When salt is spread on a snow-covered road at -2C, the snow melts. Though when the same amount of salt is spread on the snow covered road at -30C, nothing happens. Explain?
Is this diue to beinf ionic, and nonvolatile vapor and volatile vapor pressure?
Personally I think the question is too vague; therefore, I don't think it is a good question. Why does the first application at -2 degrees C melt? It is because the NaCl lowers the melting point of ice to below -2 so it melts. The second application with snow at -30 C will melt, too, if enough NaCl is added. The question might make more sense if the first application at -2 said that just enough NaCl was added to melt it, then that SAME amount of NaCl would not melt snow at -30.
DrBob222 is perfectly correct although it does say in the question that the same amount of salt is added at both temperatures. when the snow is at -2 degrees the addition of the NaCl lowers the temperature that the snow melts at to somewhere between -2 and -30 degrees. However the same amount of NaCl being added to the snow at -30 degrees does not lower the melting point below -30 degrees, which of course means that the snow will not melt.
The reason why salt is effective at melting snow on a road is primarily because of its ability to lower the freezing point of water. When salt, which is an ionic compound, is spread on a snow-covered road, it dissolves into its constituent ions, namely sodium (Na+) and chloride (Cl-). These ions then interact with the water molecules in the snow.
At -2°C, the salt ions disrupt the hydrogen bonding between water molecules, preventing them from forming a solid crystal lattice and reducing the freezing point of the solution. This means that the water can remain in the liquid phase, even at temperatures below 0°C. As a result, the snow melts when in contact with the salt.
However, at -30°C, the freezing point of the saltwater solution is much lower than the temperature itself. At this extremely low temperature, the energy required for the salt ions to disrupt the water molecules' hydrogen bonding is too high. As a result, the salt is unable to lower the freezing point of the solution significantly enough to cause melting. Consequently, the salt has minimal to no effect on the snow-covered road at -30°C.
Regarding your mention of nonvolatile and volatile vapor pressure, these concepts are not directly related to the ability of salt to melt snow. Nonvolatile refers to substances that do not readily evaporate or have low vapor pressures, while volatile substances have high vapor pressures and readily evaporate. In the case of salt melting snow, it is primarily the effect of the lowered freezing point that causes the snow to melt, rather than vapor pressure considerations.