Explanation

• Oxygen has 8 protons and typically 8 neutrons, giving it a mass number of 16.
• Mass number = Number of protons + Number of neutrons.
• Discoverers: Joseph Priestley, Carl Wilhelm Scheele
• Symbol: O
• Atomic number: 8
• Electrons per shell: 2, 6
• Discovered: 1774

Explanation

The valency of both hydrogen and potassium is 1.  

Hydrogen (H):

- Atomic number = 1 → has 1 electron.

- Needs 1 more to complete its outer shell → shares or loses 1.

- So, its valency is +1.

Potassium (K):

- Atomic number = 19 → 1 electron in outer shell.

- Easily loses 1 electron to become stable → forms K⁺.

- So, its valency is +1.

Explanation

Solids:

• Have a definite shape and volume.
• Their particles are tightly packed and vibrate in fixed positions.

Liquids:

• Have a definite volume but take the shape of their container.
• Their particles are close but can move past one another.

Gases:

• Have no definite shape or volume and expand to fill their container.
• Their particles are far apart and move freely. 

Explanation

Reflecting telescope: uses a concave mirror (primary) and a plane mirror (secondary).

• Concave mirror collects/focuses light.
• Plane mirror directs it to the eyepiece.
• No chromatic aberration; can be built larger than lenses.

Explanation

• Atoms are made up of subatomic particles.
• The three main particles are protons, neutrons, and electrons.
• Protons have a positive charge.
• Electrons have a negative charge.
• Neutrons have no electric charge.

Explanation

• Chlorine is used to kill harmful bacteria.
• This process is called chlorination.
• One of the most common methods of water disinfection. 
• Chlorine is effective because it:
• Kills or inactivates disease-causing microorganisms, such as bacteria, viruses, and parasites.
• It is relatively inexpensive and easy to apply.
• While other methods like ozone and ultraviolet (UV) light are also used for disinfection

Explanation

• Water covers approximately 71% of the Earth's surface.

Additional information

• Land Coverage: The remaining 29% of the Earth's surface consists of land, including continents and islands.
• Water Distribution: About 96.5% of Earth's water is held in the global oceans as saltwater

Explanation

• Frequency is the number of occurrences of a repeating event per unit of time.
• The SI unit of frequency is the hertz (Hz).
• 1 Hz = 1 cycle or vibration per second
• Unit: Hz = s⁻¹ (per second)
• Named after physicist Heinrich Hertz
• Used for waves, oscillations, sound, and electromagnetic radiation
• Derivations from other quantities: f = 1 / T
• Dimension T^−1

Explanation

The products of combustion include:

Hydrogen gas,

Carbon dioxide,

Water vapor,

Heat and light.

In a complete combustion of a hydrocarbon (like methane or petrol), the fuel reacts with oxygen (O₂) to produce:

CO₂ + H₂O + Heat

Example:  

CH₄ + 2O₂ → CO₂ + 2H₂O + energy

Explanation

• An exothermic reaction releases energy into the surroundings as heat or light.
• Heat released: Energy moves outward.
• Surroundings warm: Temperature rises.
• Negative enthalpy: Delta H < 0
• Lower product energy: Reactants hold more energy.

Additional information:

• In an endothermic reaction, heat is absorbed from the surroundings.
• Examples include photosynthesis and melting of ice.