why does water boil at 100 degrees c.?

Answer 1

because that is the deffinition of 100C.

Answer 2

That is dependent on the surface pressure.

On a mountain (where the pressure above the surface of the water is less than 760 mm Hg), water boils at a temperature somewhat less than 100 C. In the production of sugar, the vacuum pans drastically reduce the surface pressure, and most of the excess water boils off at a temperature far less than 100 C. In certain large refrigeration systems, water is used as the refrigerant. It boils at about 40 C.

On the other hand, in pressure cookers, the pressure above the surface is higher than 760 mm Hg so the boiling point is above 100 C.

But let us assume that you know all of THAT already. Someone is adding heat to pure water at 760 mm Hg pressure (1 atm). When the water reaches 100 C, it does NOT get hotter but begins to boil. Why? To answer that, I need to explain evaporation. Consider a bowl of water at, say, 30 C. If left standing for 24 hours, it will be noticed that the water level has dropped slightly. Some of the water has evaporated. That is to say, some of the molecules had enough energy to escape through the surface (at the surface, the molecules are packed closer, forming a "skin", somewhat similar to cream forming at the surface of whole milk when it is scald). If more heat is added, the temperature increases, that is, a greater percentage of the molecules now possess that energy required in order to escape the surface skin. Just below 100 C , almost all the molecules now have that energy. At 100 C, ALL the molecules in the water have the same energy which is required to break free. This is why boiling takes place throughout the liquid and not just at the surface. So, to answer your question, it is the temperature at which ALL molecules in the liquid have sufficient energy to break free from the liquid structure/lattice/network.

Answer 3

The centigrade scale is based upon the freezing and boiling points of water at sea level. 0 degrees centigrade was defined to be the point at which water freezes and 100 degrees the point at which water boils.

Answer 4

at normal temperature water will be in dynamic equilibrium
i.e water will be changin to vapour and vapour to water at equal rates, as u go on increasin the temperature the rate of water gettin converted to vapour increases and at 100 degrees there is no equilibrium water just gets converted to vapour

and 100 degrees is the temperature at which the molecules get sufficient energy to move apart and become gaseous
liquid - molecules r close
gas - molecules r far apart

Answer 5

Because that is the temperature at which the vapor pressure of the water equals the pressure of the surroundings.

Answer 6

That is the temperature it takes to "excite" the electrons in the water molecules to create heat by their interaction.

Answer 7

because that is what we based our temperature celcius scale on