Conservation of Mass

Lesson Plan - Get It!

Audio:

Unfortunately, most of the household trash ends up in landfills located across the United States or wherever you live.

It takes a up a great deal of space. Trash, like all matter, cannot be destroyed. This is communicated in the law of conservation of mass, that states that matter cannot be created or destroyed, only changed or altered.

You might ask, "But what if we burn it?" Burning trash changes the solid mass material into gaseous waste, like carbon dioxide, that builds up in the atmosphere. Mass is conserved, even through phase changes from solid to gas.

Chemical reactions communicate how substances change from reactants to products. Chemical reactions use chemical formulas, like H₂ or NaCl, to communicate what elements or compounds are interacting. Chemical formulas are used to show how many of each element are found in the substance. Sodium chloride, NaCl, has one sodium atom and one chlorine atom.

The law of conservation of mass must be observed within a chemical reaction, meaning that you must have the same number of each element on the reactant and product sides of the reaction.

Look at the image above. There are two hydrogen molecules on the reactant side of the reaction, with a total of four hydrogen atoms. Count the number of blue hydrogen atoms on the product side, and you should end up with four. There are two oxygen atoms on the reactant side, and two on the product side. The chemical reaction for this process contains coefficients. Coefficients are the full-size numbers placed in front of the elements and molecules in the reaction above. A coefficient of two indicates two molecules of the substance. For this reaction, the two in front of hydrogen and the two in front of water are coefficients.

• How do you determine where to put a coefficient?

First, write out the chemical reaction. Let's use the formation of NaCl:

Na + Cl₂ —> NaCl

To form sodium chloride, one atom of sodium combines with two atoms of chlorine. Be aware, you cannot ever change a subscript (the small number located on a chemical symbol)! You can only add coefficients to balance the equation!

You have one sodium atom on both sides, but two chlorine atoms on the reactant side.

• Where should you add a coefficient to balance the number of chlorine atoms?
• Maybe between the Na and Cl?

That is a no-no! Never put a coefficient in the middle of the compound, only at the front.

Let's add a two in front of NaCl:

Na + Cl₂ —> 2NaCl

Now your chlorine is balanced, but sodium is not.

• Where could you add a 2 to balance the sodium?

Just before the Na atom!

2Na + Cl₂ —> 2NaCl

Now you have a balanced chemical equation that demonstrates how the law of conservation of mass is observed in the chemical reaction above. Some chemical equations are easy to balance, while others take a far more complicated process of trial and error.

In the Got It? section, you will practice balancing several different types of reactions.