General Science Previous Year Questions (PYQ) with Answers – Page 12

Lead nitrate breaks down only when it is heated strongly, so the reaction absorbs heat and is endothermic. On heating, lead nitrate gives lead oxide, brown fumes of nitrogen dioxide and oxygen: 2Pb(NO3)2 → 2PbO + 4NO2 + O2.

Methane and chlorine react in sunlight to give chloromethane and hydrogen chloride, not hydrogen gas. This is a substitution reaction: CH4 + Cl2 → CH3Cl + HCl in the presence of sunlight.

Acetic acid boils far above methane, and ethanol gives hydrogen with sodium, while statements 1 and 4 are false. Propyne (CH3–C≡CH) is an alkyne: it has one carbon–carbon triple bond and no double bonds, so statement 1 is wrong.

Gypsum heated at 373 K becomes calcium sulphate hemihydrate, CaSO4 · ½H2O, which is plaster of Paris, not CaSO4 · H2O as the statement says.

A substance that loses hydrogen is oxidised, not reduced, so statement A is the incorrect one. In the classical definitions, oxidation means the gain of oxygen or the loss of hydrogen, and reduction means the loss of oxygen or the gain of hydrogen.

Nitrogen dioxide has an unpaired electron, so two of its molecules join to form dinitrogen tetroxide, N 2 O 4.

Xerophytes are plants adapted to live in deserts and other places with very little water; the Greek word "xeros" means dry.

Cholesterol is a sterol, a kind of lipid (fat-like substance) that does not dissolve in water. Cholesterol is made mainly in the liver and also comes from animal foods such as eggs, meat and butter; plants do not contain it.

Mammals have a four-chambered heart, so blood passes through the heart twice in each round, once for the lungs and once for the body, without oxygenated and deoxygenated blood mixing.

The coronary arteries carry oxygen-rich blood to the muscle of the heart wall itself. The heart pumps blood to the whole body, but its own thick muscle (the myocardium) needs a separate supply.

Apical meristems sit at the tips of roots and shoots, and their dividing cells make the plant grow in length.

Xylem is made of more than one type of cell working together, which is what makes a tissue complex. Plant permanent tissues are of two kinds.

In the sodium-calcium exchanger, Na+ ions enter the cell while Ca2+ ions are pushed out, so the two ions cross the membrane in opposite directions.

This is wrong, because a compound is a pure substance with a fixed composition throughout. Pure substances are of two kinds, elements and compounds.

Four grams of hydrogen gas is two moles, so it contains 2 × 6.022 × 10²³, about 1.2 × 10²⁴ molecules, not 6.022 × 10²³.

In facilitated transport, molecules cross the membrane with the help of membrane proteins, while still moving down their concentration gradient.

Spindle fibres attach to the kinetochore, a disc-shaped protein structure on the centromere of each chromosome. During cell division the spindle, made of microtubules, pulls the copies of each chromosome apart.

pH falls, not rises, as hydrogen ion concentration rises, so a more acidic solution has a lower pH. pH is defined as the negative logarithm of the hydrogen ion concentration: pH = −log[H⁺].

Solid carbon dioxide is called dry ice because it turns straight into gas without melting into a liquid, so it leaves nothing wet behind.

Weight is the gravitational force with which the Earth pulls an object towards its centre. Weight is given by W = mg, where m is the mass and g is the acceleration due to gravity, about 9.8 m/s² near the Earth's surface.