Sciences · Topic 2.2
Reactions & Acids
Learn how chemical reactions rearrange atoms to form new substances, how to balance equations, and how acids and bases interact. Understand the pH scale, indicators, neutralisation, and the reactivity series.
What You'll Learn
- Identify and classify four types of chemical reactions: synthesis, decomposition, displacement, neutralisation
- Balance chemical equations by adjusting coefficients
- Explain the pH scale and distinguish acids from bases/alkalis
- Describe neutralisation reactions and their products
- Use indicators to test pH and identify acids/bases
- Apply the reactivity series to predict displacement reactions
IB Assessment Focus
Criterion A — Knowing: State types of reactions, recall equations, and define key terms accurately.
Criterion B — Inquiring: Design experiments to test reactions, form hypotheses about displacement.
Criterion C — Processing: Balance equations, interpret pH data, analyse experimental results.
Criterion D — Reflecting: Evaluate real-world applications of neutralisation (e.g. antacids, agriculture).
Key Vocabulary
| Term | Definition |
|---|---|
| Chemical reaction | A process where reactants are transformed into products; bonds are broken and new bonds are formed |
| Reactant | A substance that undergoes a chemical reaction (on the left of the arrow) |
| Product | A substance formed as a result of a chemical reaction (on the right of the arrow) |
| Balanced equation | A chemical equation with equal numbers of each type of atom on both sides |
| Coefficient | The large number placed in front of a chemical formula to balance equations (e.g. the 2 in 2H2O) |
| Subscript | The small number within a formula showing how many atoms of an element are in a molecule (e.g. the 2 in H2O) |
| Acid | A substance with pH < 7 that releases H⁺ ions in solution |
| Base | A substance with pH > 7; an alkali is a base that dissolves in water, releasing OH⁻ ions |
| Indicator | A substance that changes colour depending on pH (e.g. litmus, universal indicator) |
| Neutralisation | Reaction between an acid and a base to form a salt and water |
Sciences · Topic 2.2
Types of Chemical Reactions
Chemical reactions can be classified into four main types based on how atoms are rearranged.
1. Synthesis (Combination)
Pattern
A + B → AB
Two or more simple substances combine to form a single, more complex product.
Examples:
- Fe + S → FeS (iron + sulfur → iron sulfide)
- 2Mg + O2 → 2MgO (magnesium burns brightly in air)
- 2Na + Cl2 → 2NaCl (sodium + chlorine → sodium chloride / table salt)
- 2H2 + O2 → 2H2O (hydrogen + oxygen → water)
2. Decomposition
Pattern
AB → A + B
A single compound breaks down into two or more simpler substances. Often requires heating (thermal decomposition).
Examples:
- CaCO3 → CaO + CO2 (calcium carbonate decomposes when heated)
- 2H2O → 2H2 + O2 (electrolysis of water)
- 2H2O2 → 2H2O + O2 (hydrogen peroxide decomposes)
3. Displacement (Single Replacement)
Pattern
A + BC → AC + B
A more reactive element displaces (pushes out) a less reactive element from a compound. The more reactive element "takes the place" of the less reactive one.
Examples:
- Zn + CuSO4 → ZnSO4 + Cu (zinc displaces copper because zinc is more reactive)
- Mg + 2HCl → MgCl2 + H2 (magnesium displaces hydrogen from acid)
- Fe + CuSO4 → FeSO4 + Cu (iron displaces copper)
Key Rule: A displacement reaction can ONLY occur if the free element is more reactive than the element in the compound. If you add copper to zinc sulfate solution, nothing happens because copper is LESS reactive than zinc.
4. Neutralisation
Pattern
Acid + Base → Salt + Water
An acid reacts with a base (or alkali) to produce a salt and water. The H⁺ ions from the acid combine with OH⁻ ions from the base to form water (H2O).
