Chemical Changes - Comprehensive Study Notes

Key Concepts

Characteristics of Chemical Change

• Chemical changes produce new substances with different properties from the original substances
• During a chemical change, chemical bonds are broken and new bonds are formed
• Chemical changes are usually difficult to reverse (unlike physical changes)

Key indicators that a chemical change has occurred:

• Colour change - A new colour appears that wasn’t present before
• Temperature change - Heat is given out (exothermic) or absorbed (endothermic)
• Gas production - Bubbles or fizzing observed (effervescence)
• Precipitate formation - A solid forms in a solution
• Permanent change - Cannot easily reverse to get original substances back
• Light emission - Some reactions produce light

Comparison with Physical Changes:

• Physical changes: No new substance formed (e.g., melting ice, dissolving sugar)
• Chemical changes: New substance(s) formed with different properties

Acids and Bases — pH and Indicators

Acids:

• Sour taste (never taste chemicals in the lab!)
• Turn blue litmus paper red
• React with metals to produce hydrogen gas
• React with carbonates to produce carbon dioxide gas
• Common examples: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), ethanoic acid (vinegar)

Bases:

• Bitter taste and feel slippery (never taste or touch!)
• Turn red litmus paper blue
• React with acids in neutralisation reactions
• Common examples: sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂), ammonia (NH₃)

The pH Scale:

• Measures how acidic or alkaline a solution is
• Scale ranges from 0 to 14
• pH 7 = neutral (pure water)
• pH 0-6 = acidic (lower number = stronger acid)
• pH 8-14 = alkaline (higher number = stronger base)
• Each step represents a 10-fold change in acidity/alkalinity

Common pH Values:

• Stomach acid (gastric juice): pH 1-2
• Lemon juice: pH 2-3
• Vinegar: pH 3
• Pure water: pH 7
• Blood: pH 7.4
• Soap solution: pH 9-10
• Bleach: pH 12-13

Indicators:

• Substances that change colour depending on pH

• Litmus paper:

  • Red litmus turns blue in alkali
  • Blue litmus turns red in acid
  • No change in neutral solutions

• Universal indicator:

  • Shows a range of colours for different pH values
  • Red/orange = acidic
  • Yellow/green = neutral
  • Blue/purple = alkaline
  • More precise than litmus paper

• Phenolphthalein:

  • Colourless in acid
  • Pink in alkali
  • Used in titrations

Neutralisation

• Neutralisation is the reaction between an acid and a base to produce a salt and water
• General word equation: Acid + Base → Salt + Water
• The reaction is exothermic (gives out heat)
• At the neutralisation point, pH = 7

Examples of neutralisation reactions:

• Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water

• HCl + NaOH → NaCl + H₂O

• Sulfuric acid + Potassium hydroxide → Potassium sulfate + Water

• H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O

Types of bases:

• Alkali: A base that dissolves in water (e.g., NaOH, KOH)
• All alkalis are bases, but not all bases are alkalis

Salt naming:

• The first part comes from the base (metal or ammonium)
• The second part comes from the acid:
  • Hydrochloric acid → chloride
  • Sulfuric acid → sulfate
  • Nitric acid → nitrate

Real-life applications:

• Treating acid indigestion with antacids (contain bases)
• Neutralising acidic soil with lime (calcium hydroxide)
• Treating bee stings (acidic) with baking soda (basic)
• Treating wasp stings (alkaline) with vinegar (acidic)
• Neutralising acid spills in laboratories

Reactions of Acids with Metals

General word equation: Acid + Metal → Salt + Hydrogen gas

• Not all metals react with acids
• Reactivity series (most reactive to least):
  • Potassium, Sodium, Calcium, Magnesium, Aluminium, Zinc, Iron, Lead, Hydrogen, Copper, Silver, Gold
• Only metals above hydrogen in the reactivity series will react with dilute acids to produce hydrogen gas
• Copper, silver, and gold do NOT react with dilute acids

Observations during reaction:

• Effervescence (fizzing/bubbling)
• Metal dissolves/disappears
• Heat produced (exothermic)
• Gas produced can be tested: “squeaky pop” with lighted splint = hydrogen

Examples:

• Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen

• Mg + 2HCl → MgCl₂ + H₂

• Zinc + Sulfuric acid → Zinc sulfate + Hydrogen

• Zn + H₂SO₄ → ZnSO₄ + H₂

Speed of reaction depends on:

• Type of metal (more reactive = faster reaction)
• Concentration of acid (more concentrated = faster)
• Temperature (higher temperature = faster)
• Surface area of metal (powder reacts faster than lumps)

Reactions of Acids with Carbonates

General word equation: Acid + Carbonate → Salt + Water + Carbon dioxide

• All carbonates react with acids
• Common carbonates: calcium carbonate (CaCO₃), sodium carbonate (Na₂CO₃), zinc carbonate (ZnCO₃)

Observations:

• Effervescence (fizzing)
• Carbonate dissolves
• Gas produced is carbon dioxide

Test for carbon dioxide:

• Bubble gas through limewater (calcium hydroxide solution)
• Limewater turns milky/cloudy if CO₂ is present
• This is the confirmatory test for carbon dioxide

Examples:

• Calcium carbonate + Hydrochloric acid → Calcium chloride + Water + Carbon dioxide

• CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

• Sodium carbonate + Sulfuric acid → Sodium sulfate + Water + Carbon dioxide

• Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂

Real-life applications:

• Limestone buildings damaged by acid rain
• Using vinegar to remove limescale (calcium carbonate)
• Baking powder (carbonate) reacting with acids in cake mixture to produce CO₂ (makes cakes rise)

Oxidation — Rusting and Burning

Oxidation:

• A chemical reaction where a substance gains oxygen
• Can also be defined as the loss of electrons (at higher levels)

Rusting:

• Rusting is the corrosion of iron in the presence of oxygen and water
• Chemical name: Iron reacts to form hydrated iron(III) oxide (rust)
• Word equation: Iron + Oxygen + Water → Hydrated iron(III) oxide (rust)

Conditions needed for rusting:

• BOTH oxygen (from air) AND water must be present
• Neither alone will cause rusting
• Salt water accelerates rusting (electrolyte effect)
• Acids accelerate rusting

Preventing rust:

• Barrier methods (prevent contact with oxygen and water):

  • Painting
  • Greasing/oiling
  • Plastic coating
  • Plating with another metal (e.g., chromium plating)

• Sacrificial protection:

  • Coating with a more reactive metal (e.g., zinc - called galvanising)
  • The more reactive metal corrodes instead of iron
  • Zinc blocks, magnesium blocks attached to ship hulls and pipelines

• Alloying:

  • Mixing iron with other metals to make stainless steel
  • Stainless steel contains chromium which forms a protective oxide layer

Burning (Combustion):

• Combustion is a chemical reaction where a substance reacts with oxygen and releases energy as heat and light
• A type of oxidation reaction that happens rapidly

Requirements for combustion (Fire Triangle):

1. Fuel - substance that burns
2. Oxygen - usually from air
3. Heat - ignition temperature must be reached

Complete combustion:

• Plenty of oxygen available
• Hydrocarbon + Oxygen → Carbon dioxide + Water
• Clean flame (blue)
• Example: Methane + Oxygen → Carbon dioxide + Water
• CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion:

• Limited oxygen supply
• Produces carbon monoxide (CO) or carbon (soot)
• Yellow/orange smoky flame
• Carbon monoxide is poisonous (colourless, odourless)
• Example: Methane + Oxygen → Carbon monoxide + Water
• 2CH₄ + 3O₂ → 2CO + 4H₂O

Extinguishing fires:

• Remove one element of the fire triangle:
  • Remove fuel (turn off gas supply)
  • Remove oxygen (cover with fire blanket, use foam, CO₂ extinguisher)
  • Remove heat (water cools below ignition temperature)

Organic chemistry (alkanes, alcohols, carboxylic acids) is a Sec 3/4 topic — not required for Sec 2 NA.

Important Definitions

Chemical change: A change that produces one or more new substances with different properties from the original substances.

Acid: A substance that turns blue litmus paper red, has a pH less than 7, and produces hydrogen ions (H⁺) in water.

Base: A substance that turns red litmus paper blue, has a pH greater than 7, and reacts with acids in neutralisation.

