Fire-Fighting Foam

Introduction

Do you have a fire extinguisher at home? Hopefully, yes! It can save your life in case there is a fire. But have you ever wondered how these extinguishers work? Some of them don't even contain water, which is what you probably think of first when it comes to stopping a fire from spreading! What else other than water can extinguish a flame? Do this activity to find out—and put a candle out as if by magic!

Background

What actually happens when something is burning? A fire, also called combustion, is the result of a chemical reaction in which oxygen gas reacts with the substance that is burnt. Luckily, not everything catches fire right away when exposed to air or oxygen. For a fire to start, certain requirements have to be met. First, you need a combustible material, or a fuel, that can burn. Second, this material needs to be hot enough so that it can catch fire. The combustion reaction then has to create enough heat to keep the fire going. And lastly, a fire needs oxygen to burn. If either heat, fuel, or oxygen are absent, there will be no fire.

Fire extinguishers are designed to remove one of the necessary "ingredients" for a fire to burn. There are several ways this can be done. For example, water can be used to remove heat from a fire. Adding water often cools down the fuel enough so that it stops burning. Some types of fire extinguishers use dry chemicals to interrupt the chemical combustion reaction. Yet another option is to cover the fire with a layer of foam, carbon dioxide gas, or dry chemicals that stop air from reaching the flames. Each of the above-mentioned methods results in the same outcome: the fire gets extinguished. You can see for yourself in this activity, and you don't even have to use a real fire extinguisher: just some baking soda and vinegar. Curious about how these two ingredients can put out a flame? Then go ahead and try it out!

Materials

Preparation

Procedure

Observations and Results

Did you successfully extinguish your flame? You don't even see what makes the candle go out, but it does—it almost looks like magic! Initially, your candle should have burnt bright and steady. After all, it had enough fuel from the candle, heat from the lighter, and oxygen from the air to sustain a fire. However, you should have observed that when you added the vinegar to the baking soda in the glass dish, after a short time, the flame suddenly went out. What happened? The answer has to do with the chemical reaction that occurs when you mix vinegar with baking soda.

Baking soda is a chemical compound called sodium bicarbonate. It reacts with any type of acid such as vinegar to form carbon dioxide (CO₂). Carbon dioxide is a gas—you can actually see it being produced. It makes the bubbles and foam that you observe when you add the vinegar to the glass dish. The same carbon dioxide reaction makes a baking soda volcano erupt or inflates a balloon when you mix an acid with sodium bicarbonate. In this experiment, the carbon dioxide extinguishes the flame of your candle. Although you can't see it, the CO₂ that is produced in the baking soda vinegar reaction starts filling the glass dish from the bottom up. Eventually, once all of the air in the glass dish is replaced by carbon dioxide, the flame has no more oxygen left to burn, it goes out. If you try to light the candle again, it won't work, because the match also goes out once it enters the layer of carbon dioxide in the glass dish.

When you add just water to the baking powder in the glass dish, no carbon dioxide is produced as water is not an acid. Therefore, the candle should have kept burning as long as enough oxygen and fuel was available.

Cleanup

More to Explore

• What is fire?, from Science Learning Hub
• How Fire Extinguishers Work, from HowStuffWorks
• Types of Fire Extinguishers, from the Fire Equipment Manufacturers' Association
• Using a Fire Extinguisher, from Fire Extinguisher: 101
• Science Activity for All Ages!, from Science Buddies

Credits