Foundations for
Chemical Reactions
Master the essential building blocks of GCSE chemistry. This guide covers acids, alkalis, the pH scale, how to identify different types of oxides, and the five key types of chemical reactions — with interactive flashcards and a quiz to test your knowledge.
Acids
H⁺ ions, pH < 7, common examples
Alkalis & Bases
OH⁻ ions, pH > 7, neutralisation
The pH Scale
0–14, indicators, measuring acidity
Types of Oxides
Acidic, basic, amphoteric, neutral
Reaction Types
Synthesis, decomposition, displacement…
Meet Buzzi!
Your energetic chemistry guide! Buzzi will help you master acids, alkalis, and chemical reactions with fun mnemonics, interactive flashcards, and timed revision sprints. Ready to spark your chemistry knowledge? Let's go! ⚡
How to use this guide
Work through each topic in order using the sidebar. Each section has explanations, key facts, and flashcards to help you memorise. When you're ready, take the Test Yourself quiz at the end.
Acids
What is an acid?
An acid is a substance that produces H⁺ (hydrogen) ions when dissolved in water. The more H⁺ ions present, the stronger the acid and the lower the pH.
🔑 Key Definition
Strong acids fully dissociate in water (e.g. HCl, H₂SO₄, HNO₃). Weak acids only partially dissociate (e.g. ethanoic acid, citric acid).
Common Acids
| Name | Formula | Found in |
|---|---|---|
| Hydrochloric acid | HCl | Stomach, labs |
| Sulfuric acid | H₂SO₄ | Car batteries, labs |
| Nitric acid | HNO₃ | Fertiliser production |
| Ethanoic acid | CH₃COOH | Vinegar |
| Citric acid | C₆H₈O₇ | Citrus fruits |
Properties of Acids
- → pH below 7
- → Produce H⁺ ions in aqueous solution
- → Turn litmus red / universal indicator red-orange
- → React with metals, bases, carbonates
- → Corrosive at high concentrations
Reactions of Acids
Acid + Metal
Acid + Metal → Salt + Hydrogen
e.g. H₂SO₄ + Zn → ZnSO₄ + H₂
Test for H₂: burning splint makes a 'squeaky pop'
Acid + Metal Oxide
Acid + Metal Oxide → Salt + Water
e.g. HCl + CuO → CuCl₂ + H₂O
Metal oxides are bases — this is neutralisation
Acid + Metal Hydroxide
Acid + Metal Hydroxide → Salt + Water
e.g. HCl + NaOH → NaCl + H₂O
Classic neutralisation — forms a salt and water
Acid + Metal Carbonate
Acid + Metal Carbonate → Salt + Water + CO₂
e.g. HCl + CaCO₃ → CaCl₂ + H₂O + CO₂
Test for CO₂: turns limewater milky
💡 Buzzi's Exam Tips
Strong vs Weak
Strong acids (HCl, H₂SO₄, HNO₃) FULLY dissociate. Weak acids (ethanoic) only PARTIALLY dissociate. Examiners LOVE this distinction!
The Squeaky Pop Test
When acid reacts with metal → hydrogen gas. Light a burning splint near the gas and you'll hear a SQUEAKY POP! This is your proof of H₂.
🃏 Acids Flashcards
Click each card to reveal the answer
Alkalis & Bases
What is a base?
A base is any substance that neutralises an acid to form a salt and water. An alkali is a base that dissolves in water to produce OH⁻ (hydroxide) ions.
🔑 Key Distinction
All alkalis are bases, but not all bases are alkalis. Copper oxide (CuO) is a base but is insoluble in water, so it is not an alkali.
Common Alkalis
| Name | Formula | Use |
|---|---|---|
| Sodium hydroxide | NaOH | Oven cleaners, soap |
| Potassium hydroxide | KOH | Alkaline batteries |
| Calcium hydroxide | Ca(OH)₂ | Treating acidic soil |
| Ammonia solution | NH₃(aq) | Fertilisers, cleaning |
| Sodium carbonate | Na₂CO₃ | Washing soda |
Properties of Alkalis
- → pH above 7
- → Produce OH⁻ ions in aqueous solution
- → Turn litmus blue / universal indicator blue-purple
- → React with acids in neutralisation reactions
- → Strong alkalis are corrosive
Neutralisation Reactions
When an acid and an alkali are mixed, the H⁺ ions from the acid react with the OH⁻ ions from the alkali to form water. This is the ionic equation for all neutralisation reactions:
H⁺(aq) + OH⁻(aq) → H₂O(l)
The ionic equation for all neutralisation reactions
HCl + NaOH → NaCl + H₂O
Hydrochloric acid + Sodium hydroxide
H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
Sulfuric acid + Potassium hydroxide
HNO₃ + NaOH → NaNO₃ + H₂O
Nitric acid + Sodium hydroxide
💡 Buzzi's Exam Tips
Alkali ≠ Base
ALL alkalis are bases, but NOT all bases are alkalis. Copper oxide is a base but NOT an alkali (it's insoluble). Don't get caught out!
The Ionic Equation
For ANY neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l). This is ALWAYS the same, no matter which acid or alkali. Memorise this!
🃏 Alkalis & Bases Flashcards
Click each card to reveal the answer
The pH Scale
The pH scale measures how acidic or alkaline a solution is. It runs from 0 to 14. A pH of 7 is neutral (pure water). Below 7 is acidic; above 7 is alkaline. Each unit on the scale represents a 10× change in H⁺ ion concentration.
