# conservation of mass in chemical reactions

## following the law

Why must chemical equations be balanced? Its the law! Matter cannot be created or destroyed in chemical reactions. This is the law of conservation of mass. In every chemical reaction, the same mass of matter must end up in the products as started in the reactants. Balanced chemical equations show that mass is conserved in chemical reactions.

## lavoisier and conservation of mass

How do scientists know that mass is always conserved in chemical reactions? Careful experiments in the 1700s by a French chemist named Antoine Lavoisier led to this conclusion. Lavoisier carefully measured the mass of reactants and products in many different chemical reactions. He carried out the reactions inside a sealed jar, like the one in the Figure 1.1. In every case, the total mass of the jar and its contents was the same after the reaction as it was before the reaction took place. This showed that matter was neither created nor destroyed in the reactions. Another outcome of Lavoisiers research was the discovery of oxygen. Click image to the left or use the URL below. URL: Q: Lavoisier carried out his experiments inside a sealed glass jar. Why was sealing the jar important for his results? What might his results have been if he hadnt sealed the jar? A: Sealing the jar was important so that any gases produced in the reactions were captured and could be measured. If he hadnt sealed the jar, gases might have escaped detection. Then his results would have shown that there was less mass after the reactions than before. In other words, he would not have been able to conclude that mass is conserved in chemical reactions.

## chemical reactions and balanced equations

A chemical reaction occurs when some substances change chemically to other substances. Chemical reactions are represented by chemical equations. Consider a simple chemical reaction, the burning of methane. In this reaction, methane (CH4 ) combines with oxygen (O2 ) in the air and produces carbon dioxide (CO2 ) and water vapor (H2 O). The reaction is represented by the following chemical equation: CH4 + 2O2 CO2 + 2H2 O This equation shows that one molecule of methane combines with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water vapor. All chemical equations must be balanced. This means that the same number of each type of atom must appear on both sides of the arrow. Q: Is the chemical equation for the burning of methane balanced? Count the atoms of each type on both sides of the arrow to find out. A: Yes, the equation is balanced. There is one carbon atom on both sides of the arrow. There are also four hydrogen atoms and four oxygen atoms on both sides of the arrow.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

chemical reactions are represented by chemical formulas.

``````a. true

-->  b. false
``````

which choice correctly represents the reaction in which methane burns?

``````-->  a) ch4 + 2o2  co2 + 2h2o

b) 2ch4 + 2o2  co2 + 2h2o

c) ch4 + o2  co2 + h2o

d) none of the above
``````

the equation h2 + o2 h2o is balanced.

``````a. true

-->  b. false
``````

which equation shows that mass is conserved in the chemical reaction?

``````a) 2al + 2o2  2al2o3

b) 2al + o2  al2o3

-->  c) 4al + 3o2  2al2o3

d) none of the above
``````

## diagram questions

No diagram questions associated with this lesson