Laws of Chemical Combination: Overview & Explanation

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  • 0:04 Laws of Chemistry
  • 0:35 Law of Conservation of Mass
  • 3:04 Law of Constant Proportions
  • 3:36 Law of Multiple Proportions
  • 4:39 Law of Reciprocal Proportions
  • 6:17 Lesson Summary
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Lesson Transcript
Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

There are four major laws in the study of chemistry. In this lesson, we'll define and see examples of each of the four chemical combination laws that govern chemical reactions.

Laws of Chemistry

When people are learning to drive, they have to learn the rules of the road such who goes first at a four way stop intersection and how to change lanes properly. When learning chemistry, you have to understand four overarching laws that govern how atoms combine:

  1. The law of the conservation of mass
  2. The law of constant proportions
  3. The law of multiple proportions
  4. The law of reciprocal proportions

Let's go through each of these laws one by one.

Law of the Conservation of Mass

In chemical reactions, pure elements or combinations of elements called compounds get rearranged. In nuclear reactions, the nucleus of atoms change. In both cases, mass is conserved even though new substances are created. Imagine an empty room with one door and five people walk into the room. Eventually five people have to leave the room. What goes in has to come out. The law of the conservation of mass states that mass before reaction has to equal mass after reaction. So, let's go through a chemical and nuclear reaction to show how mass before a reaction equals mass after a reaction.

Chemical Reaction

Let's take a quick look at how solid calcium reacts with hydrobromic acid:

Ca + HBr → CaBr2 + H2

Notice there is only one bromine and hydrogen on the left side of the reaction while there are two atoms of each on the products side of the reaction. This is an impossibility because one atom of hydrogen and bromine can't be generated out of nowhere. To show what really happens we have to balance the equation with coefficients, which are numbers in front of the compounds or elements in the reaction. Let's balance this equation:

Ca + 2HBr → CaBr2 + H2

The coefficient in front of HBr makes the equation balanced for mass. Now let's look at a nuclear reaction.

Nuclear Reaction

In a nuclear reaction, the nucleus of the atom changes in some fashion. A proton can turn into a neutron or vice versa. The nucleus can also lose four units of mass and two units of charge through the expulsion of an alpha particle. Let's look at how polonium-214 turns into lead-210. This diagram shows this transmutation:


The values on top are the mass values and the values on the bottom are is the electrical charge. Notice neither mass nor charge are conserved, which is another impossibility. To show what actually occurs, we have to add an alpha particle, which is a helium nucleus, to the products side. This next diagram shows the complete nuclear reaction:


By adding the masses on the products side of the equation, we get 214, which equals the mass on the reactants side of the equation. The charges on either side of the yields sign are equal. Now, we move on to the law of constant proportions.

Law of Constant Proportions

The law of constant proportions states that the ratio of mass for the same compound is constant. In other words, it tells us that compounds of the same type always have the same ratio of elements. For example, carbon dioxide always has the ratio of ONE Carbon atom to two oxygen atoms resulting in the formula CO2. The ratio of mass is also constant: 12 g of carbon to 32 g of oxygen, which are rounded numbers off the periodic table. The law of multiple proportions is next.

Law of Multiple Proportions

The law of multiple proportions explains how multiple elements can combine with each other in multiple ratios. Think of the ratio of males to females in your family. Some families have one mother, one father, and one daughter, giving the ratio of females to males as 2:1. The family next door might have one mother, one father, and two sons, giving the ratio of females to males as 1:3. The same idea applies with elements and compounds. For example, hydrogen and oxygen combine multiple ways to form multiple compounds.

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