ca-inspire-chemistry-sampler

Figure 10

When iron rusts, new substances

are formed due to chemical change.

Identify

the reactants and products in the

formation of rust.

Get It?

Define

chemical change.

EARTH SCIENCE

Connection

The water cycle

Phase changes associated with

water make up the water cycle, which allows life to exist on Earth. At atmospheric

pressure and at temperatures of 0°C and below, water is in its solid state, which is

known as ice. As heat is added to the ice, it melts and becomes liquid water. This

change of state is a physical change because even though ice and water have different

appearances, they have the same composition. If the temperature of the water increases

to 100°C, the water begins to boil, and liquid water is converted to steam. Melting and

formation of a gas are both physical changes and phase changes. Terms such as

boil,

freeze, condense,

vaporize,

melt

in chemistry generally refer to a phase change in

matter.

The temperature and pressure at which a substance undergoes a phase change are

important physical properties. These properties are called the melting and boiling

points of the substance. Look again at

Table 1

to see this information for several

common substances. Like density, the melting and boiling points are intensive physi-

cal properties. Tables of intensive properties, such as those given at the end of this

textbook or in the

CRC Handbook of Chemistry and Physics,

are useful tools in identify-

ing unknown substances from experimental data.

Chemical Changes

A process that involves one or more substances changing into new substances is called a

chemical change,

commonly referred to as a chemical reaction. The new substances

formed in the reaction have different compositions and different properties from the

substances present before the reaction occurred. For example, the formation of rust when

iron reacts with oxygen in moist air is a chemical change. Rust, shown in

Figure 10

, is a

chemical combination of iron and oxygen.

In chemical reactions, the starting substances are called reactants, and the new

substances that are formed are called products. Terms such as

decompose, explode,

rust, oxidize, corrode, tarnish, ferment, burn,

rot

generally refer to chemical reactions.

Module 2 • Matter—Properties and Changes

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