Looking at any substance with the naked eye or even a regular microscope, you cannot see that molecular movement. Okay, so we know that chemical reactions happen when two substances come together. Technically, a chemical reaction happens with pretty much any two substances coming together. This is a classic chemical reaction as well, but not the one we usually associate with the term chemical reaction.
Take a cup, add some water and then slowly pour some oil into the same cup. Explain that when something burns, it reacts with the oxygen in the air.
Note : Some curious students may ask what the flame is made of. This is a great question and not trivial to answer. The flame is burning wax vapor. The light of the flame is caused by a process called chemiluminescence. Energy released in the chemical reaction makes electrons from different molecules move to a higher energy state. When the electrons come back down, energy is released in the form of light.
Explain to students that wax is made of long molecules called paraffin and that paraffin is made up of only carbon atoms and hydrogen atoms bonded together. Molecules made of only carbon and hydrogen are called hydrocarbons.
Tell students that you will use the simplest hydrocarbon methane as a model to show how the wax, or any other hydrocarbon, burns. Project the image Chemical Reaction between Methane and Oxygen. Show students that there is methane and oxygen on the left side of the chemical equation and carbon dioxide and water on the right side.
Explain that the molecules on the left side are the reactants and the ones on the right side are the products.
When the candle was burning, the paraffin reacted with oxygen in the air to produce carbon dioxide and water, similar to the chemical reaction between methane and oxygen. Explain to students that the chemical formula for methane is CH 4.
This means that methane is made up of one carbon atom and four hydrogen atoms. Show students that the other reactant is two molecules of oxygen gas. Point out that each molecule of oxygen gas is made up of two oxygen atoms bonded together. It can be confusing for students that oxygen the atom, and oxygen the molecule, are both called oxygen.
Let students know that when we talk about the oxygen in the air, it is always the molecule of oxygen, which is two oxygen atoms bonded together, or O 2. Note : Leave this equation projected throughout the activity in the Explore section of this lesson. Students will need to refer to it as they model the chemical reaction. Students will record their observations and answer questions about the activity on the activity sheet. The Explain It with Atoms and Molecules and Take It Further sections of the activity sheet will either be completed as a class, in groups, or individually, depending on your instructions.
Look at the teacher version of the activity sheet to find the questions and answers. On a sheet of paper, place the atoms together to make the molecules of the reactants on the left side of the chemical equation for the combustion of methane.
After you are sure that students have made and written the formula for the reactant molecules, tell students that they will rearrange the atoms in the reactants to form the products. Tell students that in a chemical reaction, the atoms in the reactants come apart, rearrange, and make new bonds to form the products. Show students that the atoms in methane and oxygen need to come apart just like in their models.
Also point out that the atoms arrange themselves differently and bond again to form new products. This is also like their model. Be sure that students realize that the atoms in the products only come from the reactants. There are no other atoms available. No new atoms are created and no atoms are destroyed. Explain to students that chemical reactions are more complicated than the simplified model shown in the animation.
The animation shows that bonds between atoms in the reactants are broken, and that atoms rearrange and form new bonds to make the products.
In reality, the reactants need to collide and interact with each other in order for their bonds to break and rearrange. Also, the animation shows all of the atoms in the reactants coming apart and rearranging to form the products. But in many chemical reactions, only some bonds are broken, and groups of atoms stay together as the reactants form the products. Read more about the combustion of methane in the teacher background section. Note : The coefficients actually indicate the ratios of the numbers of molecules in a chemical reaction.
It is not the actual number as in two molecules of oxygen and one molecule of methane since there are usually billions of trillions of molecules reacting. The coefficient shows that there are twice as many oxygen molecules as methane molecules reacting. It would be correct to say that in this reaction there are two oxygen molecules for every methane molecule.
Project the image Balanced Equation. Explain that the balance shows the mass of methane and oxygen on one side exactly equals the mass of carbon dioxide and water on the other. It is not necessary at this stage to talk about particles such as atoms or molecules or chemical bonds.
In learning about chemical reactions students will need to describe various substances, which at this level will be materials they are familiar with the kitchen and changes involving cooking are very good starting points. They will need to be able to identify changes in these substances with the purpose of eventually recognising when new chemicals have been produced i. As mentioned above, this is can be difficult as students often fail to see the difference between an egg white going through a change from liquid to solid as it is cooked and changes such as melting chocolate or boiling water which do not involve chemical change.
Teaching will need to be focused on what happens when new substances are formed. These ideas are also explored in the focus idea Problems with classifying. Environmental effects of chemical reactions can also be considered, for example how we dispose of some chemicals once they are produced, in forms such as plastic bags. At this stage it is important that students are encouraged to put up their ideas and discuss them in small groups. All alternatives should be considered with no resolution at this stage.
A starting activity could be observing the burning of a candle and discussing the changes that take place. Here the distinction can be made between the melting of the wax and the appearance of new materials. Questions posed could include:. Activities which provide problems to be explored and challenge existing ideas are useful in encouraging students to seek new explanations for things they observe.
Students should investigate a number of changes and ask questions similar to those above. In all of these students should be encouraged to observe the changes that take place and to identify what products are formed. Discussion can also centre on how these are different from the starting materials.
Some examples could include:. Other activities can involve chocolate making. There are many other similar chemical changes that can be investigated - further cooking activities can include: making a chocolate cake, melting and browning cheese, making honeycomb, baking bread, poaching eggs and making toast.
Other changes can include the setting of two component glues like Araldite and mixing steel wool and a solution of copper sulfate available from plant nurseries. Oxygen is a very important reactant in many chemical reactions and students can investigate changes involving this component of air.
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