http://www.ck12.org Chapter 2. Matter and Change
FIGURE 2.3
Sodium vapor lamps glow with a distinc-
tive yellow color.2.4 shows the differences between the ways in which particles appear in each of these three states. Remember, any
substance can be present as a gas, liquid, or solid when placed under specific conditions.
FIGURE 2.4
The particles of a gas are very far apart
compared to the particles of a liquid or a
solid.AsFigure2.4 shows, the distance between particles is much smaller for the solid and liquid states than for the gas
state. In the solid state, particles are fixed in place, while particles are more free to move in the liquid and gas states.
The particles in the solid and liquid states “stick together,” but in the gas state, they move freely about the container.
In general, it requires energy to separate individual particles. If we want to make a solid adopt a liquid form, we
can add energy in the form of heat, increasing the temperature of the substance. Conversely, if we want to convert
a substance from a gas to a liquid or from a liquid to a solid, we remove energy from the system and decrease the
temperature. Pressure also plays an important role in changes of state, which will be discussed later on. We will
study these difference in greater detail in the chapterStates of Matter.
Pure Substances
When studying the different states that matter exhibits, we have been looking atpure substances. Pure substances
have a constant composition and can only be changed through chemical reactions. Constant composition indicates
that a sample of a pure substance always contains the same elements in the same proportions. There are two main
types of pure substances:
- elements: Substances that cannot be decomposed into simpler substances by chemical or physical means.
- compounds: Substances that can be broken down into elements through chemical means.
Figure2.5 shows pure substances in the form of elements and compounds.