Illustrated Guide to Home Chemistry Experiments

Chapter 5: Mastering Laboratory Skills 85

AvIDSO UPERHEATInG

As strange as it sounds, it’s possible for a liquid to be

heated above its boiling point without boiling. A liquid

in that unstable state is superheated, and is extremely

dangerous. A superheated liquid can begin boiling

spontaneously and explosively, ejecting large amounts

of boiling liquid from the container. Microwave ovens are

notorious for superheating liquids, but it is also possible

to superheat a liquid with an alcohol or gas burner,

particularly if the heat is focused on a small part of the

bottom of the container.

To avoid superheating, add a boiling chip to the liquid

before you begin heating it, particularly if you are heating

it in a microwave oven. A boiling chip is simply a pebble

of limestone or a similar porous material that prevents

superheating from occuring by providing a locus for

boiling to begin. If you are boiling a liquid in a beaker or

flask over an alcohol or gas burner, use a ceramic wire

gauze to spread the heat and place a stirring rod in the

beaker or flask so that the tip of the stirring rod is in

contact with the bottom of the container at the point

where the heat is most intense, as shown in Figure 5-16.

Like a boiling chip, the stirring rod provides a locus for

boiling to begin, preventing superheating from occurring.

Avoid superheating when you heat a test tube by directing

the flame toward the middle of the solution rather than the

bottom, as shown in Figure 5-17, and keep the tube moving

constantly to distribute the heat.

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FIGURE 5-15:

Using a Beral pipette to separate layers

USING HEAT SoURCES

Heat sources are essential in any chemistry laboratory, but you risk being burned any time

you use a heat source. Heat sources that use open flame add the risk of fire. Use the following

guidelines for safe use of heat sources.

• o minimize the danger of fire, use a hotplate rather than an T
  alcohol lamp or gas burner whenever possible.


• When you use flame, always have a fire extinguisher handy.
  Know how to use it.


• Before you light a burner, make absolutely certain that there
  are no flammable substances nearby, including burner fuel that
  is not in the burner.
  
  
  • Never use open flame to heat a container that contains a
    flammable substance.
  
  
  • When you finish using a heat source, turn it off or extinguish it
    immediately.
  
  
  • Remember that hot glass looks exactly like cold glass. Always
    use tongs or insulated gloves to handle glassware unless you
    are certain that it has cooled.