4.02 Chemistry Notes

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4.02 chemistry notes
Law of Conservation of Mass
One of the most important principles of chemistry is the law of conservation of mass. This law states that matter is neither created nor destroyed during an ordinary chemical reaction. In other words, all of the atoms that were present at the beginning of a reaction are present at the end of the reaction—they have just been rearranged to bond differently and form new substances. It is important that chemical equations represent this law by always having equal numbers of each type of atom on both sides of the equation; chemists call this “balanced.” In a balanced equation, each type of element must appear on both sides of the arrow the same number of times. Sometimes it can be difficult to observe what happens to all of the atoms in a chemical reaction, especially if one of the reactants or products is a gas. Complete the lab demonstration below to observe how mass is conserved in a chemical reaction.

Did You Know?
Remember that we are only allowed to change the coefficients in front of a formula, and not the subscripts within the formulas, when balancing a chemical equation. Changing the subscripts in a formula changes the identity of the compounds. The equation represents an observed chemical reaction, so the formulas themselves cannot be changed.

Helpful Hints
1. Balance each type of element one at a time.
2. Whenever you add a coefficient in front of a formula, remember that it affects the number of each atom in that formula. Check how this new coefficient affects each element in the equation before you add the next coefficient. 3. Sometimes an element is found in more than one compound on the same side of an equation, which can make it even more challenging to balance. Try balancing all of the other elements first, leaving that element for last. 4. Remember that balancing an equation is a lot like solving a puzzle. There are times when you may have to change a coefficient more than once...
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