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A hotplate is used to heat objects or containers. Flasks or beakers containing solutions can be heated directly or indirectly depending upon the shape of the container and the desired temperature. Always use the lowest temperature setting possible when heating.
Note: NEVER heat a volumetric flask.
Direct heating:
Beakers and Erlenmeyer flasks can be heated directly on a hotplate. These containers have a flat bottom and heat will be spread evenly throughout the solution when the solution is also stirred. Other containers that do not have a flat surface should be heated indirectly. Controlling the temperature of the solution is more difficult when using direct heating. Even low temperature settings on the hotplate may reach temperatures in excess of 100°C and the temperature of the plate will flucuate due to air currents or room temperature changes.
Indirect heating:
When the container does not have a flat surface or more control over the temperature is required, indirect heating should be used. To indirectly heat a container, set up a bath on the hotplate. The bath is assembled by placing a liquid or solid in a large beaker (or a container that looks like a beaker). The bath is heated and the sample is placed in the bath.
Selection of the liquid or solid can help to control the temperature of the bath. If a sample is not to be heated above 100°C, then water can be used in the bath (since liquid water can not be heated above its boiling point). Likewise, other liquids could be chosen to control the amount of heating. Liquids should not be flammable or present any other hazards. Remember to replace the liquid as it evaporates. Sand is a common solid used in heating baths. Sand can be heated to very high temperatures without having to be replaced due to evaporation.
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View the video clip. Is there a better way to check to see if the plate is hot?