Heat

• But where the altitude is high due to low atmospheric pressure, the boiling point of water remains less than 100 °C in those places.

Ignition temperature (also called autoignition temperature) is the lowest temperature at which a substance (usually a fuel) will spontaneously catch fire without any external flame or spark.

In other words, it is the minimum temperature needed to start combustion by itself.

For example:

• The ignition temperature of gasoline is around 280°C.

• The ignition temperature of paper is about 233°C.

• Thus, the evaporation process slows down.

• The radiation received by the Earth from the Sun is in the form of short waves, which cannot be absorbed by the atmosphere.
• As a result the penetrating solar radiation reaches the surface and heats the earth.
• On heating, the earth becomes a radiation body and starts radiating energy in the form of long waves in the atmosphere, this process is called Terrestrial Radiation.

• The faster molecules move, the hotter the air.
• As the molecules heat and move faster, they move apart.
• So air, like most other substances, air expands when heated and contracts when cooled.
• Because there is more space between the molecules, the air is less dense than the surrounding matter and the hot air floats upward.
• This is the concept used in hot air balloons.
• The air is heated by the burner and the expanding air becomes less dense, causing the balloon to rise through the denser, cooler surrounding air.

When ice cubes are made (water freezes), the entropy of water decreases.

Explanation:

• Liquid water has more molecular disorder because molecules move freely.

• When water freezes into ice, molecules become more ordered and fixed in a solid lattice structure, so the randomness (entropy) decreases.

Entropy is a measure of the disorder or randomness in a system.

More formally:

• It quantifies the number of possible microscopic arrangements (microstates) that correspond to the macroscopic state of a system.

• In thermodynamics, entropy (denoted as $S$) is a state function that tends to increase in spontaneous processes, reflecting the second law of thermodynamics.

• This is because the addition of an external impurity (such as salt to ice) disturbs the crystalline form of the frozen water, causing the ice to melt at the slightest heat.
• That is, the liquefaction of ice starts at a temperature of less than 0°C.

• As water cools from room temperature to 4°C, it contracts and becomes denser.
• Below 4°C, water begins to expand again due to the formation of a more structured hydrogen-bonded lattice (as it approaches freezing).
• This expansion makes ice less dense than liquid water — which is why ice floats!

• White surfaces reflect most wavelengths of visible light.
• Black surfaces absorb all wavelengths and convert them into heat.
• So when you're wearing white clothes in the sun, they reflect sunlight and absorb less heat, keeping you cooler.
  In contrast, black clothes soak up more heat, making you feel hotter.