Chemical changes | Year 9 Chemistry (Cambridge) PDF Download

Chemical Changes

Chemical changes occur when substances react to form new substances with different properties. Examples include burning wood, rusting of iron, and digestion of food.

Key Concepts

• Definition: Chemical changes involve the formation of new substances through chemical reactions.
• Examples: Burning wood to form ash and gases, digestion breaking down food into nutrients, and photosynthesis producing glucose from carbon dioxide and water.

Chemical Equations

Chemical equations represent chemical reactions using symbols and formulas. They must be balanced to show conservation of mass.

Example Reaction

Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water
HCl + NaOH → NaCl + H₂O

Displacement Reactions

Displacement reactions occur when a more reactive element displaces a less reactive one from its compound.

Example

Zinc + Copper(II) sulfate → Zinc sulfate + Copper
Zn + CuSO₄  → ZnSO₄  + Cu

Factors Affecting Reaction Rate

Several factors influence how quickly a chemical reaction proceeds:

• Concentration: Higher concentrations generally increase the rate by increasing the frequency of collisions between particles.
• Temperature: Increased temperature provides more energy to particles, speeding up reactions.
• Surface Area: Smaller particle size increases the surface area available for collisions, accelerating reactions.
• Catalysts: Catalysts speed up reactions by providing an alternative reaction pathway without being consumed themselves.

Reaction Rate Examples

• Temperature Effect: Heating food speeds up digestion due to increased enzyme activity.
• Catalytic Converter: Converts harmful vehicle emissions into less harmful gases.

Summary

Chemical changes involve the formation of new substances, often indicated by changes in properties or energy. Understanding reaction rates helps predict and control chemical processes in various applications.