Chemistry Questions and Answers

Answer: D

All the above describe glycolysis.

It is the first stage in the oxidation of glucose, and it breaks one molecule of glucose down to two of pyruvate: \[\mathrm{C_6H_{12}O_6 + 2NAD^+ + 2ADP + 2P_i \rightarrow 2\,CH_3COCOO^- + 2NADH + 2ATP + 2H^+ + 2H_2O}\] It takes place in the cytoplasm and needs no oxygen, which is why it is common to both aerobic and anaerobic respiration.

Answer: B

Estrogen is a steroid.

Steroids are lipids built on a four-ring carbon skeleton, and estrogen, testosterone and cortisol are all derived from cholesterol. Phospholipids form cell membranes and triglycerides store energy.

Answer: A

Ozone is concentrated in the stratosphere.

The ozone layer lies between about 15 and 35 km, where ultraviolet light splits \(\mathrm{O_2}\) and the free atoms combine with more \(\mathrm{O_2}\). The heat released is why the stratosphere, uniquely, grows warmer with height.

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: D

Milk is not a pure substance.

It is an emulsion of fat in an aqueous solution of proteins, lactose and salts, of variable composition. Oxygen is an element and carbon dioxide and water are compounds, each with a fixed composition throughout.

Answer: D

All the above are decomposition reactions.

In each, one compound breaks into two or more simpler substances: \[\mathrm{2HgO \rightarrow 2Hg + O_2},\quad \mathrm{2NaNO_3 \rightarrow 2NaNO_2 + O_2},\quad \mathrm{ZnCO_3 \rightarrow ZnO + CO_2}\] All three here are brought about by heat, so they are thermal decompositions.

Answer: A

The chemical changes are (a) and (b).

Rusting forms hydrated iron(III) oxide and burning magnesium forms magnesium oxide - new substances, and neither change is reversible by simple physical means. Melting ice and dissolving sugar produce no new substance and are reversed by freezing and by evaporation.

Answer: B

Carbon has four valence electrons.

Its configuration is \(1s^2\,2s^2\,2p^2\), so four electrons lie in the outermost shell. Having exactly half an octet, carbon neither gains nor loses electrons readily but shares all four - which is the root of the whole of organic chemistry.

Answer: B

\(\mathrm{CH\equiv C-CH_3}\) is commonly called methylacetylene.

Its IUPAC name is propyne. The common naming treats alkynes as derivatives of acetylene, \(\mathrm{C_2H_2}\), so replacing one hydrogen with a methyl group gives methylacetylene.

Answer: A

An element is the simplest form of matter.

It cannot be broken down into anything simpler by chemical means, since it consists of atoms of one kind only. An atom is a particle rather than a form of matter, and molecules and compounds are built from elements.