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EXPLAIN THE FOLLOWING BRIEFLY 1.gases expert pressure pressure on the walls of the container 2.water is liquid at room temperature 3.evaporation causes cooling

(i) Gases exert pressure on the walls of the container.

Because the gases have freely moving molecules present in it. As they move at a certain speed in the container, they clash with the inner walls of the container. As a result it applies a certain amount of pressure on its walls.
The molecules are in continuous random motion and during their motion continuously strike the walls of container and exert a force on walls, the force per unit area in the pressure of the gas

(ii) Water is liquid at room temperature.
Reason : Hydrogen bonding

Water is a liquid at room temperature due to the presence of hydrogen bond between water molecules.
The state of molecule is determined by (i) intermolecular forces such as dipole interactions, Vanderwaal's forces (also called London dispersion forces) and ii) Hydrogen bonds.
Water is unique in that it has extremely strong hydrogen bonds but a very low molecular weight (18gms per mole). Usually molecules with very low molecular weight are gases at room temperature.
For example: CO₂ - 44gm/mole which is gas O₂, N₂ - 32, 28gm/mole respectively
                     Propne, Butane - 44, 58 gm/mole
All of these are gases and yet have much higher molecular weights than water does. However, one of these can hydrogen bond, and that is why they are gases, whereas water is a liquid.
Off course, at high temperatures water is a gas (steam) and at low temperatures water is a solid (Ice).

(iii) Evaporation causes cooling.

Evaporation is the process by which water is converted from its liquid form to its vapor form and thus transferred from land and water masses to the atmosphere.
The atoms/molecules of the liquid can get converted into vapour if they have enough energy to free themselves from the liquid state. Liquids are closely packed than gases therefore, the energy required to loosen up from their liquid state is taken from the environment.
The atoms/molecules absorb enough energy in the form of heat from the environment and use it to free themselves from the liquid state.
By doing so, they absorb heat from the environment causing the cooling effect.
It does not end here. The atom has enough energy (heat) and therefore it will rise up (hot air being lighter rice up). And cooler (heavier air) takes its place. In this way, evaporation causes cooling.