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When salt dissolves in water, the ions (positive sodium and negative chloride) form ion-dipole interactions with the water molecules. These interactions almost perfectly balance the lattice enthalpy of NaCl. Therefore, boiling the dissolved NaCl is not exothermic. The ion-dipole bonds also have much lower energy than the hydrogen bonding between water molecules, so the vapor enthalpy of the solution is lower.
The boiling point of a compound is defined as the temperature at which its vapor pressure equals the atmospheric pressure. This is a physical property of the compound, so it does not depend on the molecular weight or structure of the compound itself. For example, ethanol has a lower molecular mass than butane, but it has a higher boiling point.
In the laboratory, you can use boiling points to determine a pure substance’s identity. Put some of the inflammable liquid in a test-tube and seal one end. Place a capillary tube into the inflammable liquid, with its sealed end in contact with the bulb of the thermometer. Heat gently until bubbles start to come out of the capillary tube in a steady stream. Read the temperature and stop heating when the bubbling stops.
During this process, you can also compare the boiling point of the different solutions you create. The solution with the highest boiling point is the most pure. Use a bar graph to display your results. For each solution, make a vertical bar with the boiling temperature at the top of the bar and the name of the solution below it.