What is Fire?

What state of matter is fire? Is it a liquid? I get that question a LOT, so let me clarify. The ancient scientists (Greek, Chinese… you name it) thought fire was a fundamental element. Earth, Air Water, and Fire (sometimes Space was added, and the Chinese actually omitted Air and substituted Wood and Metal instead) were thought to be the basic building blocks of everything, and named it an element. And it’s not a bad start, especially if you don’t have a microscope or access to the internet.

Today’s definition of an element comes from peeking inside the nucleus of an atom and counting up the protons. In a flame, there are lots of different molecules from NO, NO₂, NO₃, CO, CO₂, O₂, C… to name a few. So fire can’t be an element, because it’s made up of other elements. So, what is it?

[am4show have=’p8;p9;p18;p45;p91;p101;’ guest_error=’Guest error message’ user_error=’User error message’ ]

Fire is a combination of different gases and hot plasma. It’s a complicated exothermic (gives off heat) chemical reaction that releases a lot of heat and light (you can feel and see the flame). You need three things for a flame: oxygen, fuel, and a spark. When you take away one of these three, you snuff the flame and stop the chemical reaction. You start with fuel (usually contains carbon), and add oxygen to get carbon dioxide, carbon monoxide, nitric oxide, and many other gases and leftover ash. Most flames are hot enough to heat the gas mixture to create tiny bits of plasma within the flame, so fire is actually involved in two states of matter.

In this experiment, we’re going to see how you can protect a surface from burning using water. Are you ready?

Materials:

• Shallow baking dish
• Tongs
• Rubbing Isopropyl Alcohol (50-91%)
• Water (omit if using 50-70% alcohol)
• Dollar bill
• Fire extinguisher
• Adult help

 
Download Student Worksheet & Exercises

What’s going on? Alcohol burns with a slightly blue and orange flame (as shown in the video). The secret to keeping the dollar bill from burning is the water you mixed in with the alcohol. Water has a high heat capacity, which means that the water absorbs the energy from the flame and keep the bill from catching on fire. If you dipped the dollar bill in pure 100% alcohol, the temperature would rise high enough on the bill to burn. The reason we chose a bill instead of regular paper is that the dollar bill is a combination of linen and paper, making it much stronger and absorbent for this experiment.

You need both the water and the alcohol for this experiment. The water, as it absorbs the energy from the flame, heats up to its boiling point and then vaporizes, keeping the bill cool enough to not catch on fire. The alcohol is the fuel needed to keep the flame going. It’s a delicate balance between the two, but here are a couple of variations you can try out:

• You can change the color of the flame by adding in a sprinkling of salt (for yellow), boric acid (for green), or epsom salt (for white).

• You can also try mixing different ratios of water to alcohol, using 50%, 70% and 91% isopropyl alcohol. You can also try ethyl alcohol (which is an entirely different molecule) but will react about the same with this experiment. Note that if you decrease the water content too much, you’re going to lose your dollar bill.

[/am4show]