Examples:
- HCl + NaOH → NaCl + H2O (hydrochloric acid + sodium hydroxide → sodium chloride + water)
- H2SO4 + 2NaOH → Na2SO4 + 2H2O
- HNO3 + KOH → KNO3 + H2O
Acid + Metal Carbonate Reactions
When an acid reacts with a metal carbonate, the products are a salt + water + carbon dioxide.
Pattern
Acid + Metal Carbonate → Salt + Water + CO2
Example:
- 2HCl + CaCO3 → CaCl2 + H2O + CO2
- The fizzing observed is carbon dioxide gas being released
- Test: bubble the gas through limewater — if it turns milky/cloudy, the gas is CO2
Signs That a Chemical Reaction Has Occurred
- Colour change (e.g. copper sulfate solution turns from blue to colourless when iron is added)
- Gas produced (bubbling, fizzing)
- Temperature change (exothermic: gets hotter; endothermic: gets cooler)
- Precipitate formed (an insoluble solid appears in a solution)
- Light or sound produced (e.g. combustion)
Sciences · Topic 2.2
Balancing Chemical Equations
In a chemical reaction, atoms are rearranged but never created or destroyed. A balanced equation has equal numbers of each type of atom on both sides — this is the Law of Conservation of Mass.
Critical Rule: When balancing equations, NEVER change the subscripts in chemical formulas (e.g. don't change H2O to H3O). Only change the coefficients — the large numbers placed in front of the formulas.
Step-by-Step Method
- Write the unbalanced equation with correct chemical formulas
- Count the atoms of each element on both sides
- Add coefficients to balance one element at a time (start with the most complex molecule)
- Recount all atoms to verify the equation is balanced
- Check that coefficients are in the simplest whole-number ratio
Example 1: Simple Balancing
Balance: H2 + O2 → H2O
Step-by-step:
- Count atoms: Left: H=2, O=2. Right: H=2, O=1. Oxygen is unbalanced.
- Put a 2 in front of H2O: H2 + O2 → 2H2O. Now: Right: H=4, O=2. Hydrogen is now unbalanced.
- Put a 2 in front of H2: 2H2 + O2 → 2H2O
- Final check: Left: H=4, O=2. Right: H=4, O=2. Balanced!
Example 2: More Complex
Balance: Fe2O3 + C → Fe + CO2
- Count: Left: Fe=2, O=3, C=1. Right: Fe=1, C=1, O=2.
- Balance Fe: Put 2 in front of Fe. Fe2O3 + C → 2Fe + CO2
- Balance O: 3 on left, 2 on right. Use coefficient 3 for CO2: Fe2O3 + C → 2Fe + 3CO2. Now O: left=3, right=6. Not balanced yet.
- Try 2 in front of Fe2O3: 2Fe2O3 + C → Fe + CO2. Left: Fe=4, O=6, C=1. Need: 4Fe, 6O on right.
- Revised: 2Fe2O3 + 3C → 4Fe + 3CO2. Left: Fe=4, O=6, C=3. Right: Fe=4, O=6, C=3. Balanced!
Practice: Common Equations to Balance
| Unbalanced | Balanced |
|---|---|
| Na + Cl2 → NaCl | 2Na + Cl2 → 2NaCl |
| N2 + H2 → NH3 | N2 + 3H2 → 2NH3 |
| CH4 + O2 → CO2 + H2O | CH4 + 2O2 → CO2 + 2H2O |
| Mg + HCl → MgCl2 + H2 | Mg + 2HCl → MgCl2 + H2 |
Sciences · Topic 2.2
Acids, Bases & the pH Scale
The pH scale measures how acidic or alkaline a solution is. Understanding acids and bases is essential for chemistry and has many real-world applications.
