Science Quiz for Class 8

Which of these is a kharif crop, sown at the start of the monsoon?

• Wheat
• Gram
• Paddy
• Mustard

Answer: Paddy. Kharif crops are sown in the rainy season, from about June to September, and paddy (rice) needs plenty of water. Wheat, gram and mustard are rabi crops, sown in winter.

A mosquito's buzz and a lion's roar: which property of sound is much higher for the mosquito?

• Loudness
• Pitch
• Amplitude
• Speed

Answer: Pitch. A mosquito's wings vibrate very fast, so its sound has a high frequency and a high pitch. A lion's roar is louder (bigger amplitude) but much lower in pitch.

School bags have wide straps. Why?

• To spread the weight and reduce pressure on the shoulders
• To make the bag lighter
• To increase friction with the shirt
• To make the bag look bigger

Answer: To spread the weight and reduce pressure on the shoulders. Pressure = force ÷ area. The same weight spread over a wider strap gives less pressure, so it hurts less. Thin straps dig in.

Ball bearings are used in bicycle hubs and ceiling fans. How do they help?

• They add weight for balance
• They stop the parts from rusting
• They replace sliding friction with rolling friction, which is smaller
• They increase friction for better grip

Answer: They replace sliding friction with rolling friction, which is smaller. Rolling friction is much smaller than sliding friction, so parts that roll on ball bearings turn easily and wear out more slowly.

Which of these is stored under water because it catches fire on its own in air?

• Sodium
• Sulphur
• Iodine
• White phosphorus

Answer: White phosphorus. White phosphorus catches fire in air at room temperature, so it is kept in water. Sodium is the opposite case: it reacts with water, so it is kept in kerosene.

Bathroom taps, gas burners and bicycle handlebars often get a shiny, scratch-resistant coating by electroplating. Which metal is used?

• Chromium
• Copper
• Silver
• Gold

Answer: Chromium. Chromium is shiny, does not corrode and resists scratches. It is costly, so only a thin layer is electroplated over a cheaper metal.

Which of these fuels gives the most heat per kilogram burnt, that is, the highest calorific value?

• Petrol
• LPG
• Coal
• Hydrogen

Answer: Hydrogen. Hydrogen's calorific value is about 150,000 kJ/kg, far above LPG (about 55,000) and petrol (about 45,000). And when it burns, it gives only water.

A lightning conductor on a tall building protects it by...

• stopping clouds from forming
• carrying the electric charge safely into the ground
• pushing the lightning towards other buildings
• storing the charge for later use

Answer: carrying the electric charge safely into the ground. A metal rod at the top runs down into the earth. If lightning strikes, the charge flows through the rod into the ground instead of through the building.

During adolescence, many boys get a bump at the front of the throat called the Adam's apple. What is it?

• A swollen gland
• An extra bone
• The growing voice box (larynx)
• The thyroid gland

Answer: The growing voice box (larynx). At puberty the larynx grows, and in boys it often sticks out as the Adam's apple. A bigger voice box is why boys' voices get deeper.

A ray of light hits a plane mirror with an angle of incidence of 30°. What is the angle between the incident ray and the reflected ray?

• 30°
• 60°
• 90°
• 120°

Answer: 60°. The angle of reflection equals the angle of incidence, 30°, both measured from the normal. So the angle between the two rays is 30° + 30° = 60°.