下雪為何撒鹽在路上呢？

加鹽 (即impurities 雜質) 會令水嘅 boiling point 上升，而 melting point/freezing point 則會下降。
因為需要多啲能量去將雜質，轉化形態，即由水變水蒸氣/水變冰，所以在冰上撒鹽可以令冰快啲溶 (難啲結冰)。

下雪為何撒鹽在路上呢？
為何在冰上撒鹽，冰會慢些溶呢？
Originally, the surface of snow on the ground/ice is at about 0 degree Celcius, so compared to the surrounding tmeperature, the differemce is small, heat transfer is slow, so the snow/ice melts slowly.
When salt is added to the snow/ice, the surface temperature of snow/ice drops far below zero, the difference is enlarged, heat transfer is fasten, so the snow/ice melts more quickly, rather than slowly.
You may check this by putting soft drinks into pure ice and ice with salt added, you may see that the cans put in ice with salt will be cooler.
Also, note that, add salt to ice to snow on ground is to shorten the time for the snow to melt, and make the snow on ground much more easier to be clean.

有人說，加熱下了鹽的水，該水去不到沸點，對不對呢？
鹽對溫度有甚麼影響呢？
The salty water still have a boiling point, note the boiling point may not be at 100 degree Celcius, but higher, so the stove at home may fail to make the salty water boil. This may due to rate of heat loss by water to surroundings = rate of heat gain by water from fire. Only pure water will have boiling point at 100 degree Celcius, unpure water(inc. salty water) will have a different boiling point.

應該係,
在冰上撒鹽, D冰會易D溶 (又或者係, 難D結冰) 窩.

所以有D落雪既地方會享馬路 / 公路上撒鹽,
等D冰會易D溶 (或 難D結冰),
甘D車就唔會 " 線 tie " 囉.

希望幫到你.

其實撒鹽是有助防滑