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Law of Conservation of Matter

The law of conservation of matter says that mass can not be created or destroyed. Below we go into detail on this law, work through some example questions, and discuss the origins of the law of conservation of mass.

What is the Law of Conservation of Mass

The law of conservation of mass states that in a reaction matter can not be created or destroyed. That means that the mass of all reactants in a reaction will be equal to the mass of all the products. The mass may change forms in the reaction but the matter is neither created or destroyed.

Another way to say this is that matter is conserved in a closed system. A closed system is where nothing (gas, water, other) can enter or leave. All products are contained within the system. Matter can not enter or leave the system. A closed system could be a well-sealed test tube, or it could be the entire earth.

The law of conservation of mass is also referred to as the law of conservation of matter and the two names are often used interchangeably. Other names sometimes used for the law also include the law of matter conservation or the law of mass conservation.

Can matter be created or destroyed?

No, matter can not be created or destroyed. In a closed system, you will always start and end with the same amount of matter/mass.

It may sometimes appear that mass disappears if a gas is produced and not measured. However, all products need to be taken into account. In a closed system, the mass of the reactants will always equal the mass of the products.   

Law of Conservation of Matter Examples

All chemical reactions can demonstrate the law of conservation of matter. In some reactions, it will be easier to observe, however.

For example, a good reaction is solid KI reacting with solid Hg(NO3)2 to form solid KNO3 and solid HgI2.

KI (s) + Hg(NO3)2(s) –> KNO3 (s) + HgI2(s)

The solids are easily massed and no gas produced that must be captured. The reaction takes place in water. HgI2 precipitates out and removed with a filter. Next, the water can be evaporated off to get the KNO3.

Potassium Iodide (KI) Powder (Source: Wikipedia Commons)

Added bonus, here is a cool experiment with potassium iodide (KI)!

Example Question:

You have the reaction below:

AB + CD –> AD + CB

You start with 4.03 grams of AB and 2.09 grams of CD. You measure 4.85 grams of CB. How many grams of AD do you have?

Answer:

There must be equal masses of product and reactant. We have 4.03 g + 2.09 g = 6.12 g of reactant. Therefore, CB + AD must equal 6.12g.

4.85 g + AD = 6.12g

Solving the above equation for AD, the answer is 1.27g of AD.

History of the Law of Conservation of Matter

Antoine Laurent Lavoisier discovered the law of conservation of mass in 1789. Lavoisier lived from 1743-1794 in France and made many chemical discoveries. By performing combustion reactions in a closed container with careful measurements he discovered the law of mass conservation. In his scientific career, he generally focused on reactions with oxygen and other gasses, finding them particularly interesting. Fittingly, one of his other discoveries was the oxygen theory of combustion. Additionally, Marie Ann, Lavoisier’s wife, was also influential in his scientific experiments and ability to disseminate them.

A portrait of Lavoisier and his wife Marie Ann. Lavoisier discovered the law of conservation of mass
A portrait of Lavoisier and his wife Marie Ann. (Source: Wikipedia Commons)

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