Alkali: A base that dissolves in water to form a solution with pH greater than 7.

pH: A scale from 0 to 14 that measures how acidic or alkaline a solution is, with 7 being neutral.

Indicator: A substance that changes colour depending on whether it is in an acid or alkali.

Neutralisation: The chemical reaction between an acid and a base that produces a salt and water.

Salt: A compound formed when the hydrogen in an acid is replaced by a metal or ammonium ion.

Oxidation: A chemical reaction in which a substance gains oxygen (or loses electrons).

Rusting: The corrosion of iron in the presence of oxygen and water to form hydrated iron(III) oxide.

Combustion: A chemical reaction in which a substance reacts rapidly with oxygen, releasing heat and light energy.

Reactivity series: A list of metals arranged in order of their reactivity, from most reactive to least reactive.

Precipitate: An insoluble solid that forms in a solution during a chemical reaction.

Effervescence: The production of bubbles of gas during a chemical reaction (fizzing).

Worked Examples

Example 1: Identifying Chemical Changes

Question: For each of the following, state whether it is a chemical change or physical change, and give a reason for your answer.

a) Burning a piece of paper b) Boiling water c) Iron rusting d) Dissolving salt in water

Solution:

a) Chemical change

• Reason: New substances formed (ash, carbon dioxide, water vapour)
• Permanent change that cannot be reversed
• Heat and light given out

b) Physical change

• Reason: No new substance formed - water remains water (H₂O)
• Only state changes from liquid to gas
• Can be reversed by condensation

c) Chemical change

• Reason: New substance formed (iron oxide - rust)
• Different properties from original iron
• Cannot easily reverse to get pure iron back

d) Physical change

• Reason: No new substance formed
• Salt (sodium chloride) remains chemically the same
• Can be reversed by evaporation to get salt back

Example 2: Neutralisation and Salt Formation

Question: Complete the following neutralisation reactions and name the salt formed:

a) Hydrochloric acid + Sodium hydroxide → _____ + _____

b) Sulfuric acid + Potassium hydroxide → _____ + _____

c) Nitric acid + Calcium hydroxide → _____ + _____

Solution:

a) Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water

Step-by-step working:

• Metal/base: Sodium → First part of salt name: Sodium
• Acid: Hydrochloric → Second part of salt name: chloride
• All neutralisations produce water
• Salt formed: Sodium chloride (NaCl)

b) Sulfuric acid + Potassium hydroxide → Potassium sulfate + Water

Step-by-step working:

• Metal/base: Potassium → First part: Potassium
• Acid: Sulfuric → Second part: sulfate
• Salt formed: Potassium sulfate (K₂SO₄)

c) Nitric acid + Calcium hydroxide → Calcium nitrate + Water

Step-by-step working:

• Metal/base: Calcium → First part: Calcium
• Acid: Nitric → Second part: nitrate
• Salt formed: Calcium nitrate (Ca(NO₃)₂)

Remember the pattern:

• Hydrochloric acid → chloride salts
• Sulfuric acid → sulfate salts
• Nitric acid → nitrate salts

Example 3: Acid Reactions and Gas Tests

Question: A student adds magnesium ribbon to hydrochloric acid and observes fizzing.

a) Write a word equation for this reaction. b) Name the gas produced. c) Describe how to test for this gas. d) What would be observed in this test?

Solution:

a) Word equation: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen

Working:

• Pattern: Metal + Acid → Salt + Hydrogen
• Salt naming: Magnesium (from metal) + chloride (from hydrochloric acid)

b) Gas produced: Hydrogen

Reasoning:

• Metals above hydrogen in reactivity series react with acids
• Always produce hydrogen gas
• Magnesium is quite reactive (above hydrogen in series)

c) Test for hydrogen gas:

• Collect the gas in a test tube
• Bring a lighted/burning splint to the mouth of the test tube

Safety note: Hold test tube at arm’s length pointing away from face

d) Observation in positive test:

• A “squeaky pop” sound is heard
• This confirms the gas is hydrogen
• The pop is caused by hydrogen igniting and reacting rapidly with oxygen

Alternative answer: A small explosion or “pop” sound with a blue flame

Common Mistakes to Avoid

1. Confusing Chemical and Physical Changes

• Mistake: Thinking dissolving is always a chemical change
• Correction: Dissolving is usually physical (like salt in water) unless a reaction occurs (like sodium in water)
• Key: Ask “Has a new substance formed?” If no, it’s physical

