General Science Previous Year Questions (PYQ) with Answers – Page 14

Light from the bottom bends as it passes from water into air, so the bottom seems nearer than it really is.

If you hold a current-carrying straight wire in your right hand with the thumb pointing along the current, your curled fingers show the direction of the magnetic field lines around it.

Potassium and barium exchange their partner ions: K₂SO₄ and BaCl₂ swap ions to give KCl and BaSO₄. In a double displacement reaction two compounds exchange ions.

Both a plane mirror and a convex mirror always form an erect, virtual image, whatever the distance of the object.

Iron is a transition (d-block) element, and such elements can show more than one oxidation state. In transition metals the energies of the outer s electrons and the inner d electrons are very close, so atoms can lose different numbers of electrons.

Pulling the hands back stretches the time in which the ball stops, so its deceleration and the force on the hands become smaller.

A body sinks when its weight is greater than the buoyant force of the water it displaces, which happens when it is denser than water.

This is the statement that is not correct; the other three describe the benzene molecule correctly, and UPSC's official key also gives D. Benzene, C6H6, is a flat ring of six carbon atoms.

Heating weakens the attraction between the molecules of a liquid, so its surface pulls less tightly. Surface tension arises because molecules at the surface are pulled inward by their neighbours, making the surface behave like a stretched skin.

A liquid boils when its vapour pressure rises to match the pressure of the air above it. As a liquid is heated, more molecules escape as vapour and its vapour pressure climbs.

Plastics are long chains of carbon atoms joined by strong covalent bonds, which water, air and most microbes cannot easily break.

The heat needed to turn unit mass of a liquid into vapour at constant temperature is its specific latent heat of vaporisation.

A thermocouple has no liquid that can freeze, so suitable types can read temperatures far below −200°C. A thermocouple is made of two wires of different metals joined at a junction.

Graphene is not a wide band-gap semiconductor; it has practically zero band gap and behaves like a semimetal.

Sodium is so reactive that carbon cannot reduce its compounds, so it is obtained by passing electricity through molten sodium chloride.

Inside a long current-carrying solenoid the magnetic field has the same strength and direction at every point, shown by parallel, evenly spaced field lines.

Work is W = F × d × cos θ, and when the angle θ between force and displacement is 90°, cos 90° = 0, so the work done is zero.

A Van de Graaff generator is an electrostatic machine that builds up a very large static charge on a hollow metal sphere, raising it to millions of volts.

Starlight is bent again and again by layers of air of changing density, so its path and brightness keep shifting and the star seems to twinkle.

Planck's constant has the dimensions [ML²T⁻¹], exactly those of angular momentum. From E = hf, h = E / f. Energy has dimensions [ML²T⁻²] and frequency has [T⁻¹], so h has [ML²T⁻²] ÷ [T⁻¹] = [ML²T⁻¹], with the SI unit joule-second.