Chemical Change vs Physical Change

Chemical Change vs. Physical Change

• Chemical changes result in new substances and are typically irreversible.
• Physical changes do not create new substances and are generally reversible.

Clues of Chemical Change

• Effervescence: Gas production indicates a chemical reaction.
• Precipitate Formation: A solid that forms and separates from a liquid.
• Energy Release: Light or heat may be emitted during the reaction.

Representation of Chemical Change

• Word Equation Format: Reactant(s) + Condition → Product(s)
  • Example: Carbon dioxide + water → glucose + oxygen (with light).
• Mass Conservation: During chemical changes, mass is conserved through atom rearrangement.
• Balancing Equations: Ensure all atoms are accounted for in reactions.

Types of Chemical Changes

Thermal Decomposition

• A single substance breaks down into simpler substances upon heating.
  • Example: Metal Carbonate → Metal Oxide + Carbon Dioxide.

Oxidation

• Rusting: Iron reacts with water and oxygen to form hydrated iron(II) oxide.
• Combustion: A rapid reaction with oxygen producing heat.
  • Example: Petrol + Oxygen → Carbon Dioxide + Water.
  • Incomplete combustion produces toxic carbon monoxide.
• Respiration: Process of converting glucose and oxygen into carbon dioxide and water while releasing energy.

Reactions with Light

Photosynthesis

• Plants convert carbon dioxide and water into glucose and oxygen using light energy.

Reactions with Electricity

• Electrolysis: Uses electricity for the chemical decomposition of substances.
  • Example: Copper Chloride → Copper + Chlorine.
• Electroplating: Coating a substance with another metal through electric current.

Neutralisation

• Acid + Alkali Reaction: Results in salt and water without gas production.
  • Example: Hydrochloric acid + Sodium Hydroxide → Sodium Chloride + Water.

Acid + Metal Reactions

• Reaction produces salt and hydrogen gas without effervescence.
  • Produces a 'pop' sound extinguishing a lit splint.
  • Exceptions: Copper, Silver, and Gold do not react.
  • Example: Nitric Acid + Magnesium → Magnesium Nitrate + Hydrogen gas.

Acid + Metal Carbonate Reactions

• Reaction produces salt, water, and carbon dioxide, often producing a white precipitate in limewater.
  • Example: Sulphuric Acid + Zinc Carbonate → Zinc Sulfate + Water + Carbon Dioxide.

pH Scale

• Acidic substances taste sour with a pH < 7; examples include hydrochloric and sulfuric acids.
• Alkaline substances taste bitter with a pH > 7; examples include sodium and potassium hydroxide.
• Neutral pH is represented by a pH of 7.

Summary of Common Acids and Bases

• Common Acids: Hydrochloric acid, Sulfuric acid, Nitric acid.
• Common Bases: Sodium Hydroxide, Potassium Hydroxide, Aqueous Ammonia.