Illustrated Guide to Home Chemistry Experiments

324 DIY Science: Illustrated Guide to Home Chemistry Experiments

SBSTITUTIU oNS ANd modIfICATIoNS

• You may substitute any containers of similar size for
  the beakers.


• Sodium hydrogen carbonate is the official name for
  the 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.

LABORATORY 18 .2:

pRodUCE fIREfIGHTING foAm

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, marshmallows, the

meringue in a lemon-meringue pie, and the

head on a glass of beer.

RIREEqU d 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)

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 longer than momentarily, a foam must be
stabilized by the addition of 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:

CH 3 C(aq) + NaHCooH o 3 (aq)
→ H 2 ol) + CH( 3 Ca(aq) + CooN o 2 (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 for a festive appearance.

POCEDURER

1.f you have not already done so, put on your splash I

goggles, gloves, and protective clothing.

2. Transfer about 100 mL of vinegar to the 250 mL beaker.


3. 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.)


4. 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.


5. Dump the contents of the 150 mL beaker into the larger
   beaker, and watch what happens (preview in Figure 18-2).