Everyday Science Mixed Quiz 5: Sound, Electricity and Household Chemistry, WBPSC Clerkship GS

The loudness level of sound is measured in decibels (dB). The hertz (Hz) is the unit of frequency, which sets the pitch of a sound, not its loudness. The watt is the unit of power and the newton the unit of force.

The sensation of a sound stays in the ear for about 0.1 s, so the echo must return at least 0.1 s later to be heard separately. In that time the sound goes and comes back, so the distance is $d = \frac{v \times t}{2} = \frac{344 \times 0.1}{2} = 17.2$ m, that is about 17 m. 34 m is the total path there and back, not the one-way distance.

Household electricity in India is alternating current with a frequency of 50 Hz: it goes through 50 full cycles of reversing its direction every second. 60 Hz is used in some countries such as the USA. The number 220 belongs to the voltage (220 V), not the frequency.

One unit on the bill is 1 kilowatt-hour (kWh): the energy used when a 1,000-watt appliance runs for 1 hour. It is the commercial unit of energy, $1 \text{ kWh} = 3.6 \times 10^{6}$ joules. The kilowatt and the watt are units of power, not energy; the joule is a unit of energy but far too small for a bill.

A motor takes electrical energy and turns, that is, it converts electrical energy into mechanical energy; fans, mixers and pumps all work this way. A generator (dynamo) does exactly the reverse: mechanical energy into electrical energy. A battery or cell converts chemical energy into electrical energy.

An ant's sting contains formic acid (methanoic acid), which is also found in the hairs of nettle leaves. Applying a mild base such as baking soda neutralises the acid and eases the pain. Citric acid is in lemons, acetic acid in vinegar and lactic acid in sour curd.

Glands in the stomach wall secrete hydrochloric acid (HCl); it kills germs in food and activates pepsin, the enzyme that digests protein. Too much acid causes acidity, and a mild base such as milk of magnesia (magnesium hydroxide) neutralises it as an antacid. Sulphuric and nitric acids are strong industrial acids, not made in the body.

The chemical name of bleaching powder is calcium oxychloride (CaOCl₂); it is made by the action of chlorine on dry slaked lime. Its chlorine kills germs, so it is used to treat drinking water and well water, and also to bleach cloth. Calcium carbonate is limestone, calcium sulphate is in gypsum, and sodium chloride is common salt.

When gypsum (CaSO₄·2H₂O) is heated at about 373 K (100°C) it loses part of its water and becomes Plaster of Paris (CaSO₄·½H₂O). Mixed with water, Plaster of Paris turns back into gypsum and sets hard, which is why it holds broken bones in place and is used for statues. Limestone gives lime, bauxite aluminium and haematite iron.

Washing soda is sodium carbonate with ten molecules of water (Na₂CO₃·10H₂O); it is used for washing clothes and softening hard water. Sodium bicarbonate (NaHCO₃) is baking soda, used in cooking. Sodium chloride is common salt and sodium hydroxide is caustic soda.

Carbon dioxide does not burn and does not help other things burn, and being heavier than air it covers the fire like a blanket and cuts it off from oxygen. That is why fire extinguishers use it; it is also safe on electrical fires where water is not. Oxygen helps burning, while hydrogen and methane are themselves flammable.

LPG is mainly butane and propane, which have no smell of their own. So that a leak can be noticed, a little strong-smelling ethyl mercaptan (a sulphur compound) is added. Methane is the main part of CNG and biogas, and carbon monoxide, though poisonous, has no smell.

Breathing nitrous oxide (N₂O) brings mild drowsiness and a laughing mood, hence the name laughing gas; it has been used for mild anaesthesia in dentistry. Nitric oxide (NO) and nitrogen dioxide (NO₂) are other oxides of nitrogen, linked to air pollution and acid rain. Ammonia is a pungent, alkaline gas.

Water with dissolved salts of calcium and magnesium is hard: soap does not lather in it and scum forms. Their bicarbonates cause temporary hardness, removed simply by boiling; their chlorides and sulphates cause permanent hardness, which needs a chemical such as washing soda. Sodium and potassium salts do not make water hard.

The stapes (shaped like a stirrup), one of the three tiny bones of the middle ear, is the smallest bone in the body, only about 3 mm long. The three middle-ear bones (malleus, incus, stapes) carry the vibrations of sound to the inner ear. The femur (thigh bone) is the longest and strongest bone; the radius is in the arm and the tibia in the leg.

Ethylene is a gaseous plant hormone that ripens fruit; that is why raw fruit kept beside ripe fruit ripens faster. Auxin controls the growth of stems and roots and their bending towards light, gibberellin makes stems longer, and cytokinin helps cell division.

Burning coal, petrol and diesel releases sulphur dioxide and oxides of nitrogen into the air; these combine with water vapour to form sulphuric and nitric acids that come down with the rain. Acid rain damages marble monuments such as the Taj Mahal and makes soil and water bodies acidic. CFCs cause the depletion of the ozone layer, not acid rain.

An anemometer measures wind speed; a common anemometer has a few cups that spin in the wind, and the rate of spinning gives the speed. Air pressure is measured by a barometer, humidity by a hygrometer and rainfall by a rain gauge. A weather station keeps all of these together.

A light-year is the distance light travels in a vacuum in one year, so it is a unit of distance, not time. Light travels at about $3 \times 10^{8}$ m/s, so one light-year is about $9.46 \times 10^{15}$ m. It is used for the huge distances to the stars.