General Science GK Questions with Answers – Page 69

A wave carries energy from one place to another, while the particles of the medium only vibrate about their fixed positions.

Equal and opposite forces acting for the same time give equal and opposite changes in momentum, so the total stays the same.

Atoms get smaller along a period and larger down a group. Moving from left to right in a period, electrons are added to the same shell while the nuclear charge rises by one at each step.

The speed of the wave is frequency times wavelength, 700 m/s, and 2,100 m at 700 m/s takes 3 seconds.

Prokaryotes have no true nucleus, so there is no membrane around their genetic material. Prokaryotes are the simplest cells: bacteria and cyanobacteria (blue-green algae).

Hedgehogs and pangolins curl into a tight ball when threatened, covering the soft belly and face with spines or scales; marmots do not.

Lemongrass and its close relative citronella grass yield aromatic oils that keep mosquitoes away, so they are the base of many natural repellents.

A diode does not obey Ohm's law: the current through it is not proportional to the voltage across it. An ohmic conductor gives a straight-line graph of current against voltage, so its resistance stays constant at a fixed temperature.

Anhydrous sodium carbonate is Na₂CO₃, and 2 × 23 + 12 + 3 × 16 = 106 u. Formula mass is the sum of the atomic masses of all the atoms in one formula unit of an ionic compound.

Pitchblende, or uraninite, is the chief ore of uranium, the fuel used in most nuclear power stations. Pitchblende is a heavy, black mineral made mostly of uranium oxide.

Portland cement is made by heating a mix of limestone, which supplies lime, with clay, which supplies silica and alumina.

Traditional solder is an alloy of lead and tin, often in a ratio of about 60 parts tin to 40 parts lead.

The trapped steam raises the pressure inside the cooker, and at higher pressure water boils at a higher temperature, so food cooks faster.

Mercury(II) oxide is a red-orange solid, mercury is a liquid metal at room temperature, and oxygen is a gas. This is a thermal decomposition reaction: when mercuric oxide is heated, it breaks down into mercury and oxygen, written with states as 2HgO(s) → 2Hg(l) + O₂(g).

One mole of N₂ weighs 28 g, so half a mole weighs 14 g. A nitrogen molecule is made of two nitrogen atoms, each with an atomic mass of 14 u, so its molecular mass is 28 u and its molar mass is 28 g per mole.

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.

Soft sand gives way slowly, so the person is brought to rest over a longer time and the force on the body is smaller.

A Mach number greater than 1 means the body is moving faster than sound in the surrounding medium. The Mach number is the ratio of a body's speed to the speed of sound in the medium around it, named after the Austrian physicist Ernst Mach.

An object denser than the liquid sinks, and one less dense than the liquid floats. By Archimedes' principle, a body placed in a liquid feels an upward buoyant force equal to the weight of the liquid it displaces.

Milk is a colloid in which tiny droplets of fat are dispersed in water, so it is not uniform at the microscopic level and counts as a heterogeneous mixture.