Everyday Science

The sun is primarily composed of hydrogen (about 74%) and helium (about 24%), making up approximately 99.9% of its mass. This composition is well-established by astrophysical observations and spectroscopy.

An equinox occurs when the duration of day and night are approximately equal all over the Earth, which happens twice a year when the sun crosses the celestial equator.

Water vapor condenses into clouds when the air temperature drops to the dew point, causing the vapor to change into liquid droplets that form clouds.

Mercury is commonly known as 'Quick Silver' due to its shiny, silver appearance and its liquid, fast-moving nature at room temperature. This nickname accurately reflects its physical properties.

A 1 KWh (kilowatt-hour) unit of electricity means consuming 1000 watts for 1 hour. A 100 watt bulb consumes 100 watts per hour, so it will take 10 hours to consume 1 KWh (100 watts × 10 hours = 1000 watt-hours = 1 KWh).

The seasons change because the Earth revolves around the Sun, causing different parts of the Earth to receive varying amounts of sunlight throughout the year. This revolution, combined with the tilt of Earth's axis, leads to seasonal changes.

A substance is a form of matter that has a uniform and definite composition. Graphite is a pure form of carbon, making it a substance. Sea water and tap water are mixtures, and brass is an alloy (a mixture of metals), so they are not pure substances.

The standard quantity of fresh air required for a man is approximately 1000 cubic feet every 5 minutes to maintain adequate oxygen levels and remove carbon dioxide. This is a common ventilation guideline in everyday science and health contexts.

18 carat gold means 18 parts out of 24 are pure gold. The ratio is therefore 18/24 = 0.75, which corresponds to option D.

Galvanised iron sheets are coated with zinc to prevent rusting and corrosion. Zinc acts as a protective layer, which is why option C is correct.