Chemicals and chemical changes.

Chemical Substances

Types of Chemical Substances

1. Pure Substances: Elements or compounds.

2. Mixtures: Combination of two or more substances not chemically combined.

Elements

1

. Definition: Substance made up of only one kind of atoms.

2.

Properties: Each element has its own distinct set of properties.

3. Examples: Silver, gold, aluminium, hydrogen, oxygen.

Compounds

1. Definition: Substance formed when

two or more elements combine chemically.

2. Properties: Properties of a compound differ from those of its constituent elements.

3. Examples

 Mixtures**

1 . Definition: Combination of two or

more substances not chemically combined.

2

. Properties: Retains properties of its components.

3. Examples: Air, mixture of iron filings and sulphur.

Chemical Symbols

1. Definition: Abbreviations used to represent elements.

2. Purpose: Simplifies writing and communication in chemistry.

Chemical Symbols and Atomicity

History of Chemical Symbols

1. Early Symbols: Pictorial symbols used by John Dalton.

2. Modern System: Developed by Jons Jacob Berzelius using letters of the English alphabet.

Features of Modern Chemical Symbols

1. First Letter: First letter of the element's name is used as its symbol.

2. Additional Letters: Second letter or distinct letter added when necessary.

3. Latin Names: Some symbols derived from Latin names (e.g., Au for Gold, Ag for Silver).

Examples of Chemical Symbols

1. Elements: H (Hydrogen), C (Carbon), O (Oxygen), N (Nitrogen).

2. Symbols from Latin Names: Na (Sodium), K (Potassium), Pb (Lead).

Chemical Formulae and Equations

Chemical Formulae

1. Definition: Representation of a molecule of a compound using chemical symbols of its constituent elements.

2. Examples: H2O (water), NaCl (common salt), CaCO3 (calcium carbonate).

Valency

1. Definition: Combining capacity of an atom of an element.

2. Examples: Hydrogen (1), Oxygen (2), Carbon (4).

Writing Chemical Formulae

1. Steps:

Write symbols of constituent elements.

Write valencies of elements.

Cross over valencies and write as subscripts.

Simplify formula if possible.

Atomicity

1

. Definition: Number of atoms present in one molecule of an element.

2. Types:

Monatomic: 1 atom (e.g., noble gases).

Diatomic: 2 atoms (e.g., oxygen, 02).

Tetratomic: 4 atoms (e.g.,

phosphorus, P4).

Chemical Equations

1 . Definition: Shorthand form of

representing a chemical reaction using symbols and formulae.

2. Components:

Reactants: Substances involved initially in the reaction.

Products: New substances formed in the reaction.

Writing Chemical Equations

1. Steps:

Identify reactants and products.

Write names of reactants on the left-hand side with a '+' sign between them.

Write names of products on the right-hand side with a '+' sign between them.

Use an arrow (→) to separate reactants from products.

Chemical Equations

Writing Chemical Equations

1. Steps:

Identify reactants and products.

Write names of reactants on the left-hand side with a '+' sign between them.

Write names of products on the right-hand side with a '+' sign between them.

Use an arrow (→) to separate reactants from products.

2. Example: Magnesium burns in oxygen to form magnesium oxide.

Word equation: magnesium + oxygen → magnesium oxide

Chemical equation: 2Mg + 02 → 2MgO

Balancing Chemical Equations

1. Definition: Ensuring the number of atoms of each element is equal on both sides of the equation.

2. Steps:

Count the number of atoms of each element on both sides.

Adjust coefficients to balance the equation.

Examples of Chemical Changes

1. Rusting of Iron: Iron reacts with

oxygen and moisture to form iron oxide (rust).

Chemical equation: 4Fe + 302 + 6H2O → 4Fe(OH)3

Importance of Chemical Equations

1. Representation: Chemical equations

represent chemical reactions in a concise and symbolic way.

2. Balancing: Balancing chemical equations ensures the law of conservation of mass is obeyed

Balancing Chemical Equations

1. Definition: Ensuring the number of atoms of each element is equal on both sides of the equation.

2. Steps:

Count the number of atoms of each element on both sides.

Adjust coefficients to balance the equation.

Examples of Chemical Changes

1. Rusting of Iron: Iron reacts with

oxygen and moisture to form iron oxide (rust).

Chemical equation: 4Fe + 302 + 6H2O → 4Fe(OH)3

Importance of Chemical Equations

1. Representation: Chemical equations

represent chemical reactions in a concise and symbolic way.

2. Balancing: Balancing chemical equations ensures the law of conservation of mass is obeyed.

Chemical Changes

Examples of Chemical Changes

1. Rusting of Iron: Iron reacts with

oxygen and moisture to form iron oxide (rust).

Conditions: Oxygen and water are essential for rusting.

Prevention: Oiling or galvanizing iron objects can prevent rusting.

2. Browning of Vegetables and Fruits:

Cut surfaces of vegetables and fruits turn brown due to reaction with atmospheric oxygen.

Prevention: Soaking cut vegetables and fruits in plain water can reduce browning.

3. Reaction between Vinegar and Baking Soda: Produces carbon dioxide gas.

Chemical equation: CH3COOH + NaHCO3 CH3COONa CO2 + H2O +

4. Reaction between Copper Sulphate Solution and Iron Nails: Iron displaces

copper from copper sulphate solution,

forming iron sulphate and copper.

Chemical equation: CuSO4 + Fe → FeSO4 + Cu

Characteristics of Chemical Changes

1. Permanent Change: Chemical

changes are often irreversible.

2. New Substances Formed: Chemical changes result in the formation of new substances with different properties

Physical and Chemical Changes

Physical Changes

1. Definition: Changes in which no new substances are formed.

2. Examples: Formation of ice from water, crystallization of common salt.

3. Characteristics: Properties such as color, shape, size, or physical form may change.

Chemical Changes

1. Definition: Changes in which new substances are formed.

2. Examples: Rusting of iron, reaction

between vinegar and baking soda, reaction between copper sulphate solution and iron nails.

3. Characteristics: Permanent change, new substances formed with different properties.

Crystallization

1. Definition: Process of obtaining crystals of a substance from its solution.

2. Example: Crystallization of common salt from seawater.

3. Steps:

Dissolve impure substance in water.

Filter the solution to remove insoluble impurities.

Allow the solution to cool and form crystals.

Examples of Chemical Reactions

1. Rusting of Iron: Iron reacts with

oxygen and moisture to form iron oxide (rust).

2. Reaction between Vinegar and Baking

Soda: Produces carbon dioxide gas.

3. Reaction between Copper Sulphate Solution and Iron Nails: Iron displaces copper from copper sulphate solution, forming iron sulphate and copper.