Physics & chemistry in everyday life

Answer: (c) 1 and 3 only

The cooking temperature is the boiling point of water at the pressure held inside the cooker, and that pressure is fixed by the weight of the lid's regulator and the area of the vent hole it covers. A hotter flame only makes steam faster; it does not raise the cooking temperature.

• ✓ 1. Steam escapes through the vent hole once its push (pressure × hole area) exceeds the regulator's weight. So the hole area helps decide how much pressure builds up, and water boils at a higher temperature at higher pressure.
• ✗ 2. A bigger flame makes steam sooner, but once the regulator lifts, the extra steam simply escapes. The pressure, and hence the boiling temperature, stays the same.
• ✓ 3. The weight placed over the vent is what the steam must lift. A heavier weight lets more pressure build up, so the water boils at a higher temperature (well above 100 °C).

Remember · Water boils at a higher temperature under higher pressure. In a pressure cooker the vent weight and vent-hole area set the pressure; the flame only sets how fast it is reached.

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Answer: (a) it is dipolar in nature

Water dissolves so many substances because its molecule is dipolar (polar): the oxygen end carries a slight negative charge and the hydrogen end a slight positive charge. These charged ends attract ions and other polar molecules and pull them apart, so they dissolve.

• ✓ (a) The two charged ends of a water molecule attract the particles of many solids, such as the ions in common salt, and separate them into the liquid.
• ✗ (b) Conducting heat well has nothing to do with dissolving. Water is in fact a poor conductor of heat compared with metals.
• ✗ (c) High specific heat means water needs a lot of heat to change its temperature. It explains slow heating and cooling, not solvent power.
• ✗ (d) Being an oxide of hydrogen only describes water's composition (H2O). Many other compounds are oxides and dissolve little or nothing.

Remember · Water is called the universal solvent because its molecule is polar (dipolar), with a positive and a negative end that pull other substances apart.

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Answer: (c) 1 and 3 only

Sodium lamps throw light out in all directions and give a narrow spectrum weighted towards yellow and red, whereas LEDs emit from one side of the chip in a directional beam and can produce broad-spectrum white light. LEDs also last longer than sodium lamps, so statement 2 is the wrong one.

• ✓ 1. Lamp-type sources, such as sodium lamps, give light in nearly all directions, so reflectors and lenses are needed to send it to the road. An LED emits from one side of its chip and is directional.
• ✗ 2. LED products typically last far longer than conventional lamps, with good-quality ones rated at 30,000 to 50,000 hours or more. Sodium lamps do not outlast them.
• ✓ 3. Sodium lamps are narrow-spectrum sources that put most of their output into longer wavelengths such as yellow and red, so the visible light is almost monochromatic. LEDs can give a broad white spectrum, which is the colour advantage the statement claims.

Remember · Against sodium lamps, LED street lights are directional, longer-lasting, more efficient and broad-spectrum; sodium lamps radiate in all directions with a narrow yellow-red-weighted spectrum.

Question and answer: UPSC's official GS Paper I (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). Permalink · Report an error

Answer: (b) Polycarbonate

BPA is the building-block chemical of polycarbonate, the hard, clear plastic used in reusable bottles and food containers. It is also used in epoxy resins, such as the lining of food cans. The official agencies name only these two plastics as its main uses, so the other options fail.

• ✓ (b) Polycarbonate is made by joining BPA units, so BPA is a structural part of it. Small amounts can leach into food or drink, which is why BPA is a health concern (it can act on the hormone system).
• ✗ (a) Low-density polyethylene (carry bags and films) is not among the plastics that US and EU agencies name as made with BPA; they name polycarbonate and epoxy resins.
• ✗ (c) PET, the plastic of soft-drink and water bottles, is not among the plastics that US and EU agencies name as made with BPA; polycarbonate is the BPA-based bottle plastic.
• ✗ (d) Polyvinyl chloride is not among the plastics that US and EU agencies name as made with BPA; they name polycarbonate and epoxy resins.

Remember · BPA goes into polycarbonate plastics and epoxy resins (can linings). It is a suspected hormone disruptor.

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Answer: (d) Speed of light is always same.

A light-year is the distance light covers in one year, and it works as a fixed yardstick because the speed of light in vacuum is constant. Space distances are so large that a unit with a fixed, known value is far more convenient than kilometres.

• ✓ (d) Light travels at about 3 × 10^8 m/s in vacuum, always the same value. So the distance it covers in one year is a fixed length, roughly 9 trillion km.
• ✗ (a) Distances between stars and galaxies do change, because they move; the light-year is chosen for another reason.
• ✗ (c) Light does not always go straight: it bends when it passes from one medium to another, so straight-line travel is not the basis of the unit.
• ✗ (b) The strength of gravity of a star has no role in defining a unit of length.

Remember · A light-year is a distance (about 9.46 trillion km), not a time. It is usable because the speed of light in vacuum is constant.

📘 Read it in NCERT: Class 10 Science, Ch 9 (practise this chapter)

Question and answer: UPSC's official GS Paper I (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error