Burning through Money
In this experiment, we’ll freak Dad out by setting a five-dollar bill on fire and laughing madly as it burns. The flames will mysteriously snuff themselves out and the five-dollar bill will be left unharmed. Hmmmm… Is this science or magic? Well, given the name of this website we all know that it is pure science but we’ll tell Dad it’s magic just to see his eyes go big. Oh yeah, and we’ll tell you how to make $20 extra dollars in the deal too…
Before we begin, ask your Dad for a five-dollar bill and a twenty-dollar bill. Tell him the twenty-dollar bill is insurance to ensure the five-dollar bill does not get harmed. Tuck the twenty-dollar bill into your pocket. If Dad cocks an eye and that puzzled expression spreads across his face, that’s ok. Tell him that insurance has to do with risks and ratios and all that stuff and he just wouldn’t understand. Trust Reeko, you’re going to end up with twenty extra bucks when we’re done.
- Prepare an alcohol and water solution that consists of 50% alcohol and 50% water. If that is too difficult to measure, you can use a 1:1 ratio instead (heh, that’s a little inside joke for the math geeks).
- Add a sprinkle of salt to the solution. If Dad’s watching, add a little flare by snapping your wrist when adding the salt – it will enhance the “magic” effect…
- Soak the paper currency (the five-dollar bill) in the solution so that it is soaked really well
- Use the metal tongs to pick up the bill. “Tongs”, not “thongs”. Boys, stop grinning…
Note: it’s very important to use the metal tongs to pick up the bill. We’re eventually going to light this baby up so if you are holding the bill in your hands… Well, whether or not you get to keep the $20-dollar-bill will become irrelevant….
- Move away from the solution. We repeat, hold your hands above your head and move away from the solution.
- While holding the five-dollar bill with the tongs, light it with a match and grin sinisterly as it burns. Mad laughter adds to the effect.
The five dollar bill will appear to burn with a colorful flame. The fire will extinguish itself leaving the dollar-bill unscathed but slightly wet.
What happened here? Fire is a chemical reaction called combustion. In this case, the reaction occurs between alcohol and oxygen in the air and produces light and heat (the flame) and carbon dioxide and water.
Alcohol is represented by the molecular formula c2h2oh. It combines with oxygen in the air to produce the carbon dioxide and water. The equation looks like this:
C2h2oh (alcohol) + 4 oh (oxygen in the air) = 2 co2 (carbon dioxide) + 3 h20 (water)
The water in the solution is actually untouched and only serves to “insulate” the dollar bill from damage.
Given that a dollar bill is more like cloth than paper (which you should already know if you’ve ever forgotten and left a dollar in your pocket before mom put it in the washing machine), it’s a bit harder to burn anyway – it soaks up the water pretty well. When the bill is soaked in the solution, the alcohol has a higher vapor pressure than the water and is located mainly on the outside of the bill.
Compounds with high vapor pressures are volatile and form a high concentration of vapor above the liquid (which in the case of some materials, poses an extreme fire hazard). When you light the five-dollar bill with a match, the alcohol is what burns – not the water. Also, alcohol burns at a low relative temperature which means the temperature at which the alcohol burns is not high enough to evaporate the water out of the bill.
Water has an unusually high specific heat too. That means that water will change its temperature less when it absorbs or loses a given amount a heat. Thus, the bill remains wet and is not able to catch on fire on its own.
And what about the salt in the solution? It simply adds a bit of color which is only needed to add to the effect.
As you can see, although this appears to be “magic”, it’s actually pure science in action.
And yes, Dad will be so freaked out by this experiment he will have temporarily forgotten all about the twenty in your pocket. Tip your hat, hand Dad the soggy five-dollar bill, and ask politely for applause. Tell Dad you hope he enjoyed the show and then disappear quietly out the door while he gapes puzzled at the soaked five-dollar bill he’s holding in his hand.
The vapor pressure of a liquid is the pressure exerted by its vapor when the liquid and vapor are in dynamic equilibrium (see below). If we were to place a substance in closed container, some of it would vaporize. The pressure in the space above the liquid would increase from zero and eventually stabilize at a constant value, the vapor pressure. If a liquid is not in a closed container it still has a vapor pressure, however, the liquid would eventually all evaporate away (turn into gas).
Even though the pressure in our closed container is constant, molecules of the vapor are still condensing into the liquid phase and molecules of the liquid are still evaporating into the vapor phase. However, the rate of these two processes is equal, so there is no net change in the amount of vapor or liquid. This process is called dynamic equilibrium. For this reason, the term equilibrium vapor pressure is sometimes used.
Vapor pressure and boiling point have an intimate relationship. The boiling point is the temperature at which the vapor pressure of the liquid equals the external pressure. For example, because the air pressure is lower in a city far above sea level such as Denver, the boiling point of water is lower than in a sea level city such as New York.
The most common unit for vapor pressure is the torr. 1 torr = 1 mm Hg (one millimeter of mercury).
Most materials have very low vapor pressures. For example, water has a vapor pressure of approximately 15 torr at room temperature. But remember that vapor pressures increase with temperature; water will have a vapor pressure of 760 torr = 1 atm at its boiling point of 100 oC (212 oF).
Solids are bound together much tighter and stronger than liquids. As a general rule, the vapor pressure of solids is much lower than the vapor pressure of a liquid.
In general, the higher the vapor pressure of a material at a given temperature, the lower the boiling point. In other words, compounds with high vapor pressures are volatile, forming a high concentration of vapor above the liquid; this can sometimes pose a fire hazard.
Specific heat is the ratio of the quantity of heat required to raise the temperature of a body one degree to that required to raise the temperature of an equal mass of water one degree. The term is also used in a narrower sense to mean the amount of heat, in calories, required to raise the temperature of one gram of a substance by one Celsius degree. The Scottish scientist Joseph Black, in the 18th century, noticed that equal masses of different substances needed different amounts of heat to raise them through the same temperature interval, and, from this observation, he founded the concept of specific heat.
The ability of water to stabilize temperature depends on its relatively high specific heat. The specific heat of a substance is defined at the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1º C. The specific heat of water is 1.00 cal/g ºC. Compared with most other substances, water has an unusually high specific heat. For example, ethyl alcohol, the type in alcoholic beverages, has a specific heat of 0.6 cal/g ºC.
Because of the high specific heat of water relative to other materials, water will change its temperature less when it absorbs or loses a given amount of heat. The reason you can burn your finger by touching the metal handle of a pot on the stove when the water in the pot is still lukewarm is that the specific heat of water is ten times greater than that of iron. In other words, it will take only 0.1 cal to raise the temperature of 1 g of iron 1ºC. Specific heat can be thought of as a measure of how well a substance resists changing its temperature when it absorbs energy.