Explanation

When the moon isn't visible from Earth, it's the New Moon phase, occurring when the Moon is between the Earth and Sun, so the sunlit side faces away from us, making it appear dark or invisible. 

New Moon

Full Moon (fully visible)

 First Quarter (half visible)

 Last Quarter (half visible) 

Explanation

Wind blows from areas of high pressure to areas of low pressure.

This is because air molecules move from areas of higher concentration (high pressure) to areas of lower concentration (low pressure) to equalize the pressure difference.

This movement of air is what we feel as wind.

Explanation

Explanation

The major cause of ocean acidification is the absorption of excess atmospheric carbon dioxide CO2 into the ocean,

Primarily from human activities like burning fossil fuels, leading to a drop in pH (increased acidity).

Explanation

• Nitrogen makes up about 78% of the Earth’s atmosphere.
• Oxygen is about 21%,
• Other gases like argon and carbon dioxide are in very small amounts.

Last updated by NA on 28April 2026

Explanation

Because sound is just vibrating air, space has no air to vibrate and therefore no sound in a vacuum.

Sound waves below 20 Hz are known as infrasound.

Sound waves above 20 kHz are known as ultrasound

Sound waves transmit a medium such as a gas, liquid, or solid.

• The speed of sound in air is about 343 meters per second
• The sound travels at 1,481 m/s in water 
• The speed of sound in a vacuum is zero meters per second.

Explanation

• The scientist who first discovered that the earth revolves around the sun was Copernicus.
• Earth's Rotational Theory:
  • Rotation can be defined as the spin of Earth on its own axis from west to east direction.
  • It must also be noted that the speed of rotation of the earth is not constant throughout latitudes.
    
  • It differs at different latitudes.

Explanation

Geothermal, Solar, and Hydropower all come from naturally replenished sources.

These energies are renewable because they do not deplete with use and are environmentally friendly.

Renewable sources of energy include:

• Solar energy
• Wind energy
• Hydropower
• Geothermal energy
• Biomass

Explanation

Explanation

Cooking is difficult in the mountains primarily due to low atmospheric pressure, which affects the boiling point of water. 

Decreased Pressure at High Altitude:

As altitude increases, the atmospheric pressure decreases because there is less air pushing down from above.

Lower Boiling Point:

Water boils when its vapor pressure equals the surrounding atmospheric pressure.

With lower pressure on mountains, water needs less heat to reach its boiling point, so it boils at a lower temperature than the 100°C (212°F) it does at sea level.

Slower Cooking:

Since food cooks based on temperature, boiling it in water that is not as hot (e.g., 92°C at 8,000 feet) means the cooking process takes significantly longer to achieve the same result as at sea level.