General Science Previous Year Questions (PYQ) with Answers – Page 57

When a wire is stretched its volume stays the same, so the length must grow in the same ratio as the cross-sectional area shrinks.

The lemon looks bigger because light from it bends as it passes from water through the curved glass into air.

For a spherical mirror of small aperture, the radius of curvature is twice the focal length, so f = R/2.

The English physician showed in 1796 that deliberate inoculation could protect a person against smallpox. Jenner noticed that milkmaids who had caught the mild disease cowpox did not get smallpox.

Sound is a mechanical wave that needs a medium, so it cannot pass through empty space. Sound travels as a series of compressions and rarefactions passed from particle to particle of a medium.

Liquid pressure at a depth is P = hρg, so a taller column presses harder on the base. In P = hρg, h is the height of the liquid above the point, ρ is the density of the liquid and g is the acceleration due to gravity.

Mitochondria carry their own small, circular DNA and their own ribosomes. Mitochondria are the powerhouses of the cell, where aerobic respiration makes ATP.

The pituitary is a single, pea-sized gland at the base of the brain, while the other three come in pairs.

Vinegar is a dilute solution of ethanoic acid, commonly called acetic acid, in water. Vinegar contains about 5 to 8 per cent ethanoic acid (CH 3 COOH) and is made by the fermentation of sugar or alcohol by acetic acid bacteria.

The N shell is the fourth shell (n = 4), and a shell can hold at most 2n² electrons, which gives 2 × 16 = 32.

Urine is made in the kidneys, carried down by the ureters, stored in the bladder and passed out through the urethra.

Resistivity equals resistance multiplied by area and divided by length, which leaves the unit ohm × length.

Chalk, limestone and marble are all forms of calcium carbonate, CaCO₃. Chalk is a soft, porous rock built from the tiny shells of marine plankton.

A temperature in kelvin is the Celsius temperature plus 273 (more exactly 273.15). The Kelvin and Celsius scales have degrees of the same size, so a change of 1 °C equals a change of 1 K; only their zero points differ.

The black layer on old silver is silver sulphide (Ag₂S). Air contains traces of sulphur compounds, mainly hydrogen sulphide (H₂S) from burning fuels and decaying matter.

Plant cells have a cellulose wall and one large central vacuole, but they do have a plasma membrane, just like animal cells.

The vaccine protects against several pneumococcal diseases and so cuts antibiotic use, but no vaccine is free of side effects.

Digestion begins in the mouth, where saliva adds salivary amylase to the food as it is chewed. Salivary amylase, also called ptyalin, starts breaking starch into maltose while food is still in the mouth.

Adenosine triphosphate stores energy in its phosphate bonds and releases it wherever the cell needs it, so it is called the energy currency of the cell.

The air we breathe out carries carbon dioxide, which reacts with limewater to form a white, insoluble solid that makes it look milky.