Which of the following is an amphoteric oxide?

The correct answer is B. In a double displacement reaction two compounds exchange their ions, so both reactants must be compounds. Here hydrogen and oxygen are elements combining to form a single product, water, which makes it a combination reaction and also a combustion and redox reaction, but certainly not a double displacement. Option A is wrong as a choice because barium chloride and sulphuric acid swap partners to give a white precipitate of barium sulphate and hydrochloric acid, the classic precipitation type of double displacement. Option C is wrong because copper sulphate and hydrogen sulphide exchange ions to give black copper sulphide and sulphuric acid. Option D is wrong because sodium hydroxide and hydrochloric acid exchange ions to give salt and water, which is neutralisation, a special case of double displacement. The test to apply is simple: look for two compounds exchanging partners.

The correct answer is D, ethylene. Ethylene, also called ethene, has the formula C2H4, and its two carbon atoms share two pairs of electrons, which is a carbon-carbon double bond made of one sigma and one pi bond; it is the simplest alkene. Option A is wrong because ethane, C2H6, is an alkane in which the two carbons are joined by a single bond only. Option B is wrong because methane, CH4, has just one carbon atom joined to four hydrogens by single bonds and can have no carbon-carbon bond at all. Option C is wrong because acetylene, or ethyne, C2H2, has a triple bond, three shared pairs made of one sigma and two pi bonds, not a double bond. Learn the family in order: ethane single, ethylene double and acetylene triple, and remember that the double and triple bonded compounds are unsaturated and decolourise bromine water.

The correct answer is A, 12 and 3. Benzene has the formula C6H6 and a ring of six carbon atoms, each carrying one hydrogen. Count the sigma bonds first: six carbon-carbon bonds around the ring and six carbon-hydrogen bonds give twelve sigma bonds. The ring also carries three carbon-carbon double bonds in the Kekule structure, and since every double bond is one sigma plus one pi, the extra component is three pi bonds. Option B is wrong because three counts only the pi bonds twice over and ignores the twelve sigma framework. Option C is wrong because six counts only the carbon-carbon sigma bonds and leaves out the six carbon-hydrogen bonds. Option D is wrong because nine is simply a distractor with no basis in the structure. The general rule to carry into the exam is that a single bond is one sigma, a double bond one sigma and one pi, and a triple bond one sigma and two pi.

The correct answer is B, Ni, that is, nickel. Vegetable oils contain unsaturated fatty acids with carbon-carbon double bonds, and when hydrogen is passed through the oil at about 60 degrees Celsius in the presence of finely divided nickel, the double bonds add hydrogen and become saturated, turning the liquid oil into semi-solid vanaspati. Option A is wrong because lead is not a hydrogenation catalyst; it is in fact used as a catalyst poison in some reactions. Option C is wrong because hydrogen is the reactant being added across the double bond, not the catalyst, and the question asks for the substance that speeds the reaction without being consumed. Option D is wrong because helium is a noble gas, chemically inert, and takes no part in any such reaction. Remember the set of industrial catalysts: nickel for hydrogenation, iron for the Haber process and vanadium pentoxide for the contact process.

The correct answer is B, ionic bond. In an ionic or electrovalent bond a metal atom loses one or more electrons to a non-metal atom; the metal becomes a positive ion and the non-metal a negative ion, and the electrostatic attraction between the two holds the compound together, as in sodium chloride and magnesium oxide. Option A is wrong because a covalent bond is formed by the sharing of electron pairs between two atoms, typically two non-metals, with no transfer at all. Option C is wrong because a coordinate bond is a special covalent bond in which both shared electrons come from one atom, as in the ammonium ion; the electrons are still shared, not transferred. Option D is wrong because a hydrogen bond is a weak attraction between a hydrogen atom already bonded to a very electronegative atom and a lone pair on another such atom, and it is not a true chemical bond of this kind.