Explanation

• The audible range of a normal human ear is 20 Hz to 20,000 Hz (20 kHz).
• Sounds below this range are called infrasound,
• Sounds above it are called ultrasound. 
• Lower limit: 20 Hz (deep rumbles)
• Upper limit: 20,000 Hz (high-pitched squeaks)

Additional information

• Adult change: The upper limit often drops to 15,000–17,000 Hz as people get older. 
• Unit of frequency: Hertz (Hz)

Last updated 27-07-2026 by NA

Explanation

• Damage to alveoli due to smoking leads to reduced gas exchange efficiency in the lungs.
• Alveoli are tiny air sacs where oxygen enters blood and carbon dioxide is removed.
• Smoking destroys alveolar walls, reducing surface area for gas exchange.
• This causes breathing problems and diseases like emphysema and COPD.

Last verified on 22-06-2026

Explanation

• The mass of a proton is approximately 1836 times greater than the mass of an electron.
• A proton is a positively charged particle found in the nucleus of an atom.
• It has almost the same mass as a neutron (slightly less).
• Electrons are much lighter compared to protons.

Last verified on 22-06-2026

Explanation

• Neon gas was discovered by William Ramsay and Morris Travers in 1898.
• It was found during the study of liquefied air and noble gases.
• Neon is a noble gas.
• It is chemically inert (does not react easily).
• It is famous for producing bright reddish-orange light in neon signs.
• It is present in very small amounts in Earth’s atmosphere.

Last verified on 21-06-2026

Explanation

• Theory proposed by: Charles Darwin
• Book published: 1859
• Book name: On the Origin of Species
• Key concept: Natural Selection
• Voyage that inspired him: HMS Beagle expedition (1831–1836)

Last verified on 21-06-2026

Explanation

• The density of gases increases when pressure is increased because gas molecules come closer together.
• Density is the mass per unit volume of a substance.
• When pressure increases, the volume of gas decreases (at constant temperature), causing density to increase.
• This relationship follows Boyle’s Law.

Last verified on 21-06-2026

Explanation

• Lead pencil is made up of graphite.
• Graphite is a form (allotrope) of carbon.
• It is soft and leaves marks on paper easily.
• It is used in pencils because of its slippery layered structure.
• Despite the name “lead pencil,” it contains no real lead.
• Graphite was named by Abraham Gottlob Werner in 1789.

Last verified on 21-06-2026

Explanation

• Diamond is a naturally occurring allotropic form of carbon.
• Allotropy means a single element can exist in more than one form with different structures and properties.
• It is extremely hard.
• It has a high melting point.
• It is widely used in jewelry and cutting tools.

Additional information:

• Graphite – soft, good conductor
• Fullerenes (C60) – spherical molecules
• Carbon nanotubes – tubular, high strength
• Amorphous carbon – coal, soot
• Lonsdaleite – hexagonal diamond

Last verified on 17-01-2026

Explanation

• Alcohol is soluble in water (can dissolve in water).
• It forms hydrogen bonds with water molecules.
• Common example: ethanol mixes completely with water.

Last verified on 20-06-2026

Explanation

• Discoverer: Victor Hess
• Year: 1912
• Method: High-altitude balloon experiments
• Nobel Prize: 1936 (Physics)
• Field: Astrophysics / Space radiation

Last verified on 20-06-2026