Law of Conservation of Mass

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Law of Conservation of Mass

The Law of Conservation of Mass says that in any chemical reaction, the total mass of all atoms in the reactants that you started with must be the same as the total mass of all the atoms in the products. Extra atoms (or mass) are not created or destroyed - just rearranged from those at the start of the reaction. It can be hard to observe this when one of the products is a gas, such as in a burning candle.

If we could collect all the gas products of carbon, carbon dioxide and water vapour and add their mass to the mass of the leftover candle, then it would be the same as the unburned candle mass.

Balancing Equations to Show Conservation of Mass

Atoms are not created or destroyed in chemical reactions. Chemical equations show this by having the same number of atoms on each side of the equation. This is called balancing an equation.

Write the unbalanced chemical equation
Count atoms of each element on both sides.
- - Count polyatomic ions as one item.
Add coefficients (whole numbers in front of formulas) to equalize atom counts.
- - Only change coefficients, not subscripts.
  - Balance one element at a time.
  - Consider balancing elements in "MaNHO" order (Metals, Nonmetals (except H, O), Hydrogen, Oxygen).
  - Treat unchanged polyatomic ions as a unit.
Check your work by recounting atoms on both sides.

For example, take the reaction that is occuring inside a rocket motor:

hydrogen + oxygen → water

Below are some simple rules for writing and balancing equations:

Small numbers after an atom's symbol tell you how many atoms are combined. For example: O₂ means two oxygen atoms combined; O₃ means three oxygen atoms combined.
Large numbers in front (called the coefficient) mean more than one of whatever follows. For example 3O₂ means three oxygen molecules, each with the formula O₂.
You can't alter a formula to balance an equation. You can only change the numbers (co-efficient) in front.

Example: Magnesium reacting with oxygen to produce magnesium oxide. The reactants are magnesium metal (Mg) and oxygen gas (O₂).

The large 2 in front of Mg (in the balanced equation) means two magnesium atoms are needed to react with one molecule of oxygen gas (O₂). It is not written as Mg₂ because the magnesium atoms are not joined together in molecules like the oxygen atoms are.

The large 2 in front of MgO (in the balanced equation) shows that two olecules of magnesium oxide are formed. This balances the number of magnesium atoms and oxygen atoms on each side of the equation.

Magnesium oxide is made up of one magnesium ion bonded to one oxygen ion. If a subscript number is not shown, there is one atom or ion present.

To be able to write a chemical equation in words or symbols, you have to know what the reactants and products are. The reactants are easy to name because they are the chemicals used to produce a reaction.

It is harder to know exactly what the products will be, but there are some rules that will help you and you will learn these as you study different aspects of chemistry.

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