water was used to set the scale.. ( well one scale of many.. )
0 being when it froze and 100 when it boiled..
and that's pure water ( ie H2O and nothing else.. )
the addition of pollutants does indeed lower the freezing point, which is why slat works.
that's also at sea level as air pressure affects the boiling point ( not sure about freezing point though... )
as a side note, water is at it's densest at 4°C.. either side of that and it expands.. which is why ice floats..