahn and Strassmann discovered it in 1938.

Answer: D

The mixture is separated by fractional distillation.

Ethanol boils at 78 °C and water at 100 °C, a difference too small for simple distillation, so a fractionating column is needed. Even then only 95.6 per cent ethanol can be obtained, because the mixture forms an azeotrope at that composition.