Physical and Chemical Properties
A physical property is one that a sample of matter displays without changing its composition.

• Color
• Odor
• Taste
• Solubility
• Hardness
• Volume
• thermal and electrical conductivities
• physical state of matter
• melting and boiling points

are physical properties of a substance.

A chemical property is the ability (or inability) of a sample of matter to undergo a change in composition. Some examples to chemical properties are

• Combustibility
• Flammability
• Acidity
• Basicity
• Reactivity with water and so on.

Physical Change: is the change in the condition or state of a substance. Physical change do not result with formation of new substances. Ex: melting of ice, breaking of glass

Chemical Change: in a chemical change a substance is converted to a new substance with different composition. Ex: rusting of iron, frying of an egg, burning of paper

Indicators of a Chemical Change:

• Color change
• Formation of gas
• Formation of precipitate
• Formation of smell
• Change in temperature

Chemical Equations
Chemical changes are represented with chemical equations. Chemical equations are always written with the reactants on the left and an arrow pointing to the products on the right.
Reactants→Products

Chemical equations contain the symbols of the elements and the formulas of the compounds in both reactants and products. Physical states of the substances are shown with some abbreviations:
(s) -solid
(l) -liquid
(g) -gas
(aq) -aqueous, water solution.
Formations of solids or gases are often shown with arrows. An upward arrow is used for gases. For solids, the direction of the arrow is downwards.
↑- gas formation
↓- solid formation

Example:
Solid copper (II) oxide reacts with hydrogen gas to give free copper metal and water vapor.
CuO(s)+H₂(g) →Cu(s)+H₂O(g)

 
In a chemical equation:

• Number of molecules
• Physical states
• Color, smell, taste…

may change!!!

• Number of atoms
• Types of atoms
• Total mass

do not change!!!

Balancing Chemical Equations

• Chemical reactions neither create nor destroy matter and therefore the quantity of atom on both sides of the equation should always be equal.
• This can only be done by writing a number in front of the formulas.
• You must never balance an equation by changing the formulas of the compounds!!!

Remember the molecular elements: hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine. If these elements are in free form, they should be written as:
H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂.

Here some examples:

• H₂(g)+O₂(g)→ H₂O(l) unbalanced equation

    2H₂(g)+O₂(g)→ 2H₂O(l)  balanced equation

• CO(g)+O₂(g) →CO₂(g) unbalanced equation

    2CO(g)+O₂(g) →2CO₂(g) balanced equation

• N₂+  H₂  →  NH₃ unbalanced equation

    N₂+  3H₂  →  2NH₃ balanced equation

• N₂+O₂→  NO unbalanced equation

    N₂+O₂→  2NO balanced equation

• Si+ F₂→  SiF₄ unbalanced equation

    Si+ 2F₂→  SiF₄ balanced equation

• K+Cl₂→  KCl unbalanced equation

    K+Cl₂→  2KCl balanced equation

• Al(OH)₃ +  H₂SO₄ →  Al₂(SO₄)₃ + H₂O unbalanced equation

    2Al(OH)₃ +  3H₂SO₄ →  Al₂(SO₄)₃ + 6H₂O balanced equation