The basis of chemistry is producing chemical reactions. When chemists write down reactions that occur, they use chemical equations. A chemical equation looks like this:
Al + O2 → Al2O3
In a chemical equation, the two chemicals being mixed together- the reactants- are on the left hand side of the equation, while the products are on the right. However, the equation listed above is not a balanced equation. A balanced equation simply means that the number of atoms for each element must equal each other on both sides of the equation. This relates to the Law of Conservation of Mass, which states that in a chemical reaction, the mass of the products must equal the mass of the reactants.
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There are a few basic steps to create a balanced equation. As an example, we’ll use the equation listed above.
1. Write the Unbalanced Equation.
a. Be sure to put the reactants on the left and the products on the right. Our example is aluminum adding to oxygen and creating aluminum oxide:
Al + O2 → Al2O3

2. Balance The Equation

a. In order to balance an equation, you must add numbers in front of each molecule to try and get the number of atoms to equal each other. Never change the subscript (behind the molecule): only add coefficients in front of each molecule.
b. Another thing to think about is diatomic molecules, or atoms that naturally come in pairs. An example is hydrogen (H2). These atoms have just the right amount of valence electrons to be filled by its partner.
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c. Here’s how to balance our example.
Al + O2 → Al2O3
The first thing we do is put a 2 in front of aluminum. Now there are two aluminum molecules on each side.
2 AL + O2 → Al2O3
However, we cannot turn two oxygen molecules into three. So we must multiply the oxygen reactants by 3 and the product by 2.
2Al + 3O22AlO3
Now the amount of oxygen molecules are balanced, but aluminum is not- there are two on the reactant side and four on the products. To fix this, we simply replace the 2 with a 4.
4Al + 3O22Al2O3
Now we have a balanced equation!
3. Indicate the States of Matter
a. To complete a correct chemical equation, you must write what states of matter each of the products and reactants are in. The states are as follows:
G- Gas
S- Solid
L- Liquid
Aq- Aqueous
So the balanced equation SnO2 + 2H2→Sn + 2H2O becomes SnO2(s) + 2H2(g)→Sn(s) + 2H2O(g).
Once you have completed all of these steps, the equation is now completely correct. For a super fun review game, go here! It's wonderful practice. http://education.jlab.org/elementbalancing/index.html
PERIODIC TABLE:
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QUESTIONS:
1. What are the chemicals called on the left hand side of the equation?
A) Products B) Reactors C) Reactants D) Mixers
2. Why must chemical equations be balanced? Explain using the law of conservation of mass.
3. What are paired atoms called?
A) Dipole Atoms B) Double Atoms C) Diatomic Atoms D) Bonded Atoms
4. Why are some atoms always found paired together in nature?
SOURCES: http://richardbowles.tripod.com/chemistry/balance.htm
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http://chemistry.about.com/cs/stoichiometry/a/aa042903a.htm