2. pH Scale Errors

• Mistake: Saying pH 8 is acidic or pH 6 is alkaline
• Correction: pH 7 is the EXACT neutral point. Below 7 = acid, Above 7 = alkali
• Remember: The further from 7, the stronger the acid/alkali

3. Litmus Paper Confusion

• Mistake: “Blue litmus turns red in alkali” or “Red litmus turns blue in acid”
• Correction:
  • Blue litmus turns red in ACID
  • Red litmus turns blue in ALKALI
  • Memory aid: “RED with RAGE in acid” or “Blue turns red when it’s sad (acid)”

4. Incomplete Neutralisation Equations

• Mistake: Writing “Acid + Base → Salt” (forgetting water)
• Correction: ALWAYS write “Acid + Base → Salt + Water”
• Remember: Neutralisation produces TWO products: salt AND water

5. Wrong Gas Products

• Mistake: Saying acids + metals produce carbon dioxide
• Correction:
  • Acid + Metal → Salt + HYDROGEN
  • Acid + Carbonate → Salt + Water + CARBON DIOXIDE
• Don’t mix these up!

6. Salt Naming Errors

• Mistake: “Hydrochloric acid + magnesium → magnesium hydrochlorate”
• Correction: Hydrochloric acid makes chloride salts (not “hydrochlorate”)
• Pattern to memorize:
  • Hydrochloric → chloride
  • Sulfuric → sulfate
  • Nitric → nitrate

7. Rusting Conditions

• Mistake: Saying iron rusts in air OR water (listing them separately)
• Correction: Iron needs BOTH oxygen AND water simultaneously
• Key word: Use “AND” not “OR” when describing conditions for rusting

8. Incomplete Combustion Product Confusion

• Mistake: Saying incomplete combustion produces no carbon dioxide at all
• Correction: Incomplete combustion produces carbon monoxide (CO) and/or carbon (soot) in addition to water. Unlike complete combustion, not all the carbon is converted to CO₂ — but CO₂ can still be produced alongside CO.
• Complete combustion → CO₂ + H₂O (all carbon converts to CO₂)
• Incomplete combustion → CO and/or soot © + H₂O (some carbon forms CO or soot instead of CO₂)

9. Fire Triangle Misconceptions

• Mistake: Saying only two factors are needed for fire
• Correction: ALL THREE factors (fuel, oxygen, heat) must be present
• To extinguish: Remove ANY ONE of the three

10. Gas Test Descriptions

• Mistake: Saying “Hydrogen burns” or “It explodes” (not specific enough)
• Correction: Use precise phrases:
  • Hydrogen: “Lighted splint produces a squeaky pop sound”
  • Carbon dioxide: “Limewater turns milky/cloudy”

11. Oxidation Definition

• Mistake: Saying oxidation only means “reacting with oxygen”
• Correction: At this level, oxidation means “gaining oxygen” (substance combines with oxygen)
• Example: Iron oxidises means iron gains oxygen to form iron oxide

Exam Tips

Keywords and Mark-Earning Phrases

For identifying chemical changes (1-2 marks):

• Always state: “New substance(s) formed”
• Add: “Cannot be easily reversed” or “Permanent change”
• Mention observable signs: “colour change,” “temperature change,” “gas evolved,” “precipitate formed”

For pH and acids/bases questions (1-2 marks):

• Use specific numbers: “pH less than 7” (not just “low pH”)
• State colour changes precisely: “Blue litmus turns red” (specify which colour litmus and the colour change)
• For universal indicator: Give both colour AND pH number

For neutralisation (2-3 marks):

• Always write the complete word equation with all four parts: “Acid + Base → Salt + Water”
• Name the specific salt using the correct pattern
• State that the reaction is “exothermic” (if asked about energy)
• Mention “pH 7 at neutralisation point” or “pH moves toward 7”

For acid + metal reactions (2-3 marks):

• Write complete word equation: “Metal + Acid → Salt + Hydrogen”
• For gas test: “Lighted splint produces a squeaky pop sound with hydrogen”
• Mention observations: “effervescence,” “metal dissolves,” “solution may become warm”
• If asked about rate: Mention reactivity (more reactive metal = faster reaction)

For acid + carbonate reactions (2-3 marks):

• Complete word equation: “Carbonate + Acid → Salt + Water + Carbon dioxide”
• For gas test: “Bubble through limewater, which turns milky/cloudy”
• Remember THREE products (not two!)

