Fire is a chemical chain reaction which takes place with the evolution of heat and light. In order for a fire to take place there are 3 main ingredients that must be present: Oxygen, Heat and Fuel.
In chemistry we call the type of reaction that produces fire a combustion reaction. Combustion is a high-temperature exothermic (heat releasing) redox (oxygen adding) chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke.
Whenever we complete a combustion reaction a hydrocarbon (compound of C and H) there are generally the same products formed: CO2 and H2O.
The fuel you burn in your car's engine contains octane, C8H18. When octane is burned, the products are CO2 and H2O.
2C8H18(l) + 25O2(g) → 16CO2(g) + 18H2O(g)
The key ingredient to the process is the availability of oxygen. Combustion cannot take place in an atmosphere devoid of oxygen.
So if you have a bottle of gasoline (octane) sitting around and open to the atmosphere which contains oxygen, why doesn�t it just burst into flames?
The answer to this question is the need to overcome the activation energy of the reaction, which means that it requires energy at first to "jump start" the process. In your car, the distributor and battery provide this starting energy by creating an electrical "spark". Other sources of initial energy can come from the Sun, matches, friction, etc.
The combustion reaction itself is quite exothermic.
When heat is produced in the process of a chemical reaction this is known as an Exothermic Reaction.
N2 + 3H2 → 2NH3 + Heat
C + O2 → CO2 + Heat
When heat is absorbed from the reacting substances this is known as an Endothermic Reaction.
2C + H2 - Heat → C2H2
3O2 - Heat → 2O3
But remember, whether endothermic or exothermic, both types of reactions still require an Activation Energy to begin.
It is important to understand the difference between chemical and physical changes. Some changes are obvious, but there are some basic ideas you should know. Physical changes are usually about physical states of matter. Chemical changes happen on a molecular level when you have two or more molecules that interact. Chemical changes happen when atomic bonds are broken or created during chemical reactions.
No Change to Molecules
When you step on a can and crush it, you have forced a physical change. However, you only changed the shape of the can. It wasn't a change in the state of matter because the energy in the can did not change. Also, since this was a physical change, the molecules in the can are still the same molecules. No chemical bonds were created or broken.
When you melt an ice cube (H2O), you have a physical change because you add energy. You added enough energy to create a phase change from solid to liquid. Physical actions, such as changing temperature or pressure, can cause physical changes. No chemical changes took place when you melted the ice. The water molecules are still water molecules.
Changing the Molecules
Chemical changes happen on a much smaller scale. While some experiments show obvious chemical changes, such as a color change, most chemical changes are not visible. The chemical change as hydrogen peroxide (H2O2) becomes water cannot be seen since both liquids are clear. However, behind the scenes, billions of chemical bonds are being created and destroyed. In this example, you may see bubbles of oxygen (O2) gas. Those bubbles are evidence of the chemical changes.
Melting a sugar cube is a physical change because the substance is still sugar. Burning a sugar cube is a chemical change. Fire activates a chemical reaction between sugar and oxygen. The oxygen in the air reacts with the sugar and the chemical bonds are broken.
Iron (Fe) rusts when it is exposed to oxygen gas in the air. You can watch the process happen over a long period of time. The molecules change their structure as the iron is oxidized, eventually becoming iron oxide (Fe2O3). Rusty pipes in abandoned buildings are real world examples of the oxidation process.
Some chemical changes are extremely small and happen over a series of steps. The resulting compounds might have the same number of atoms, but they will have a different structure or combination of atoms.
The sugars glucose, galactose, and fructose all have six carbon atoms, twelve hydrogen atoms, and six oxygen atoms (C6H12O6). Even though they are made of the same atoms, they have very different shapes and are called isomers. Isomers have atoms bonded in different orders.
Each of the sugars goes through different chemical reactions because of the differences in their molecular structure. Scientists say that the arrangement of atoms allows for a high degree of specificity, especially in the molecules of living things. Specificity means the molecules will only work in specific reactions, not all of them. For example, your body uses glucose as an energy source. If you eat galactose molecules, they need to be converted into glucose before your body can use them.
Recipes for Chemists (NASA/NASA Connect Video)
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The teacher will use a small candle flame to demonstrate a chemical reaction between the candle wax and oxygen in the air. Students will see a molecular animation of the combustion of methane and oxygen as a model of a similar reaction. Students will use atom model cut-outs to model the reaction and see that all the atoms in the reactants show up in the products.
Students will be able to explain that for a chemical reaction to take place, the bonds between atoms in the reactants are broken, the atoms rearrange, and new bonds between the atoms are formed to make the products. Students will also be able to explain that in a chemical reaction, no atoms are created or destroyed.
Download the student activity sheet, and distribute one per student when specified in the activity. The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan.
Be sure you and the students wear properly fitting goggles. Be careful when lighting the candle. Be sure that the match and candle are completely extinguished when you are finished with the demonstration.
Materials for the Demonstration
Materials for Each Student