KEY2CHEM

Phases of Matter

The different phases of matter (solids, liquids, and gases) differ in the organization of their particles. Solids are highly ordered, having particles that are close together with limited motion. Particles in the liquid state are also packed relatively close together, but the particles are continuously moving and colliding. In these condensed phases, particles are highly attracted to one another due to close proximity. Energy input is required to break up the interparticle forces in moving from the solid to the liquid phase.

Example 1.

A substance contains particles that are packed very closely together in an orderly arrangement. The particles have very limited motion and have strong interparticle forces. The substance is most likely a(n) _____________________.

A. crystalline solid

B. amorphous solid

C. liquid

Solution

A. crystalline solid

Crystalline solids have their particles packed in a regular 3-D arrangement. As a solid, the particles are close together with limited motion.

Example 2.

Which compound is expected to be the most viscous (have the highest viscosity)? Assume they are all at the same temperature.

A. $$\require{mhchem}\ce{H2S}$$

B. $$\require{mhchem}\ce{HF}$$

C. $$\require{mhchem}\ce{H2O2}$$

Solution

C. $$\require{mhchem}\ce{H2O2}$$

Viscosity is defined as resistance to flow and increases with the strength of interparticle forces. The stronger the interparticle forces between molecules, the more force is required to separate the particles, and the more resistant the substance is to flowing (the more viscous it is).

Example 3.

In order to convert $$50.0\text{ g}$$ of ice to liquid water at the normal melting point, which statement is true?

A. Energy is released because interparticle forces are being broken.

B. Energy is absorbed because interparticle forces are being broken.

C. Energy is neither released for absorbed because no temperature change occurs.

Solution

B. Energy is absorbed because interparticle forces are being broken.

The phase change from solid to liquid requires the input of energy to break up the stronger interparticle forces holding the solid particles close together and restricting their motion. Although the kinetic energy of the particles is unchanged (because temperature is constant), there is an increase in the potential energy of the system due to the disruption of the interparticle forces in the solid state.