Explanation:

Isopropanol (rubbing alcohol) is used to create a "mini explosion" inside an empty water jug. Here we learn about the scaling of surface area with respect to volume and witness why a firefighter might steer clear of a "backdraft".

Lesson:

In this video, we see that the coffee stirrer burns faster if it's broken into smaller pieces. When the alcohol in the jug evaporates (turns into a gas), its almost like we're breaking it into millions of smaller pieces. Sure enough, when the match is dropped into the jug, the alcohol vapor burns very quickly and we hear a loud "swoosh" sound.

Advanced Lesson:

Here is the general equation for a combustion reaction where the fuel is a hydrocarbon.

C_xH_y + (x +y/4)O₂ -> xCO₂ + (y/2)H₂O + energy

We see that combustion can only occur where there is plenty of oxygen (this is why you have to allow plenty of room for air when building a campfire). This tells us that combustion occurs more readily near the surface of matter- because that's where the air is- and that it occurs more quickly for things with a large surface area to volume ratio. For example, the surface area of a cube is 6a² while its volume is a³. So the ratio of the two (6/a) increases as the volume decreases. This inverse relationship is seen in the plot below.

In this demonstration, we let isopropyl alcohol evaporate in a 5-gallon water jug. After some time, the vapor expels much of the oxygen in the jug, and since the jug has a small opening, very little oxygen can get back in. At this point, the inside of the jug resembles a situation that a firefighter would want to avoid. If there's a fire in a room where the windows and doors are all closed, the fire burns until all of the oxygen is used up. A rapid re-introduction of oxygen can occur if a window breaks or someone opens a door. The outcome of this is an explosive event called a backdraft.

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