Matter exists in any of three phases, or states—solid, liquid, or gas.

The state in which a compound exists under a given set of conditions depends on the relative strength of the attractive forces between particles compared to the kinetic energy of the particles.

In gases, the attractive forces between particles are very weak compared to their kinetic energy, so the particles move about freely, are far apart, and have almost no influence on one another.

In liquids, the attractive forces between particles are stronger, pulling the particles close together but still allowing them considerable freedom to move about.

In solids, the attractive forces are much stronger than the kinetic energy of the particles, so the atoms, molecules, or ions are held in a specific arrangement and can only vibrate in place.

The transformation of a substance from one state to another is called a phase change or a change of state. Every change of state is reversible and is characterized by changes in enthalpy and entropy.

The enthalpy and entropy associated with phase changes are contrary; the melting of ice, is unfavoured by a positive ∆H but favoured by a positive ∆S. Similarly, the freezing of water is favoured by a negative ∆H but is unfavoured by a negative ∆S.

The exact temperature at which these two factors (∆H and ∆S) exactly balance out is called the melting point and represents the temperature at which solid and liquid coexist in equilibrium. In the corresponding change from a liquid to a gas, the two states are in equilibrium at the boiling point.

The stronger the intermolecular forces in a substance, the more difficult it is to separate the molecules, and the higher the melting and boiling points of the substance.

There are three major types of intermolecular forces: London dispersion, dipole– dipole, and hydrogen bonding. Collectively, these attractive forces are also known as van der Waals forces.

London dispersion force are the short-lived attractive force due to the constant motion of electrons within molecules.

• London dispersion forces are the only intermolecular force available to nonpolar molecules; they are relatively weak but they increase with molecular mass and amount of surface area available for interaction between molecules.

• The larger the molecular mass, the more electrons there are moving about and the greater the temporary polarization of a molecule. The larger the amount of surface contact, the greater the close interaction between different molecules.

Dipole–dipole force are attractive force between positive and negative ends of polar molecules.

• Dipole–dipole forces are typically stronger than London dispersion forces. Although still significantly weaker than covalent bonds, the effects of dipole–dipole forces are, nevertheless, important.

A hydrogen bond is an attractive interaction between an H-bond acceptor (an electronegative O or N atom having unshared electron pairs) and an H-bond donor (a positively polarized hydrogen atom bonded to another electronegative atom (N, O, or F).

The behaviour of gases can be explained by a group of assumptions known as the kinetic–molecular theory of gases.

• A gas consists of many particles, either atoms or molecules, moving about at random with no attractive forces between them. Because of this random motion, different gases mix together quickly.

• The amount of space occupied by the gas particles themselves is much smaller than the amount of space between particles. Most of the volume taken up by gases is empty space, accounting for the ease of compression and low densities of gases.

• The average kinetic energy of gas particles is proportional to the Kelvin temperature. Thus, gas particles have more kinetic energy and move faster as the temperature increases.

• Collisions of gas particles, either with other particles or with the wall of their container, are elastic; that is, no energy is lost during collisions so the total kinetic energy of the particles is constant.

• A gas that obeys all the assumptions of the kinetic–molecular theory is called an ideal gas.

In scientific terms, pressure (P) is defined as a force (F) per unit area (A) pushing against a surface; that is, P = F/A.

• One of the most commonly used units of pressure is the millimetre of mercury, abbreviated mmHg and often called a torr.

Gas pressure inside a container is often measured using an open-ended manometer.

The gas laws make it possible to predict the influence of pressure (P), volume (V), temperature (T), and molar amount (n) on any gas or mixture of gases.

• Boyle’s law :The volume of a gas is inversely proportional to its pressure for a fixed amount of gas at a constant temperature. That is, P times V is constant when the amount of gas n and the temperature T are kept constant.

• Charles’s law :The volume of a gas is directly proportional to its kelvin temperature for a fixed amount of gas at a constant pressure. That is, V divided by T is constant when n and P are held constant.

• Gay-Lussac’s law: The pressure of a gas is directly proportional to its Kelvin temperature for a fixed amount of gas at a constant volume. That is, P divided by T is constant when n and V are held constant.

Since PV, V/T, and P/T all have constant values for a fixed amount of gas, these relationships can be merged into a combined gas law, which holds true whenever the amount of gas is fixed.

(PV)/T = constant(k)

According to Avogadro’s law, the volume of a gas is directly proportional to its molar amount at a constant pressure and temperature.

Standard temperature and pressure (STP): 0 °C (273.15 K); 1 atm (760 mmHg).

Standard molar volume: Volume of one mole of any ideal gas at STP, 22.4 L>mol.

The relationships among the four variables P, V, T, and n for gases can be combined into a single expression called the ideal gas law.

PV=nRT

• The constant R in the ideal gas law (instead of the usual k) is called the gas constant. Its value depends on the units chosen for pressure.

The contribution of each gas in a mixture to the total pressure of the mixture is called the partial pressure of that gas.

According to Dalton’s law, the total pressure exerted by a gas mixture is the sum of the partial pressures of the components in the mixture.

The composition of air does not change appreciably with altitude, but the total pressure decreases rapidly. The partial pressure of oxygen in air therefore decreases with increasing altitude, and it is this change that leads to difficulty in breathing at high elevations.

If a molecule happens to be near the surface of a liquid, and if it has enough energy, it can break free of the liquid and escape into the gas state, called vapor.

Vapor pressure is the partial pressure of vapor molecules in equilibrium with a liquid.

Normal boiling point :The boiling point at a pressure of exactly 1 atm.

Surface tension is caused by the different forces experienced by molecules in the interior of a liquid and those on the surface. Molecules on the surface are less stable because they feel fewer attractive forces, so the liquid acts to minimize their number by minimizing surface area.

Crystalline solid is solid whose atoms, molecules, or ions are rigidly held in an ordered arrangement.

Ionic solids are those whose constituent particles are ions.

Molecular solids are those like sucrose or ice, whose constituent particles are molecules held together by the intermolecular forces discussed.

Covalent network solids are those like diamond or quartz, whose atoms are linked together by covalent bonds into a giant three-dimensional array. In effect, a covalent network solid is one very large molecule.

Metallic solids, such as silver or iron, can be viewed as vast three-dimensional arrays of metal cations immersed in a sea of electrons that are free to move about. This continuous electron sea acts both as a glue to hold the cations together and as a mobile carrier of charge to conduct electricity.

The fact that bonding attractions extend uniformly in all directions explains why metals are malleable rather than brittle. When a metal crystal receives a sharp blow, no spatially oriented bonds are broken; instead, the electron sea simply adjusts to the new distribution of cations.

Amorphous solid is a solid whose particles do not have an orderly arrangement.

Heat of fusion: The quantity of heat required to completely melt 1 g of a substance once it has reached its melting point.

Heat of vaporization: The quantity of heat needed to completely vaporize 1 g of a liquid once it has reached its boiling point.