Liquids and Solids
Liquids and Solids
The atoms or molecules in a liquid have enough kinetic energy to partially overcome the forces of attraction between them. Therefore, they are in constant random motion (as in a gas) but they are still relatively close together. However, they are not as tightly packed, or as well ordered, as in a solid. There is not as much free space in a liquid as in a gas. The atoms or molecules may aggregate together to form chains or rings that readily move relative to one another; this gives a liquid its fluid (flow) properties. Liquids generally occur as compounds. For example, water, ethanol, and carbon tetrachloride are liquids at room temperature. However, a few elements are also liquids at room temperature: bromine, cesium, gallium, mercury, and rubidium. A liquid is characterized by the following physical properties: boiling point and freezing point, density, compressibility, surface tension, and viscosity. These properties of a liquid are greatly influenced by the strength of its intermolecular forces. In summary: • Liquids have definite volume but no definite shape. They take on the shape of their containers. • Liquids are characterized by low compressibility, low rigidity, and high density relative to gases. • Liquids diffuse through other liquids. • Liquids can vaporize into the space above them and produce a vapor pressure. Polar molecules possess an electric dipole moment, μ, defined as the product of the magnitude of the partial charges Q+ and Q− on the molecule and the distance r separating the charges. In mathematical terms, it is given by the equation: μ = Qr The unit for μ is debyes (D), and 1 D = 3.336×10−30 coulomb meter (C-m). No interatomic bonds are completely ionic. Knowing the dipole moment of a compound, though, lets us differentiate ionic from covalent bonds by calculating the percent ionic character for the bonds. The percent ionic character of a bond is found by comparing the measured dipole moment of the molecule of the type A−B with the calculated dipole moment for the 100% ionized compound A+B−.
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