The pH Scale
The pH scale runs from 0 to 14:
| pH Range | Classification | Examples |
|---|---|---|
| 0–6 | Acidic (lower = stronger acid) | Stomach acid (pH 1–2), lemon juice (pH 2), vinegar (pH 3), tomato (pH 4) |
| 7 | Neutral | Pure water, blood (pH 7.4 — slightly alkaline) |
| 8–14 | Alkaline/Basic (higher = stronger alkali) | Baking soda (pH 8–9), soap (pH 9–10), bleach (pH 12–13), oven cleaner (pH 14) |
Acids in Detail
An acid is a substance that releases H⁺ (hydrogen) ions when dissolved in water. The more H⁺ ions, the stronger the acid and the lower the pH.
Common acids:
- Hydrochloric acid (HCl) — found in stomach acid; used in cleaning
- Sulfuric acid (H2SO4) — used in car batteries; strong acid
- Nitric acid (HNO3) — used to make fertilisers
- Citric acid — found in citrus fruits (lemons, oranges)
- Ethanoic acid (CH3COOH) — found in vinegar; a weak acid
Bases and Alkalis
A base is a substance that neutralises an acid. An alkali is a base that dissolves in water, releasing OH⁻ (hydroxide) ions.
Common bases/alkalis:
- Sodium hydroxide (NaOH) — used in soap-making; strong alkali
- Potassium hydroxide (KOH) — used in fertilisers
- Calcium hydroxide Ca(OH)2 — slaked lime; used to treat acidic soil
- Ammonia (NH3) — found in cleaning products; weak base
Important Distinction: All alkalis are bases, but not all bases are alkalis. An alkali specifically dissolves in water. For example, copper oxide is a base (it neutralises acids) but it does not dissolve in water, so it is NOT an alkali.
Indicators
Indicators are substances that change colour to show whether a solution is acidic, neutral, or alkaline.
| Indicator | In Acid | Neutral | In Alkali |
|---|---|---|---|
| Litmus paper | Red | Purple | Blue |
| Phenolphthalein | Colourless | Colourless | Pink |
| Methyl orange | Red | Orange | Yellow |
| Universal indicator | Red/Orange/Yellow | Green | Blue/Purple |
Universal indicator is the most useful because it shows the approximate pH value using a range of colours (red = strong acid through green = neutral to purple = strong alkali).
Neutralisation in Detail
General Equation
Acid + Base → Salt + Water
Ionic Equation
H⁺ + OH⁻ → H2O
The name of the salt depends on the acid used and the metal in the base:
| Acid Used | Salt Produced |
|---|---|
| Hydrochloric acid (HCl) | Chloride salts (e.g. NaCl, MgCl2) |
| Sulfuric acid (H2SO4) | Sulfate salts (e.g. Na2SO4, MgSO4) |
| Nitric acid (HNO3) | Nitrate salts (e.g. KNO3, Ca(NO3)2) |
Real-World Applications of Neutralisation
- Antacids — neutralise excess stomach acid (e.g. magnesium hydroxide)
- Treating acidic soil — farmers add lime (calcium hydroxide) to neutralise acidic soil
- Toothpaste — is slightly alkaline to neutralise acids produced by bacteria in the mouth
- Treating insect stings — bee sting (acidic) treated with baking soda (alkaline); wasp sting (alkaline) treated with vinegar (acidic)
Sciences · Topic 2.2
The Reactivity Series
The reactivity series ranks metals in order of how vigorously they react with water, acids, and oxygen. It is essential for predicting displacement reactions.
The Reactivity Series of Metals
| Metal | Reactivity | Reaction with Water | Reaction with Dilute Acid |
|---|---|---|---|
| Potassium (K) | Most reactive | Violent reaction, catches fire | Too dangerous |
| Sodium (Na) | Very reactive | Vigorous, floats and fizzes | Too dangerous |
| Calcium (Ca) | Reactive | Steady reaction, fizzes | Very vigorous |
| Magnesium (Mg) | Fairly reactive | Very slow with cold water; reacts with steam | Vigorous fizzing |
| Aluminium (Al) | Fairly reactive | Reacts with steam (oxide layer protects) | Reacts |
| Zinc (Zn) | Moderate | Reacts with steam only | Steady fizzing |
| Iron (Fe) | Moderate | Reacts very slowly (rusts) | Slow reaction |
| Copper (Cu) | Low | No reaction | No reaction with dilute acid |
| Silver (Ag) | Very low | No reaction | No reaction |
| Gold (Au) | Least reactive | No reaction | No reaction |
Memory Aid
Please Stop Calling Me A Zebra, I'm Clearly Something Gorgeous
(Potassium, Sodium, Calcium, Magnesium, Aluminium, Zinc, Iron, Copper, Silver, Gold)
Predicting Displacement Reactions
A more reactive metal will displace a less reactive metal from its compound:
- Zn + CuSO4 → ZnSO4 + Cu — Zinc is MORE reactive than copper, so it displaces copper. The blue solution turns colourless and copper metal appears.
