Urea, a commonly used nitrogen-based fertiliser, is prepared by the reaction between ammonia and ______.

The correct answer is C. Lead and mercury are in fact the poorest conductors of heat among the metals, so calling them the best is wrong.

Metals conduct heat and electricity because of the free electrons that move through their lattice. Silver is the best conductor of heat, followed by copper, which is why cooking vessels and heat sinks are made of copper and aluminium. Lead and mercury conduct so weakly that they are the usual textbook example of poor metallic conductors.

Option A is a correct statement: most metals melt at high temperatures, tungsten at about 3400 degrees Celsius, though gallium and caesium are exceptions. Option B is correct, since good thermal conduction is a defining property of metals. Option D is correct too, as a fresh metal surface has lustre and is polished for that reason, while tarnishing is due to a layer of oxide.

Exam tip: best conductors silver then copper; poorest among metals lead and mercury.

The correct answer is C, Epithelial tissue. Epithelium covers the body and lines its organs, so it is the protective tissue.

It forms the outer layer of the skin, the lining of the mouth, stomach, intestine, blood vessels and kidney tubules. Its cells sit tightly packed with almost no material between them, resting on a basement membrane, and they act as a barrier against germs, injury and water loss. Epithelium also takes part in absorption and secretion, which is why glands are made of it. Its types are squamous, cuboidal, columnar, ciliated and glandular.

Option A is wrong because muscular tissue contracts to produce movement. Option B is wrong as nervous tissue carries impulses and controls responses. Option D is wrong because connective tissue, such as bone, cartilage, blood and adipose tissue, joins and supports other tissues rather than covering the body.

Exam tip: the four animal tissues - epithelial protects, muscular moves, nervous conducts, connective supports.

The correct answer is A, Moth. Silk comes from the cocoon spun by the caterpillar of a moth.

The common silk moth is Bombyx mori, and rearing it is called sericulture. The female lays hundreds of eggs, the larva or silkworm feeds on mulberry leaves, and when it is ready to pupate it secretes a protein thread from its salivary glands and wraps itself in a cocoon. The cocoons are treated with steam or hot water and the single long fibre is reeled off. Karnataka leads India in silk output, and India is the second largest producer after China.

Option B is wrong because goats give mohair, pashmina and cashmere, which are types of wool. Option C is wrong as sheep give wool. Option D is wrong because butterflies do not spin commercial silk cocoons; both belong to the order Lepidoptera, but only moths are reared for silk.

Exam tip: silk from the moth Bombyx mori and mulberry leaves; wool from sheep and goat.

The correct answer is D, gold. Calaverite is gold telluride, with the formula AuTe2.

It is one of the very few minerals in which gold occurs chemically combined rather than as the free metal, because gold is a noble metal that usually resists reaction. Calaverite was named after Calaveras County in California, where it was found in 1861, and it is an important gold ore in Kalgoorlie in Australia and in Cripple Creek in Colorado. It looks brassy yellow to silvery white and is brittle.

Option A is wrong because the common silver telluride is hessite, and silver's chief ore is argentite. Option B is wrong as zinc is mined mainly as zinc blende, a sulphide. Option C is wrong because copper comes largely from copper pyrites, malachite and cuprite.

Exam tip: link ore to metal - calaverite gold, argentite silver, zinc blende zinc, bauxite aluminium, haematite iron, cinnabar mercury.

The correct answer is D, green. Barium compounds such as barium nitrate give fireworks and signal flares a bright green flame.

The colour comes from a flame test. Heat excites the electrons of the metal ion, and as they fall back they release light of a fixed wavelength that is typical of that metal. Barium gives apple green, so barium nitrate is the standard green agent in pyrotechnics, where it also supplies oxygen to the burning mixture.

Option A is wrong because orange comes from calcium salts such as calcium chloride. Option B is wrong because blue is produced by copper compounds, notably copper chloride. Option C is wrong because yellow is the signature of sodium, which is why a sodium vapour lamp glows yellow.

Exam tip: remember the flame colours - sodium yellow, potassium lilac, calcium brick red, copper blue-green, barium apple green, strontium crimson.