Elements and Compounds

• Chemical substances are composed of atoms and molecules.
• An atom is the smallest particle of a substance, which may or may not exist independently.
• Molecules form when two or more atoms join; they can exist independently.
• Pure substances can be elements or compounds.
• Chemical substances are divided into pure substances and mixtures.

Elements

• An element consists of only one kind of atom and cannot be broken down chemically into simpler substances.
• Each element has its own distinct properties and the atoms of an element will have all the properties of that particular element.
• Examples of elements: silver, gold, aluminum, hydrogen, and oxygen.
• There are over 114 known elements on Earth.

Compounds

• A compound is formed when two or more elements combine chemically.
• The smallest particle of a compound is a molecule.
• The properties of a compound differ entirely from those of its constituent elements.
• Water (a liquid) is formed when hydrogen and oxygen (both gases) combine.

Mixtures

• Mixtures consist of two or more substances simply mixed together, not chemically combined.
• A mixture retains the properties of its components.
• Air is a mixture of gases like nitrogen, oxygen, carbon dioxide, water vapor, and argon.
• In an iron filings and sulfur mixture, iron is still attracted by a magnet, and sulfur remains as a powder.

Chemical Symbols

• Chemical symbols are abbreviations for elements used to avoid writing full names.
• Sir John Dalton created pictorial symbols for elements.
• Jons Jacob Berzelius used letters of the English alphabet to represent elements.
• The first letter of an element's English name is often used as its symbol.
• Examples: H for Hydrogen, C for Carbon, O for Oxygen, N for Nitrogen.
• Symbols for some elements are derived from their Latin names.
• When multiple elements begin with the same letter, another letter is added to the symbol.
• In some cases, another distinct letter is used (e.g., Chlorine (Cl), Magnesium (Mg)).

Atomicity

• Many elements exist as molecules, as their atoms cannot exist independently.
• Atomicity refers to the number of atoms in one molecule of an element.
• Molecules can be monatomic (1), diatomic (2), or tetratomic (4).

Chemical Formulae

• A chemical formula represents a chemical compound.
• It uses chemical symbols of constituent elements to represent a molecule of a compound
• If formulas are known, the constituent elements (and their symbols) and the valency of the elements can be derived.

Valency

• Valency refers to the combining capacity of an atom of an element.
• Hydrogen (H) has a valency of 1, Carbon (C) has a valency of 4, and Oxygen (O) has a valency of 2

Writing a Chemical Formula

• Symbols of constituent elements are noted.
• Valencies of elements are written down.
• Valencies are crossed over and written as subscripts.
• Formula is simplified if valencies are divisible by a common number.
• Example: Water's chemical formula is H₂O
• Example: Carbon dioxide's chemical formula is CO₂

Chemical Equations

• A chemical equation is a shorthand representation of a chemical reaction using symbols and formulae.
• Chemical reactions convert certain chemical substances into other chemical substances.
• Reactants are the elements/compounds involved initially in the reaction.
• Products are the new elements/compounds formed in the reaction.

Writing a Chemical Equation

• Identify reactants and products.
• List reactants on the left with a '+' sign.
• List products on the right with a '+' sign.
• Place an arrow between reactants and products.
• Word equations are made.
• Replace names with symbols and formulae to create the chemical equation.

Balancing an Equation

• Count the atoms of each element on both sides of the equation
• Ensure the number of atoms of each element is equal on both sides to balance the equation.
• Magnesium burns in oxygen to give magnesium oxide as an example of forming a chemical equation.

Chemical Changes

• A chemical change, also called a reaction, occurs when two or more chemical substances react to produce a new substance/substances
• Change is permanent.

Reddish-Brown Deposition on Iron Objects

• Rusting: A reddish-brown layer (rust) is deposited on iron objects left in the rain.
• Rusting is a chemical change as the reaction is irreversible.
• Oxygen and water are essential for rusting.
• Oiling iron objects prevents rusting by blocking contact with air and moisture.

Browning of Vegetable and Fruit Surfaces

• Cut surfaces turn brown due to a reaction between compounds and atmospheric oxygen.
• Melanin, a brown pigment, is produced, causing browning.
• Soaking cut produce in water reduces browning by restricting oxygen contact.

Reaction Between Vinegar and Baking Soda

• Carbon dioxide gas is released when baking soda solution reacts with vinegar.
• The gas can be verified as carbon dioxide.
• Limewater turns milky when carbon dioxide is passed through it.

Reaction Between Copper Sulphate Solution and Iron Nails

• Copper sulphate solution, initially blue, turns sea-green when iron nails are added.
• Iron nails acquire a brown color.
• Iron sulphate which is sea-green in color, is formed.
• Copper deposits on the iron nails.

Physical Changes

• Physical changes do not form new substances.
• The formation of ice from water is a physical change, and is reversible.

Crystallization

• Crystals are the purest solid form of a substance with a definite geometrical shape.
• Crystallization converts an impure compound into pure crystals.
• This process is used to obtain common salt from seawater.
• Undesirable substances removed through the process of crystallization.
• Maximum salt is dissolved in boiling water, then filtered to remove impurities.
• The filtered solution is left undisturbed to form sodium chloride crystals.