General Science Previous Year Questions (PYQ) with Answers – Page 54

The cooking temperature is set by the pressure inside the cooker, and that pressure is fixed by the weight sitting on the vent and the area of the hole under it.

Water is called the universal solvent because its molecule is polar: the oxygen end carries a partial negative charge and the two hydrogen ends carry partial positive charges.

The pineal gland, a small pine-cone-shaped gland deep in the brain, secretes melatonin. Melatonin controls the body's day-night rhythm, called the circadian rhythm, and helps regulate sleep.

Roasting means heating a sulphide ore strongly in plenty of air, so that the sulphide turns into an oxide and the sulphur escapes as sulphur dioxide gas.

Pellagra is caused by a lack of vitamin B3, also called niacin or nicotinic acid. The body can also make niacin from the amino acid tryptophan, so the disease is common where people live mainly on maize, which is poor in both.

Vitamins fall into two groups: the water-soluble ones, which are vitamin C and the whole B-complex, and the fat-soluble ones, A, D, E and K. Vitamin C is ascorbic acid, found in citrus fruits, amla, guava and tomatoes; its deficiency causes scurvy, with bleeding gums and slow healing of wounds.

A pyrometer measures very high temperatures, such as those of furnaces, kilns and molten metal, without touching the hot object.

Lead nitrate decomposes on heating into lead(II) oxide, nitrogen dioxide and oxygen. The balanced equation is 2Pb(NO3)2 → 2PbO + 4NO2 + O2.

Phosphorus has five electrons in its outer shell and shows two common valencies, 3 and 5. Phosphorus, atomic number 15, has the electronic configuration 2, 8, 5.

Silver sits near the bottom of the reactivity series and does not combine with oxygen even when heated in air.

It is very cold, chemically inert and cheap, which makes it the standard coolant for preserving cells and tissues. Liquid nitrogen boils at about −196 °C. At such low temperatures the chemical and enzyme activity inside cells almost stops, so tissues can be kept unchanged for years.

Tritium is the only radioactive isotope of hydrogen; its nucleus is unstable and decays by giving out beta particles. Hydrogen has three natural isotopes, all with one proton but different numbers of neutrons.

Penicillin stops bacteria from building their peptidoglycan cell wall, so the weakened cells burst and die. Penicillin binds to and blocks the transpeptidase enzymes, also called penicillin-binding proteins, that cross-link the peptidoglycan chains of the wall.

Sclerenchyma cells lay down thick walls hardened with lignin, and once the wall is complete the protoplasm dies. Sclerenchyma is the plant tissue that gives strength and rigidity.

Phloem carries the sugars made in the leaves to every part of the plant that needs food. Phloem is a complex permanent tissue made of four kinds of cells: sieve tubes, companion cells, phloem parenchyma and phloem fibres.

In a thunderstorm we see the lightning first and hear the thunder later, although both are produced at the same moment.

An angle of incidence of 0° means the ray falls along the normal, and a ray along the normal passes into the next medium without bending.

For an object between F and P, the image forms behind the mirror, not at infinity. When an object is placed between the pole and the principal focus of a concave mirror, the reflected rays spread out and never meet in front of the mirror.

The object lies between F and 2F of the convex lens (25 cm and 50 cm), and such an object always gives a real, inverted and enlarged image beyond 2F. Using the lens formula 1/v − 1/u = 1/f with u = −40 cm and f = +25 cm gives…

The power of a lens measures how strongly it bends light, and +2.5 D has the largest magnitude among the four options.