Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Changes in Matter

Changes in Matter is an alteration in the form or composition of matter. In science, matter is defined as anything that occupies space and possesses the attributes of gravity and inertia. Matter occurs in three forms: solid, liquid, or gas. Changes in matter may be of two types: physical or chemical. See also States of Matter


A physical change is a change in matter that involves no chemical reaction. When a substance undergoes a physical change, the composition of its molecules remains unchanged, and the substance does not lose its chemical identity. Melting, evaporating, and freezing are three types of physical change. For example, water is a liquid that freezes to form the solid ice, which may again be melted into water. Because molecules of water and ice are composed of the same chemical elements in the same proportions, the change from water to ice is a physical change. Physical changes include any alteration in the shape and size of a substance. For example cutting, grinding, crushing, annealing, dissolving, or emulsifying produce physical changes. Still another physical change is sublimation, the change from a solid to a gas.


When a substance undergoes a chemical change, the composition of its molecules changes. The properties of the original substance are lost, and new substances with new properties are produced. An example of a chemical change is the production of rust (iron oxide) when oxygen in the air reacts with iron. Chemical changes may also result in physical changes. For example, when wood (a solid) is burned, it is combined with oxygen gas to produce gaseous carbon dioxide , liquid water, and solid carbon.

Some of the various chemical changes that matter may undergo are classified below. For a more detailed discussion of chemical reactions, See Chemistry and Chemical Reaction.

A. Combination Reactions -> Combination reactions occur when two substances unite to form a third substance. For example, combining magnesium (Mg) and oxygen results in the production of magnesium oxide (MgO): . This reaction can be accomplished by burning magnesium in air, which supplies the oxygen.

B. Decomposition Reactions -> Decomposition reactions occur when a single compound breaks down into two or more simpler substances. In the decomposition of mercuric oxide (HgO), the elements mercury (Hg) and oxygen are produced: .

C. Displacement Reactions -> When one element replaces another in a compound, it is known as a displacement reaction. For example, iron (Fe) may displace copper (Cu) in a solution of cupric sulphate : .

D. Double Decomposition Reactions -> When two compounds interact to form two other compounds, it is known as a double decomposition reaction. For example, sodium iodide (NaI) and lead nitrate react to form lead iodide and sodium nitrate : .

E. Hydrolysis -> Hydrolysis is a double decomposition reaction in which water reacts with a second substance. When ammonium chloride is combined with water , it undergoes hydrolysis, yielding ammonium hydroxide and hydrochloric acid (HCl): .

F. Neutralization Reactions -> Neutralization is the interaction of an acid with the equivalent quantity of a base (see Acids and Bases). If the process is carried out in an aqueous solution (dissolved in water), the products are water and a salt. For example, hydrochloric acid (HCl) and sodium hydroxide (NaOH) neutralize each other when dissolved in water, forming sodium chloride (NaCl), a salt, and water : HCl + NaOH → NaCl + .

G. Substitution Reactions -> Substitution reactions occur when an element, such as chlorine (Cl), replaces one or more hydrogen atoms in a hydrocarbon, such as methane : .