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In the laboratory, nacl melting and boiling point can be measured using a capillary tube and thermometer. The melted or evaporated substance is allowed to cool and the temperature of the cooling surface of the liquid or vapour is recorded. The resulting curve is a plot of temperature versus time and can be used to identify the substance being tested. For example, if solder filings are heated in a crucible with powdered charcoal the melting point behaviour of the mixture can be observed. The solder melts and forms a liquid below the charcoal. This can be poured out and is then tested to determine whether the solution is pure.
The molal concentration of a solution is defined as the number of moles of solute per kilogram of solvent. A solution with a non-volatile solute has a higher boiling and melting point than the pure solvent. This occurs because of the interaction between the solute and the solvent molecules. A non-volatile solute also exerts an ionic charge on the solvent molecules and this can have a bigger effect than van der Waals forces. For example, adding a tablespoon of sodium chloride (table salt) to five litres of water increases the boiling point by about 0.5°C. This is because sodium chloride dissociates into Na and Cl ions.
This experiment is an ideal way to illustrate a point about boiling and melting points. The nacl melting and boiling point of a substance is dependent on the type of solute, the nature of the solvent and its surrounding atmospheric pressure. For example, tungsten has high melting and boiling points on the Celsius scale but lower ones on the Kelvin scale.
