Many hands-on STEM activities and demonstrations require the use of a heat source. The challenge is to determine the appropriate heat source based on safety while still meeting the needs of the activity. For example, the Bunsen burner is perhaps the most common heat source found in school science labs. However, it can be difficult to control the temperatures of Bunsen burners compared to electrical heaters (e.g., hot plates). This blog post describes different kinds of heat sources and the safety precautions for each source.
What Are The Options?
The following list describes the safety concerns associated with each heat source.
• Alcohol Burners: Some states have prohibited the use of traditional alcohol lamps with metal caps and wicks. This is with good reason! The vapors from these burners can explode and cause burns. If alcohol lamps are to be used, wickless alcohol burners are a much safer alternative. I recommend that you not use alcohol burners in K–12 classrooms.
• Candles: Candles can be unsafe and dangerous because clothing can catch fire and hot wax can cause burns. However, candles can be used to teach students about heat and fire prevention techniques.
• Electric Hot Plate: Hot plates are electrical appliances made of ceramic, cast iron, and other types of material. Hot plates are generally used at temperatures above 100°C (212°F) and are considered to be much safer than open-flame heaters such as gas burners. Hot plates should only be plugged into a circuit protected by a ground fault interrupter (GFI), which can protect users from electrocutions caused by a spill and exposed wire. Hot plates are the safer alternative for heating materials in middle school science labs.
• Gas Burners: The most common heating source used in academic science laboratories is the gas burner (e.g., Bunsen burners, Tyrell burners). The down side is that it is hard to control the exact temperature of gas burners, and the use of flammable gas in the lab can lead to accidents. Heating organic, flammable liquids such as alcohol with active flames can cause a potential fire. As such, gas burners should be used primarily for heating nonflammable solvents such as water or aqueous salt solutions. A safer gas burner alternative is the portable butane lab burner, which is safer because of it is less likely to fall over. It also delivers trigger ignition, an easy-grip handle, and a simple on/off control.
• Hot Water Bath: This heat source uses a hot plate or Bunsen burner to heat a beaker of water. A second beaker containing a material to be heated is placed in the bath of water created by the first beaker. The water bath transfers heat to the material in the inner beaker. Safety hazards/risks include burns resulting from splashing of hot water on the skin and burns from the active flame.
• Laboratory Incubator: Laboratory incubators are designed to heat biological samples at a specific temperature. For instance, a class can use an incubator to optimize the growth of bacteriological samples. Gas and microbiological incubators are the two main types of incubators.
• Laboratory Oven: Laboratory ovens are generally used to heat samples such as solids at a specific temperature over time. The ovens are used across the scientific disciplines for annealing, drying, and sterilization. Unlike standard cooking ovens, laboratory ovens offer accuracy and uniformity to set temperatures. If the thermostat fails, however, plastic objects could melt and cause a fire. Also, if the combustion temperature of paper is exceeds (80°C), it could also cause a fire.
• Microwave Oven: Though limited in applications/use, microwave ovens can be used in labs to heat liquids or melt solids. There are, however, several safety issues such as potential leaks from containers, ignition of flammable vapors created by the heated samples, and potential for explosion if containers have sealed covers.
Safety Protocols for Using Heat Sources
After selecting the appropriate heating source, be sure to follow the necessary safety precautions. Before lighting each heat source, tie back long hair, wear short sleeves or tight-fitting clothing, and use safety goggles.
• Use only the appropriate burner type for the gas source—e.g., natural gas versus bottled gas.
• Know the location of the master gas shut-off control. Make sure it is operational before using the gas.
• Use only burner tubing connectors that meet the American Gas Association standards. Do not use latex tubing!
• Inspect the burner and hose for any defects.
• Use only ceramic-centered wire gauze on the tripod, not an asbestos-centered pad.
• Use a safety lighter or match to light the burner. Carefully bring the flame up the side toward the top of the barrel while slowly turning on the gas.
• If the gas lights at the base of the burner, shut it down immediately.
• Adjust the flame to the appropriate height and color—i.e., a medium blue flame.
• Remember the gas burner is metal and will get hot. Do not handle it until it cools.
• Never lean forward or reach over the flame.
• Never leave the flame unattended.
Electric Hot Plates
• Make sure the hot plate is plugged into a GFI-protected outlet.
• Only use hot plates with grounded or three-prong plugs with an Underwriters Laboratories listing.
• The hot plate must be clean and dry.
• Inspect the wiring for damage.
• Use caution when handling the hot plate; it could still be hot from a previous user.
• Keep all electrical cords away from water and the hot plate.
• Unplug the hot plate when finished working.
• Never light or burn a candle on or near anything that can catch fire.
• Keep the wick trimmed to about ¼ inch each time before burning.
• Place candle in a stable support on a heat-resistant surface.
• Candles should not be used where there are strong air currents, drafts, or vents.
• Extinguish a candle if it smokes or the flame becomes too high. Check the size of the wick and for air currents.
• Only burn candles in a well-ventilated laboratory.
• Discontinue use of the candle once there is about 2 inches of wax remaining (1/2 inch if in a container).
• Never touch liquid wax.
• Lighted candles should always be within direct view. Never leave a burning candle unattended.
Hot Water Bath
• Follow the same precautions for electric hot plate or gas burner.
• Use caution when working around hot water to prevent a splash and skin burn.
• Immediately wipe up any water spills on the floor.
Laboratory Incubators and Ovens
• Make sure the incubator or oven is plugged into a GFI-protected outlet.
• Only use incubator or oven with grounded/three prong plugs or polarized plug with an Underwriters Laboratories listing.
• Use caution when handling heated objects like glassware; wear heat resistant gloves with elevated temperatures.
• If the materials are being dried in paper bags do not select a temperature that exceeds the combustion temperature of paper (80°C).
• Plastic objects should not be dried in glassware drying ovens. If the thermostat fails, the objects can melt and cause a fire.
• For incubators, do not add water to it until it reaches operating temperature.
• Use distilled water only.
• As soon as incubation or hatching is complete, remove all water from the unit and dry the area that had water in it.
• Do not attempt to heat flammable liquids or solids, hazardous substances, or radioactive materials in any type of microwave oven.
• Do not attempt to remove the interlock switches that prevent a microwave oven from operating with the door open.
• Do not place any wires, cables, or tubing between the door and the seal.
• Do not modify in any way the mechanical or electrical systems of a microwave oven.
• Do not use a microwave oven in a laboratory for food preparation (or vice versa).
• Do not place heat-sealed containers in a microwave oven.