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Evidence of a chemical reaction
Take another look at
Figure 10.
As shown, rust is a brownish-orange powdery sub-
stance. It looks very different from the elements iron and oxygen that compose it. Rust
is not attracted to a magnet, whereas iron is. The observation that the product (rust) has
different properties than the reactants (iron and oxygen) is evidence that a chemical
reaction has taken place. A chemical reaction always produces a change in properties.
Spoiled food, such as rotten fruit and bread, is another example of chemical reactions.
The properties of spoiled food differ from the properties of fresh food. How the food
tastes, how it smells, and its digestibility might all change as food rots. Examples of food
that have undergone chemical reactions are shown in
Figure 11
.
Conservation of Mass
It was only in the late eighteenth century that scientists began to use quantitative tools
to study and monitor chemical changes. The analytical balance, which was capable of
measuring small changes in mass, was developed at that time. By carefully measuring
mass before and after many chemical reactions, it was observed that, although chemical
changes occurred, the total mass involved in the reaction remained constant. Assuming
this was true for all reactions, chemists summarized this observation in a scientific law.
The
law of conservation of mass
states that mass is neither created nor destroyed
during a chemical reaction—it is conserved. In other words, the mass of the reactants
equals the mass of the products. The equation form of the law of conservation of mass
is as follows:
The Law of Conservation of Mass
mass
reactants
=
mass
products
Mass is conserved in a chemical reaction; products have the same mass as reactants.
Figure 11
When food rots, new substances
are formed due to chemical change.
Lesson 2 • Changes in Matter
57
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