why cant water, when boiled, reach 100 C exactly?

Various minerals that exist in the water could cause the boiling point to fluctuate.

As an example, people add salt to water that they are boiling in order to lower the boiling point of the water. On the converse scale, salt is also used to lower the freezing point as well, which is why roads are salted to avoid icing of the surface.

Aside from sodium, water can contain other natural minerals, which will also affect the boiling and freezing points. Elevation and pressure will also have an effect on this as well, depending upon whether you are at or below sea level.

I imagine that if you were perfectly at sea level, boiling only distilled water, it would probably be the closest to 100 degrees that you can get.

The boiling temparature depends on surface atmosphere. The atmosphere on the surface of the water is not fixed, but changing continously, that's why the boiling temperature in the water is not fixed too. and if it is not fixed, how can it is exactly 100C ?

Even when the water is boiling in an (unreal, even in lab) ideal outside condition, the temperature is fluctuating up and down the 100C level, in order to balance with the evaporating process, something like a dynamic balancing process.

Impurities, like salts, dissolved in water RAISE the boiling point, not lower it.

At 1 atmosphere of presssure, pure water CAN and DOES reach 100 degrees C before it stars to boil. The temperature water boils at is 100 C, the temperature steal condenses at is 100 C.

A liquid boils when its vapor pressure is equal to the pressure above the liquid. A liquids vapor pressure depends on its temperature.

A reason why you may not get 100 degree boiling water is because the air pressure above the water is lower than 1 atmposphere.

The water is not pure (tap water contains impurities) or the pressure is not at atmospheric level. Impurities in the tap water affects its properties, including boiling point, freezing point, solubility, etc. Pressure can also affect a substance's properties. That is why it is best to check the barometer (measures pressure) before every experiment. Data in books with water having a boiling point of 100 Celsius have been based on pure water at 1 atmospheric pressure.

Because that is the exact boiling point. When it's just below, it's not quite boiling, but once it passes that 100 deg mark, it begins to boil and dissipate steam and energy, lowering the temperature. That process to go from steam to water and water to steam takes a lot of energy, so to maintain that 100 deg exectly is virtually impossible.

Water starts to turn into gas at 100C. The "boiling" only occurs at the surface and that is why water must get warmer than 100C to boil.

Impurities in the water will make it "easier" for the water to boil, either by lowering its boiling point or by providing multiple points of nucleation.

Your teacher may also be theorising that the actual vaporisation of the water will consume energy (Latent Heat of Vaporisation, they used to call it in the stone age when I was at school), thus lowering the temperature of the liquid at the point of boiling...

Minerals in the water and atmospheric pressure affect the boiling point.

Cause it's already boiling, and it's vaporizing. If it wasn't boiling, it'll keep getting hotter till it does.

tough one