Study guide: Chemical Reactions and Changes

📋 Course Outline

1. Physical and Chemical Changes
2. Evidence of Chemical Reactions
3. Reaction of Metals and Salts
4. Light and Heat Emission
5. Formation of New Substances

📖 1. Physical and Chemical Changes

★ Must-know

📌 A physical change alters the state or shape of a substance without changing its chemical composition.

📌 A chemical change produces new substances with chemical properties different from those of the original substances.

Further detail

• Examples of physical changes include:

  • melting ice
  • dissolving sugar in water
  • melting wax
  • expansion of water during freezing
  • adding food coloring to water

• Examples of chemical changes include wood combustion, yogurt formation from milk, wax combustion, rusting iron, burning bread, limewater becoming milky with carbon dioxide, a universal indicator changing color near ammonia gas, cooking food, fireworks, soap formation from oil and caustic soda, and food rotting.

💡 Memory Hook

Physical changes alter state or shape, whereas chemical changes form new substances.

📖 2. Evidence of Chemical Reactions

🔑 Key Concepts & Definitions

• Chemical reaction: the process of breaking bonds between atoms of reactants and forming new bonds between atoms of products

★ Must-know

• The main indications of a chemical reaction are:

  • precipitate formation
  • gas evolution
  • color change
  • change in consistency
  • odour emission
  • emission of light and heat

• Adding diluted hydrochloric acid (HCl) to magnesium (Mg) produces hydrogen gas bubbles, which burn with a pop sound when tested with a nearby burning match.

Further detail

• Adding silver nitrate (AgNO3) to sodium chloride (NaCl) forms a white silver chloride (AgCl) precipitate that changes to violet when exposed to sunlight.

• Passing carbon dioxide gas into clear limewater, which is calcium hydroxide solution, makes the limewater milky or turbid.

• Mixing baking soda, scientifically called sodium bicarbonate, with diluted acetic acid or vinegar causes effervescence and produces carbon dioxide, which is detected using clear limewater.

💡 Memory Hook

PCGCOH: precipitate, color, gas, consistency, odour, heat/light.

📖 3. Reaction of Metals and Salts

📝 Essential Points

• Placing a zinc plate in blue copper sulphate (CuSO4) solution makes the blue color disappear as colorless zinc sulphate (ZnSO4) forms and reddish-brown copper precipitates on the zinc plate.

• During a chemical reaction, reactants may combine, decompose, or replace one another as bonds between them break.

• Every chemical reaction consists of reactants and products and is represented by a chemical equation.

💡 Memory Hook

Zinc replaces copper → blue solution becomes colorless and reddish-brown copper forms.

📖 4. Light and Heat Emission

📝 Essential Points

• Burning a magnesium strip in air makes it glow brightly with heat and converts it into a white magnesium oxide (MgO) powder that is soluble in water.

💡 Memory Hook

Magnesium burns → bright light and heat → white magnesium oxide.

📖 5. Formation of New Substances

★ Must-know

• 🔄 The formation of products involves:

  1. breaking bonds in reactants
  2. rearranging atoms
  3. forming new bonds between product atoms

• 🔄 Soap formation involves: adding 100 ml of oil drop by drop to 30 ml of concentrated NaOH solution, heating the mixture to 40–50 degrees, stirring the mixture well in one direction

• The reaction of oil with concentrated sodium hydroxide changes the mixture's consistency and forms a solid substance called soap.

Further detail

• Burning sugar changes a white, odourless solid into a brown molten substance with a caramel smell and finally into a solid black carbon substance with strong odour, smoke, and color change.

💡 Memory Hook

Reactants bond-breaking → atom rearrangement → product bond-making.

📊 Synthesis Tables

Physical and Chemical Changes

Physical changes	Chemical changes
State or shape changes without changing composition	New substances form with different chemical properties
Melting ice	Wood combustion
Dissolving sugar in water	Rusting iron
Melting wax	Soap formation from oil and caustic soda