This article incorporates, in modified form, material from Illustrated Guide to Home Chemistry Experiments: All Lab, No Lecture.
A foam is a colloidal gas phase dispersed in a liquid continuous phase. Foams are commonplace in everyday life. The lather produced by shampoo is a foam, as are sea foam, shaving cream, marshmallow, the meringue in a lemon-meringue pie, and the “head” on a glass of beer.
Foams are an example of an unstable colloidal system. In the ordinary course of things, colloidal gas bubbles dispersed in a liquid quickly coalesce into larger and larger gas bubbles until the gas bubbles are large enough to be displaced by the liquid phase. If it is to persist more than momentarily, a foam must be stabilized by adding a detergent, soap, protein, or other stabilizer to the mixture. Even when stabilized, a foam inevitably collapses into its component liquid and gas, so in that respect a foam can be thought of as a suspension that takes on the characteristics of a colloid for a short time.
Firefighters use foams made up of a carbon dioxide gas phase dispersed in a liquid water phase. Such foams suppress fires in three ways. First, the carbon dioxide dispersed in the foam does not support combustion and is heavier than air. When a foam layer covers a fire, the carbon dioxide covers and smothers the fire as the water cools it. Second, the foam itself presents a physical barrier that prevents air (and oxygen) from reaching the flame. Third, because the foam is elastic and has very low density, it covers and floats upon any burning solid or liquid. These characteristics mean that foam is effective in fighting nearly any type of fire, including burning oils and fats, for which liquid water simply spreads the fire.
In this lab, we’ll produce a foam of carbon dioxide gas in water. We’ll produce the carbon dioxide by reacting vinegar (acetic acid) and an aqueous solution of sodium hydrogen carbonate (sodium bicarbonate or baking soda), which is represented by the following equation:
CH3COOH(aq) + NaHCO3(aq) → H2O(I) + CH3COONa(aq) + CO2(g)
The carbon dioxide produced by this reaction constitutes the gas phase of the colloidal foam, and the water the liquid phase. We’ll use ordinary liquid dishwashing detergent as the stabilizing agent. And, just to make our foam more attractive, we’ll use food coloring to give it a festive appearance.
Required Equipment and Supplies
- goggles, gloves, and protective clothing
- balance and weighing papers
- beaker, 150 mL
- beaker, 250 mL
- graduated cylinder, 100 mL
- stirring rod
- vinegar (~ 100 mL)
- sodium hydrogen carbonate (~ 7.5 g)
- dishwashing liquid (~ 1 mL)
- food coloring (a few drops; optional)
All of the specialty lab equipment and chemicals needed for this and other lab sessions are available individually from Maker Shed or other laboratory supplies vendors. Maker Shed also offers customized laboratory kits at special prices, including the Basic Laboratory Equipment Kit, the Laboratory Hardware Kit, the Volumetric Glassware Kit, the Core Chemicals Kit, and the Supplemental Chemicals Kit.
Although none of the chemicals used in this laboratory session are ordinarily considered hazardous, it’s always good practice to wear splash goggles, heavy-duty gloves, and protective clothing.
Substitutions and Modifications
- You may substitute any containers of similar size for the beakers.
- Sodium hydrogen carbonate is the official name for the same substance available in grocery stores as baking soda or sodium bicarbonate.
- The food coloring is optional. Use it if you want the foam you produce to be colorful instead of white. Note that food coloring will stain clothes and surfaces.
- If you have not already done so, put on your splash goggles, gloves, and protective clothing.
- Transfer about 100 mL of vinegar to the 250 mL beaker.
- Add about 1 mL (20 drops) of liquid dishwashing detergent to the vinegar and stir gently until it is thoroughly mixed. (You don’t want to produce a lather, just to mix the vinegar and detergent.)
- Transfer about 100 mL of tap water to the 150 mL beaker, and add about 7.5 g of sodium hydrogen carbonate to the water. Stir until the solid dissolves.
- Dump the contents of the 150 mL beaker into the larger beaker, and watch what happens.
Figure 18-2. Homemade firefighting foam
Disposal All waste solutions from this lab can be flushed down the drain with plenty of water.
Q1: Why are foams like this particularly effective at putting out fires? Q2: Would a foam of carbon dioxide dispersed in water be a good choice for putting out burning sodium metal? Why or why not? Q3: Old-style soda-acid fire extinguishers produce not a stream of foam, but a stream of liquid. Propose an explanation.