For rusting questions (3-4 marks):

• “Both oxygen AND water are required” (use “AND” not “OR”)
• “Neither alone will cause rusting”
• For prevention: State the method AND explain why it works
  • Example: “Painting forms a barrier that prevents contact with oxygen and water”
• For galvanising: “Zinc is more reactive than iron, so it corrodes preferentially/instead”

For combustion questions (2-3 marks):

• State the three requirements: “Fuel, oxygen, heat” (fire triangle)
• For complete combustion: “Produces carbon dioxide and water”
• For incomplete: “Produces carbon monoxide (poisonous) or carbon/soot”
• Reason for incomplete: “Insufficient oxygen supply”

Question-Answering Strategies

1. For “Describe and explain” questions (usually 3-4 marks):

• Describe = What happens (observations)
• Explain = Why it happens (science behind it)
• Write 2-3 points for describe, 2-3 points for explain

2. For practical/experimental questions:

• Always include:
  • What you would observe (specific colours, sounds, states)
  • How you would test for gases (precise procedure)
  • Expected result of the test
• Use scientific terminology

3. For word equation questions:

• Check you have the correct number of products:
  • Neutralisation = 2 products (salt + water)
  • Metal + acid = 2 products (salt + hydrogen)
  • Carbonate + acid = 3 products (salt + water + carbon dioxide)
• Check salt name follows the pattern

4. For “suggest” questions about rusting prevention:

• Name the method clearly
• Explain the scientific principle (usually “prevents contact with oxygen and water”)
• Give a specific example if asked

5. When asked to “compare”:

• Make direct comparisons (use words like “whereas,” “while,” “but”)
• Give differences for both items being compared
• Example: “Acid has pH below 7 whereas alkali has pH above 7”

6. For safety questions:

• Mention protective equipment if relevant (goggles, gloves)
• State the hazard clearly (corrosive, flammable, toxic)
• Suggest a precaution that specifically addresses that hazard

Marks Allocation Guide

• 1 mark: A simple fact or definition
• 2 marks: Two separate facts OR one fact with explanation
• 3 marks: Three facts OR two facts with explanation OR complete word equation with salt name
• 4+ marks: Extended answer with multiple points, observations, and explanations

Common Question Types to Prepare For

1. Complete the word equation (2-3 marks)
2. Describe a test for a gas (2 marks)
3. Explain the conditions needed for rusting (2-3 marks)
4. Suggest a method to prevent rusting and explain how it works (2-3 marks)
5. State what happens to pH during neutralisation (1-2 marks)
6. Identify whether changes are chemical or physical with reasons (2-4 marks)
7. Name the products of combustion (complete vs incomplete) (2 marks)
8. Explain observations during acid-metal/carbonate reactions (3-4 marks)
9. Use of indicators to test pH (2-3 marks)

Quick Summary

Essential points to remember for exams:

• ✓ Chemical changes produce new substances with different properties; signs include colour change, temperature change, gas production, and precipitate formation

• ✓ pH scale ranges from 0-14: acids (0-6), neutral (7), alkalis (8-14); universal indicator shows colours (red→orange→yellow→green→blue→purple)

• ✓ Litmus test: Blue litmus turns red in acid; red litmus turns blue in alkali

• ✓ Neutralisation equation: Acid + Base → Salt + Water (produces salt and water, pH becomes 7)

• ✓ Salt naming: First part from base/metal, second part from acid (hydrochloric→chloride, sulfuric→sulfate, nitric→nitrate)

• ✓ Acid + Metal → Salt + Hydrogen; test with lighted splint (squeaky pop sound confirms hydrogen)

• ✓ Acid + Carbonate → Salt + Water + Carbon dioxide; test by bubbling through limewater (turns milky/cloudy)

• ✓ Rusting requires BOTH oxygen AND water; prevention methods include barrier coatings (paint, oil), galvanising (zinc coating),