Lesson 5.1 – States of Matter Chemistry 1 Honors Dr. J. Venables Northwestern High School

A Comparison of Solids, Liquids, and Gases Physical properties of substances are understood in terms of kinetic molecular theory: Gases are highly compressible, assume shape and volume of container: Gas molecules are far apart and do not interact much with each other. Liquids are almost incompressible, assume the shape but not the volume of container: Liquids molecules are held closer together than gas molecules, but not so rigidly that the molecules cannot slide past each other.

Solids are incompressible and have a definite shape and volume: Solid molecules are packed closely together. The molecules are so rigidly packed that they cannot easily slide past each other.

London dispersion forces Converting a gas into a liquid or solid requires the molecules to get closer to each other: cool or compress. Converting a solid into a liquid or gas requires the molecules to move further apart: heat or reduce pressure. The forces holding solids and liquids together are called intermolecular forces. Hydrogen Bonds Dipole-dipole forces London dispersion forces

Phase Changes

Energy Changes Accompanying Phase Changes All phase changes are possible under the right conditions. The sequence heat solid  melt  heat liquid  boil  heat gas is endothermic. cool gas  condense  cool liquid  freeze  cool solid is exothermic.

Heating and Cooling Curves Plot of temperature change versus heat added is a heating curve. Plot of temperature change versus heat removed is a cooling curve. During a phase change, adding heat causes no temperature change. These points are used to calculate Hfus and Hvap. Supercooling: When a liquid is cooled below its melting point and it still remains a liquid. Achieved by keeping the temperature low and increasing kinetic energy to break intermolecular forces.

From origin to “a” = From a to b = From b to c = From c to d = From d up =

From origin to a = solid From a to b = melting/freezing point From b to c = liquid From c to d = boiling/condensation point From d up = gas

Vapor Pressure and Boiling Point Liquids boil when the external pressure equals the vapor pressure. Temperature of boiling point increases as pressure increases. Two ways to get a liquid to boil: increase temperature or decrease pressure. Pressure cookers operate at high pressure. At high pressure the boiling point of water is higher than at 1 atm. Therefore, there is a higher temperature at which the food is cooked, reducing the cooking time required. Normal boiling point is the boiling point at 760 mmHg (1 atm).