- Cu + ZnSO4 → NO REACTION — Copper is LESS reactive than zinc, so it cannot displace zinc.
- Mg + CuSO4 → MgSO4 + Cu — Magnesium is more reactive than copper.
Key Rule: A displacement reaction ONLY occurs if the free metal is higher in the reactivity series than the metal in the compound. If it is lower, no reaction occurs.
Extraction of Metals
The reactivity series also determines how metals are extracted from their ores:
| Position in Reactivity Series | Extraction Method | Examples |
|---|---|---|
| Very reactive metals (above carbon) | Electrolysis (using electricity) | Potassium, sodium, aluminium |
| Moderately reactive (below carbon) | Reduction with carbon (heating with carbon/coke) | Zinc, iron, copper |
| Unreactive metals | Found as native metals (no extraction needed) | Gold, silver, platinum |
Sciences · Topic 2.2
Worked Examples
These examples show the structured reasoning expected at Grade 8 level. Always identify the reaction type and justify your answer.
EXAMPLE 1Balance the equation: Al + O2 → Al2O3
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Full Solution
Step 1: Count atoms. Left: Al=1, O=2. Right: Al=2, O=3.
Step 2: Balance Al: put 2 in front of Al. 2Al + O2 → Al2O3. Left: Al=2, O=2. Right: Al=2, O=3. Oxygen unbalanced.
Step 3: We need 3 O on left. Since O2 comes in pairs, the LCM of 2 and 3 is 6. Use 3O2 on the left (giving 6 O) and 2Al2O3 on the right (giving 6 O).
Step 4: Now balance Al: 2Al2O3 has 4 Al, so put 4Al on left.
Answer: 4Al + 3O2 → 2Al2O3
Check: Left: Al=4, O=6. Right: Al=4, O=6. Balanced!
Step 2: Balance Al: put 2 in front of Al. 2Al + O2 → Al2O3. Left: Al=2, O=2. Right: Al=2, O=3. Oxygen unbalanced.
Step 3: We need 3 O on left. Since O2 comes in pairs, the LCM of 2 and 3 is 6. Use 3O2 on the left (giving 6 O) and 2Al2O3 on the right (giving 6 O).
Step 4: Now balance Al: 2Al2O3 has 4 Al, so put 4Al on left.
Answer: 4Al + 3O2 → 2Al2O3
Check: Left: Al=4, O=6. Right: Al=4, O=6. Balanced!
EXAMPLE 2Explain what happens when you add zinc to copper sulfate solution. Include observations and a balanced equation.
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Full Solution
Type of reaction: Displacement
Explanation: Zinc is more reactive than copper (higher in the reactivity series), so zinc displaces copper from copper sulfate solution.
Balanced equation: Zn + CuSO4 → ZnSO4 + Cu
Observations:
• The blue colour of the copper sulfate solution fades and becomes colourless (zinc sulfate is colourless)
• A reddish-brown coating of copper metal appears on the zinc
• The zinc gradually dissolves
Why it happens: Zinc atoms lose electrons more readily than copper atoms (zinc is more reactive), so zinc goes into solution as Zn2+ ions while Cu2+ ions come out of solution as copper metal.
Explanation: Zinc is more reactive than copper (higher in the reactivity series), so zinc displaces copper from copper sulfate solution.
