General Science Previous Year Questions (PYQ) with Answers – Page 17

The green light that chlorophyll does not use is reflected back to our eyes, so leaves look green. White sunlight contains all the colours of the visible spectrum.

For lenses in contact the powers add: 2.5 D + (−2.0 D) = +0.5 D, and the focal length is 1/0.5 = +2.0 metres.

Sodium hydroxide and hydrochloric acid react to form new substances, sodium chloride and water, so this is a chemical change.

Diamond is an allotrope of carbon, made of carbon atoms alone and nothing else. In diamond each carbon atom is joined to four other carbon atoms by strong covalent bonds, forming a rigid three-dimensional network.

Egg white is mostly water and protein, and heat denatures these proteins so that they set into an opaque white solid.

This is false: aluminium has three electrons in its outer shell and loses all three, so its valency is 3.

Urea, CO(NH2)2, contains about 46 per cent nitrogen, the highest of any common solid fertiliser, and is the main nitrogen fertiliser used in India.

Platelets, or thrombocytes, release the factors that start blood clotting at a wound. Platelets are tiny cell fragments made from large cells called megakaryocytes in the bone marrow; a healthy adult has about 1.5 to 4.5 lakh of them per cubic millimetre of blood.

Dengue virus is a flavivirus, an RNA (ribonucleic acid) virus of the family Flaviviridae. Dengue is spread mainly by the bite of the female Aedes aegypti mosquito, which breeds in clean, stagnant water and bites during the day.

In rusting, iron combines with oxygen in the presence of water to form hydrated iron oxide, and the addition of oxygen is oxidation.

Nitrogen has five electrons in its outer shell and needs three more to complete an octet, so its valency is 3.

Magnesium oxide melts at about 2,850 °C, far higher than the other three, because its small Mg2+ and O2− ions carry double charges and hold each other with a very strong ionic lattice.

Borax is sodium tetraborate decahydrate, Na2B4O7·10H2O, so it contains only sodium, boron, oxygen and hydrogen. Its chemical name is sodium tetraborate, and the ten molecules of water of crystallisation supply the hydrogen and part of the oxygen.

Soap in water forms a lyotropic liquid crystal, a phase whose ordered structure appears because of the amount of solvent mixed in, not because of temperature alone.

The stinging hairs on nettle leaves inject methanoic acid (formic acid, HCOOH) into the skin when touched, and this acid causes the burning pain and itching.

Calcium chloride is an ionic compound, so the sum of the atomic masses in its formula is called the formula unit mass rather than the molecular mass.

In the rock-salt structure of sodium chloride, every Na+ ion is surrounded by six Cl− ions and every Cl− ion by six Na+ ions, so the coordination number is 6 for both.

The statement is false because the latent heat of fusion of ice is high, about 334 joules per gram, or roughly 80 calories per gram.

NCERT's Class 10 chemistry chapter on acids, bases and salts states that our body works within the narrow pH range of 7.0 to 7.8, which is slightly basic.

Water gas is made by passing steam over red-hot coke, and the reaction C + H₂O → CO + H₂ gives carbon monoxide and hydrogen in roughly equal volumes.