Chemical Changes

Physical Change

A change in matter that is usually reversible and does not form new substances.

Chemical Change

A change in matter that is usually irreversible and results in the formation of new substances.

Reversible Change

A change that can be undone, typically seen in physical changes.

Irreversible Change

A change that cannot be undone, typically seen in chemical changes.

Energy Change in Physical Change

Some physical changes involve energy changes due to heating or cooling.

Energy Change in Chemical Change

Chemical changes always involve energy changes, often in the form of heat or light.

Chemical Bonds

Physical changes do not affect the chemical bonds of substances, while chemical changes do.

Examples of Physical Changes

Melting of ice, evaporation, and sublimation of dry ice.

Examples of Chemical Changes

Formation of water (H2O), combustion of wood, and photosynthesis.

Word Equations

Used to represent chemical reactions, showing reactants, products, and conditions.

Acids

Substances that release hydrogen ions (H+) when dissolved in water.

Bases

Substances that react with acids to form salt and water.

Alkali

A soluble base that produces hydroxide ions (OH-) when dissolved in water.

Neutralisation

The reaction between an acid and a base to produce salt and water.

Salt

A compound formed from the reaction of an acid and a base, consisting of metal positive ions and non-metal negative ions.

Test for Neutralisation

Universal Indicator turns green; Methyl orange turns orange.

Acid + Metal Reaction

Produces salt and hydrogen gas; some metals do not react with dilute acids.

Effervescence

The production of gas (hydrogen or carbon dioxide) during reactions, observed as bubbling.

Test for Hydrogen Gas

A lighted splint extinguishes with a pop sound when hydrogen is present.

Acid + Metal Carbonate Reaction

Produces salt, water, and carbon dioxide; all metal carbonates react with acids.

Test for Carbon Dioxide Gas

Passing gas through limewater produces a white precipitate if carbon dioxide is present.

Sulfate Salts

Formed from sulfuric acid.

Nitrate Salts

Formed from nitric acid.

Chloride Salts

Formed from hydrochloric acid.