Balanced equation: Zn + CuSO4 → ZnSO4 + Cu
Observations:
• The blue colour of the copper sulfate solution fades and becomes colourless (zinc sulfate is colourless)
• A reddish-brown coating of copper metal appears on the zinc
• The zinc gradually dissolves
Why it happens: Zinc atoms lose electrons more readily than copper atoms (zinc is more reactive), so zinc goes into solution as Zn2+ ions while Cu2+ ions come out of solution as copper metal.
EXAMPLE 3A student adds universal indicator to a solution and it turns red. They then add sodium hydroxide drop by drop. Describe what they would observe and explain why.
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Full Solution
Initial observation: The solution is red, indicating it is a strong acid (pH 1–2).
As NaOH is added: A neutralisation reaction occurs. The acid (H⁺ ions) reacts with the alkali (OH⁻ ions) to form water. The pH gradually increases.
Colour changes: Red → Orange → Yellow → Green → Blue → Purple
• When it reaches green (pH 7), the solution is neutral — all the acid has been neutralised.
• If more NaOH is added beyond this point, the solution becomes alkaline (blue/purple).
Equation: HCl + NaOH → NaCl + H2O
As NaOH is added: A neutralisation reaction occurs. The acid (H⁺ ions) reacts with the alkali (OH⁻ ions) to form water. The pH gradually increases.
Colour changes: Red → Orange → Yellow → Green → Blue → Purple
• When it reaches green (pH 7), the solution is neutral — all the acid has been neutralised.
• If more NaOH is added beyond this point, the solution becomes alkaline (blue/purple).
Equation: HCl + NaOH → NaCl + H2O
EXAMPLE 4Identify the type of reaction and name the products: HNO3 + KOH → ?
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Full Solution
Type: Neutralisation (acid + base → salt + water)
Acid: HNO3 (nitric acid) → produces nitrate salts
Base: KOH (potassium hydroxide) → the metal is potassium
Salt name: Potassium + nitrate = potassium nitrate (KNO3)
Balanced equation: HNO3 + KOH → KNO3 + H2O
Check: Left: H=2, N=1, O=4, K=1. Right: K=1, N=1, O=4, H=2. Balanced!
Acid: HNO3 (nitric acid) → produces nitrate salts
Base: KOH (potassium hydroxide) → the metal is potassium
Salt name: Potassium + nitrate = potassium nitrate (KNO3)
Balanced equation: HNO3 + KOH → KNO3 + H2O
Check: Left: H=2, N=1, O=4, K=1. Right: K=1, N=1, O=4, H=2. Balanced!
EXAMPLE 5Predict whether a reaction occurs: Cu + FeSO4 → ? Explain your reasoning.
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Full Solution
Answer: No reaction occurs.
Reasoning: For a displacement reaction to occur, the free metal must be MORE reactive than the metal in the compound. Looking at the reactivity series:
• Iron (Fe) is HIGHER in the reactivity series than copper (Cu)
• Therefore, copper is LESS reactive than iron
• Copper cannot displace iron from iron sulfate
Comparison: If we reversed it — Fe + CuSO4 → FeSO4 + Cu — this WOULD work because iron is more reactive than copper.
Reasoning: For a displacement reaction to occur, the free metal must be MORE reactive than the metal in the compound. Looking at the reactivity series:
• Iron (Fe) is HIGHER in the reactivity series than copper (Cu)
• Therefore, copper is LESS reactive than iron
• Copper cannot displace iron from iron sulfate
Comparison: If we reversed it — Fe + CuSO4 → FeSO4 + Cu — this WOULD work because iron is more reactive than copper.
Sciences · Topic 2.2
Practice Q&A
Attempt each question before revealing the model answer. Always state the reaction type and justify your reasoning.
EXPLAINExplain what happens in a neutralisation reaction. Include the word equation and an example.
