Explanation

Temporary hardness is caused by:

• Calcium bicarbonate — Ca(HCO₃)₂
• Magnesium bicarbonate — Mg(HCO₃)₂
• Release Ca²⁺ and Mg²⁺ in water.
• Removed by boiling → form insoluble CaCO₃, MgCO₃.
• Permanent hardness (sulfates/chlorides) can't be removed by boiling.

Explanation

The permanent hardness of water is due to the presence of MgSO₄ (magnesium sulfate).

Permanent hardness is caused by dissolved minerals like calcium sulfate (CaSO₄) and magnesium sulfate, which cannot be removed by boiling the water.

Explanation

Water molecules are held together by hydrogen bonds,

It is a type of strong intermolecular force

Where the slightly positive hydrogen of one molecule attracts the slightly negative oxygen of another, creating a "sticky" network that gives water unique properties like surface tension.

Explanation

Polio is also known as poliomyelitis.

It is a disease caused by the poliovirus.

The virus is usually spread through the fecal-oral route, which includes through contaminated water and food.

Explanation

The bottom of the stratosphere is around 10 km (6.2 miles or about 33,000 feet) above the ground at middle latitudes.

The top of the stratosphere occurs at an altitude of 50 km (31 miles).

Explanation

The composition of dry air is:

1. Nitrogen (N2) - approximately 78.08% by volume
2. Oxygen (O2) - approximately 20.95% (21%) by volume
3. Argon (Ar) - approximately 0.93% by volume
4. Carbon Dioxide (CO2) - approximately 0.04% by volume (varies with location and time)

Explanation

Oxygen (O2) is not a greenhouse gas.

Greenhouse gases are gases in the Earth's atmosphere that absorb and emit radiation, trapping heat and contributing to the Greenhouse Effect.

The main greenhouse gases are:

• Carbon dioxide (CO2)
• Methane (CH4)
• Nitrous oxide (N2O)
• Water vapor (H2O)
• Ozone (O3)
• Chlorofluorocarbons (CFCs)
• Hydrofluorocarbons (HFCs)
• Perfluorocarbons (PFCs)

Explanation

The tropopause is the layer that separates the stratosphere and troposphere.

It's a boundary layer located at an average altitude of around 12 km (7.5 miles) above the Earth's surface, where the troposphere ends and the stratosphere begins.

Explanation

Atmospheric pressure decreases as altitude increases.

As you go higher, the air pressure decreases because there are fewer air molecules above you, exerting less pressure.

This is why aircraft pilots must adjust their altimeters to account for changes in air pressure as they ascend or descend.

Explanation

Aeroplanes fly in the troposphere, which is the lowest layer of the Earth's atmosphere, extending up to about 12 km (7.5 miles) above the Earth's surface.

The troposphere is where weather occurs, and it's the layer where aircraft operate, typically cruising at altitudes between 30,000 to 40,000 feet (9,100 to 12,200 meters).