Chapter 6

### INTRODUCTION

Matter in the universe, defined as anything that occupies space and has mass, exists in three phases—gases, liquids, and solids. All substances are made of atoms or molecules that are constantly in motion, although not necessarily seen by the naked eye. This assumption is essential to account for many of the properties of gases and liquids.

Phases are defined as a distinct and homogeneous state of a system with no visible boundary separating it into parts. A conversion from one phase to another is given a specific name and is associated with various standard properties. For example, condensation occurs when a substance transitions from a gas to a liquid phase and vaporization occurs when a substance transitions from a liquid to gaseous phase.

### LIQUIDS

1. Volume—Liquids occupy a definitive volume and will take on the shape of the vessel in which they are contained. Unlike a gas, the volume of a liquid does not change much, if at all, as pressure increases. The volume occupied by a given amount of a liquid is much less than that of the corresponding gas at the same temperature, because the constituent particles are much closer together in the liquid phase.

2. Surface tension—Surface tension is a unique property of liquids that allows them to assume a shape that has the least amount of surface area. Liquids generally form spherical droplets because spheres are a solid shape with the least surface area per unit volume. Surface tension is created by Van der Waals’ forces, which are the sum of the attractive or repulsive forces between molecules. Particles in the bulk of the liquid are pulled in all directions by intermolecular forces, whereas particles on the surface are only pulled from molecular forces below, leading to an unbalanced force on the surface of the liquid.

3. Boiling point—The boiling point, or vaporization point, of a liquid occurs when its vapor pressure equals the external pressure (ambient pressure) acting on the surface of the liquid. The stronger the intermolecular forces, the lower the vapor pressure and the higher the boiling point. Vapor pressure is created by the pressure exerted on the environment from vapor that is in equilibrium with its liquid phase. The molar mass of the substance and the intermolecular forces acting on the substance influence vapor pressure. The pressure of the atmosphere at sea level is 1 atm (760 mm Hg), but at higher altitudes, the ambient air pressure is much lower—for example, in Denver theair pressure is approximately 630 mm Hg because it is 1 mile above the sea level. This means that a liquid will boil at a lower temperature in Denver because the vapor pressure has to equal a lower external pressure before boiling compared to the usual 1 atm at sea level.

4. Cohesive and adhesive forces—Liquids demonstrate cohesive forces as well as adhesive forces. Cohesive forces are the attraction between ...

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