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Model Answer
In a neutralisation reaction, an acid reacts with a base to produce a salt and water. The H⁺ ions from the acid combine with OH⁻ ions from the base to form water (H2O), effectively cancelling each other out. The remaining ions form the salt. The pH moves toward neutral (7).
Word equation: Acid + Base → Salt + Water
Example: HCl + NaOH → NaCl + H2O
(hydrochloric acid + sodium hydroxide → sodium chloride + water)
Word equation: Acid + Base → Salt + Water
Example: HCl + NaOH → NaCl + H2O
(hydrochloric acid + sodium hydroxide → sodium chloride + water)
BALANCEBalance the equation: Fe + Cl2 → FeCl3
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Model Answer
Unbalanced: Fe + Cl2 → FeCl3
Count: Left: Fe=1, Cl=2. Right: Fe=1, Cl=3.
Chlorine is unbalanced. LCM of 2 and 3 is 6. Need 6 Cl on each side.
Put 3 in front of Cl2 (left: 6 Cl) and 2 in front of FeCl3 (right: 6 Cl).
Now Fe: 2 on right, so put 2 on left.
Balanced: 2Fe + 3Cl2 → 2FeCl3
Check: Left: Fe=2, Cl=6. Right: Fe=2, Cl=6. Balanced!
Count: Left: Fe=1, Cl=2. Right: Fe=1, Cl=3.
Chlorine is unbalanced. LCM of 2 and 3 is 6. Need 6 Cl on each side.
Put 3 in front of Cl2 (left: 6 Cl) and 2 in front of FeCl3 (right: 6 Cl).
Now Fe: 2 on right, so put 2 on left.
Balanced: 2Fe + 3Cl2 → 2FeCl3
Check: Left: Fe=2, Cl=6. Right: Fe=2, Cl=6. Balanced!
CLASSIFYClassify each reaction: (a) 2H2O2 → 2H2O + O2 (b) Mg + 2HCl → MgCl2 + H2
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Model Answer
(a) Decomposition — one compound (H2O2) breaks down into two simpler substances (water and oxygen). Pattern: AB → A + B.
(b) Displacement — magnesium (a more reactive metal) displaces hydrogen from hydrochloric acid. Pattern: A + BC → AC + B. Magnesium is higher in the reactivity series than hydrogen.
(b) Displacement — magnesium (a more reactive metal) displaces hydrogen from hydrochloric acid. Pattern: A + BC → AC + B. Magnesium is higher in the reactivity series than hydrogen.
PREDICTWill a reaction occur if copper is placed in silver nitrate solution? Explain and write the equation if applicable.
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Model Answer
Yes, a reaction will occur.
Copper is MORE reactive than silver (higher in the reactivity series), so copper will displace silver from silver nitrate solution.
Equation: Cu + 2AgNO3 → Cu(NO3)2 + 2Ag
Observations: Silver crystals would form on the copper surface, and the solution would turn blue (copper nitrate is blue in solution).
Copper is MORE reactive than silver (higher in the reactivity series), so copper will displace silver from silver nitrate solution.
Equation: Cu + 2AgNO3 → Cu(NO3)2 + 2Ag
Observations: Silver crystals would form on the copper surface, and the solution would turn blue (copper nitrate is blue in solution).
APPLYA farmer's soil has a pH of 4. Explain what the farmer could do to improve crop growth and why this works.
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Model Answer
The soil pH of 4 is acidic, which is too low for most crops to grow well. The farmer should add lime (calcium hydroxide or calcium carbonate) to the soil.
Why it works: The lime is alkaline/basic and neutralises the excess acid in the soil. The neutralisation reaction raises the pH toward 7 (neutral), creating better growing conditions.
Equation: Ca(OH)2 + H2SO4 → CaSO4 + 2H2O
(The lime reacts with acids in the soil to form a salt and water.)
Why it works: The lime is alkaline/basic and neutralises the excess acid in the soil. The neutralisation reaction raises the pH toward 7 (neutral), creating better growing conditions.