🎚️ Interactive pH Explorer
Neutral
Closest example: 💧 Pure water (pH 7)
Equal H⁺ and OH⁻ ion concentrations
Common pH Values
Battery acid
pH 1
Stomach acid
pH 2
Lemon juice
pH 2.5
Vinegar
pH 3
Coffee
pH 5
Rainwater
pH 5.6
Pure water
pH 7
Blood
pH 7.4
Baking soda
pH 9
Milk of magnesia
pH 10
Bleach
pH 12
Drain cleaner
pH 14
Click any card to explore that pH on the scale above
Indicators
An indicator is a dye that changes colour depending on the pH of a solution. They are used to test whether a substance is acidic, neutral, or alkaline.
| Indicator | In Acid | Neutral | In Alkali |
|---|---|---|---|
| Litmus | 🔴 Red | 🟣 Purple | 🔵 Blue |
| Universal Indicator | 🔴 Red/Orange | 🟢 Green | 🔵 Blue/Purple |
| Phenolphthalein | ⚪ Colourless | ⚪ Colourless | 🩷 Pink/Red |
| Methyl orange | 🔴 Red | 🟠 Orange | 🟡 Yellow |
💡 Exam Tip
Universal indicator gives a range of colours so you can estimate the exact pH. Litmus only tells you if something is acidic or alkaline (not the exact pH). A pH meter gives the most accurate measurement.
Types of Oxides
An oxide is a compound formed when an element reacts with oxygen. Oxides are classified by their acid-base behaviour into four types: acidic, basic, amphoteric, and neutral. The key rule: metals form basic oxides, non-metals form acidic oxides.
🔍 Quick Identification Guide
Non-metal element + O₂
→ Acidic oxide
Reacts with alkalis; dissolves to form acid
Metal element + O₂
→ Basic oxide
Reacts with acids; may dissolve to form alkali
Zn, Al, Pb + O₂
→ Amphoteric oxide
Reacts with BOTH acids and alkalis
CO, NO, N₂O, H₂O
→ Neutral oxide
Does NOT react with acids or alkalis
Acidic Oxides
React with bases/alkalis to form a salt and water. Dissolve in water to form acidic solutions.
Basic Oxides
React with acids to form a salt and water. Some dissolve in water to form alkaline solutions.
Amphoteric Oxides
React with BOTH acids AND alkalis to form a salt and water. They can act as either acidic or basic.
Neutral Oxides
Do NOT react with either acids or alkalis. They are neither acidic nor basic.
🃏 Oxides Flashcards
Click each card to reveal the answer
Types of Chemical Reactions
Chemical reactions can be classified into five main types based on what happens to the reactants and products. Recognising the type of reaction helps you predict products and write equations.
💡 Exam Tip
Some reactions can be classified as more than one type! For example, combustion is also a synthesis reaction (fuel + O₂ → products). Always look at the overall pattern of reactants and products.
🔗 Synthesis (Combination)
Two or more reactants combine to form a single, more complex product. Think of it as 'building' a new substance.
Example
Sodium + Chlorine → Sodium chloride
2Na + Cl₂ → 2NaCl
🔑 Key Fact
The number of reactants is always greater than the number of products.
Summary Table
| Type | Pattern | Key Sign |
|---|---|---|
| 🔗Synthesis | A + B → AB | The number of reactants is always greater than the number of products |
| 💥Decomposition | AB → A + B | Thermal decomposition uses heat; photodecomposition uses light |
| 🔄Displacement | A + BC → AB + C | Only works if the displacing element is MORE reactive than the one being displaced |
| ⚖️Neutralisation | Acid + Base → Salt + Water | The ionic equation is always: H⁺ + OH⁻ → H₂O |
| 🔥Combustion | Fuel + O₂ → CO₂ + H₂O | Incomplete combustion (limited O₂) produces toxic carbon monoxide (CO) |
🃏 Reaction Types Flashcards
Click each card to reveal the answer
Reactivity Series
The reactivity series is an ordered list of metals arranged by their reactivity. More reactive metals are placed higher and can displace less reactive metals from compounds. This series helps predict which reactions will occur and which won't.
💡 Memory Aid
"Please Keep Licking Candy, Magnesium Always Zips In For Copper And Gold"
Remember: The first 6 metals (K, Na, Li, Ca, Mg, Al) are VERY reactive and react with water. Metals after Aluminium are much less reactive.
🔑 Displacement Rule
A more reactive metal will displace a less reactive metal from a compound. Zn + CuSO₄ → ZnSO₄ + Cu (Zn is more reactive than Cu)
⚡ Hydrogen Position
Metals above hydrogen displace it from acids (H₂ gas produced). Metals below hydrogen do NOT react with dilute acids.
Metals Ranked by Reactivity
Potassium
K — very high
Sodium
Na — very high
Lithium
Li — very high
Calcium
Ca — high
Magnesium
Mg — high
Aluminium
Al — moderate
Zinc
Zn — moderate
Iron
Fe — moderate
Copper
Cu — low
Silver
Ag — very low
Gold
Au — very low
💡 Buzzi's Exam Tips
Above vs Below Hydrogen
Metals ABOVE hydrogen in the reactivity series displace H₂ from acids (H₂ gas bubbles). Metals BELOW hydrogen do NOT react with dilute acids. This is crucial!
Water Reactivity
The TOP 6 metals (K, Na, Li, Ca, Mg, Al) react with water. K and Na are so reactive they catch FIRE! After Al, metals barely react with water.
🃏 Reactivity Series Flashcards
Click each card to reveal the answer
Test Yourself
10 questions covering all topics. Take your time and think carefully!
Which of the following is a property of all acids?
Revision Mode
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💡 How it works
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