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Increasing the boiling point of water by adding salts or other compounds would be very useful when working with a water bath while refluxing, because with pure water you cannot reach temperatures over 70°C (at that point too much evaporates from the bath). I know that simple table salt would work, but are there other and better compounds/salts available?

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2 Answers 2

For relatively dilute solutions of non-volatile solutes, the following equation (which assumes ideal behavior) can be used to predict boiling point elevation with reasonable accuracy:

$$\Delta T_b = K_b \cdot b \cdot i$$

Here, $K_b$ is the solvent-specific ebullioscopic constant, $b$ is the molality of the solute, and $i$ is the so-called van 't Hoff factor, which accounts for the extent of solute dissociation. For example, if one were to dissolve $\ce{NaCl}$ in water and assume total dissociation, $i = 2$, since two ions are produced in solution. It follows that, ceteris paribus, a salt that dissociates to yield a greater number of ions will produce a greater boiling point elevation. In practice, the magnitude of boiling point elevation is capped by solubility and deviation from ideal behavior at concentration extremes.

Water also forms negative (i.e., higher-boiling) azeotropes with various substances. Addition of a miscible higher-boiling liquid compound can also effect elevation of the boiling point (assuming the mixture does not form a positive azeotrope), a colligative property similar to the above case for salts. Various polyols (e.g., glycerol, propylene glycol, etc.) are commonly used for such applications.

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Hmm, water has a very bad ebullioscopic constant. With table salt I would only add an extra 6°C :( , according to that formula. –  Jori Jul 9 at 19:05
    
With calcium chloride I can get up to ~18 °C using the formula (but that is at extreme concentration 1kg/L and this formula only works with dilute concentration, right?). Would this work well enough to heat water up to 100 °C without evaporation? –  Jori Jul 9 at 19:46
    
@Jori, it should allow the solution to reach a higher temperature before boiling, so with carefully controlled heating you can certainly reduce the evaporation somewhat, but some will always occur. I can't guarantee this will be effective. Personally, I'm of the opinion that an alternative solvent might be preferable (such as ron's suggestion). If you have access to it, you can also consider use of a steam bath. –  Greg E. Jul 10 at 21:47

We always used (silicone) oil baths in the lab for refluxing and distilling. It offers a much wider temperature range than water. Here's two links that describe oil bath usage and lab reaction heating in general.

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