Equation: Ca(OH)2 + H2SO4 → CaSO4 + 2H2O
(The lime reacts with acids in the soil to form a salt and water.)
EXPLAINWhy can aluminium not be extracted from its ore by heating with carbon?
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Model Answer
Aluminium is more reactive than carbon in the reactivity series. For carbon to reduce a metal oxide (extract the metal), carbon must be more reactive than the metal so it can displace it.
Since aluminium is above carbon, carbon cannot displace aluminium from its ore. Instead, aluminium must be extracted by electrolysis — using electrical energy to decompose molten aluminium oxide (Al2O3).
Since aluminium is above carbon, carbon cannot displace aluminium from its ore. Instead, aluminium must be extracted by electrolysis — using electrical energy to decompose molten aluminium oxide (Al2O3).
Sciences · Review
Flashcard Review
Tap each card to reveal the answer. Try to answer from memory first.
What are the four types of chemical reaction?
Synthesis (A+B→AB), Decomposition (AB→A+B), Displacement (A+BC→AC+B), Neutralisation (Acid+Base→Salt+Water).
Tap to reveal
What is the Law of Conservation of Mass?
In a chemical reaction, atoms are never created or destroyed. The total mass of reactants equals the total mass of products. This is why equations must be balanced.
Tap to reveal
How do you balance a chemical equation?
Change the coefficients (numbers in front of formulas) until each element has equal atoms on both sides. NEVER change the subscripts within formulas.
Tap to reveal
What is the pH scale?
A scale from 0-14 measuring acidity/alkalinity. pH < 7 = acidic, pH 7 = neutral, pH > 7 = alkaline. Lower pH = stronger acid; higher pH = stronger alkali.
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What ions do acids and alkalis release?
Acids release H⁺ (hydrogen) ions in solution. Alkalis release OH⁻ (hydroxide) ions in solution. In neutralisation: H⁺ + OH⁻ → H₂O.
Tap to reveal
What is neutralisation?
Acid + Base → Salt + Water. The H⁺ and OH⁻ ions combine to form water, and the pH moves toward 7. Example: HCl + NaOH → NaCl + H₂O.
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Which acid produces chloride salts?
Hydrochloric acid (HCl) produces chloride salts. Sulfuric acid (H₂SO₄) produces sulfates. Nitric acid (HNO₃) produces nitrates.
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What is the reactivity series?
A ranking of metals from most to least reactive: K, Na, Ca, Mg, Al, Zn, Fe, Cu, Ag, Au. More reactive metals react more vigorously with water and acids.
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When does a displacement reaction occur?
Only when the free metal is MORE reactive than the metal in the compound. E.g. Zn + CuSO₄ → ZnSO₄ + Cu works because Zn > Cu in reactivity.
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What colour does universal indicator turn in acid?
Red (strong acid, pH 1-2), orange (pH 3-4), yellow (pH 5-6). In neutral (pH 7) it is green. In alkali: blue (pH 8-10), purple (pH 11-14).
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What is the difference between a base and an alkali?
A base is any substance that neutralises an acid. An alkali is a base that dissolves in water (releasing OH⁻ ions). All alkalis are bases, but not all bases are alkalis.
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What are the five signs of a chemical reaction?
Colour change, gas produced (bubbling), temperature change, precipitate formed (insoluble solid appears), light or sound produced.
Tap to reveal
Balance: N₂ + H₂ → NH₃
N₂ + 3H₂ → 2NH₃. Check: Left: N=2, H=6. Right: N=2, H=6. Balanced!
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What is thermal decomposition?
When a compound breaks down into simpler substances when heated. Example: CaCO₃ → CaO + CO₂ (calcium carbonate decomposes into calcium oxide and carbon dioxide).
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Why is gold found as a native metal?
Gold is at the bottom of the reactivity series (very unreactive). It does not react with water, air, or acids, so it exists naturally in its pure metallic form — it never needs extraction.
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Sciences · Practice Test
Practice Test — 20 Questions
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