The Characteristic Properties Of Acids & Bases

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Notes

Properties of Acids

Acids have pH < 7, sour taste (if edible), and are corrosive.
Acids neutralise bases to form a **salt** and **water**.
In water, acids produce **H⁺ ions** (e.g., HCl → H⁺ + Cl⁻).
Acids react with metals (above H in reactivity series) to give **salt + hydrogen**.
Acids react with metal carbonates to give **salt + CO₂ + water**.
Acids react with bases (metal oxides/hydroxides) to give **salt + water** (neutralisation).

Properties of Bases & Alkalis

Bases have pH > 7; a water‑soluble base is an **alkali**.
Bases neutralise acids to form a **salt** and **water**.
In water, alkalis produce **OH⁻ ions** (e.g., NaOH → Na⁺ + OH⁻).
Alkalis react with ammonium salts on warming to produce **ammonia gas** (damp red litmus turns blue).
Common bases are metal oxides and metal hydroxides.

Indicators

Indicators change colour in acid or alkali; **litmus** (red in acid, blue in alkali) is from lichens.
**Methyl orange**: red in acid, yellow in alkali.
**Thymolphthalein**: colourless in acid, blue in alkali.
**Universal indicator** is a mixture giving a range of colours to estimate pH.
Synthetic indicators (methyl orange, thymolphthalein) have sharp colour changes for titrations; litmus is not suitable for titrations.

The Ions in Acids & Alkalis & Neutralisation

Acids are sources of **H⁺** ions; alkalis are sources of **OH⁻** ions.
Neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l).
Not all acid reactions are neutralisations (e.g., acid + metal produces no water).
The **pH scale** runs 1–14; pH < 7 = acid, pH = 7 = neutral, pH > 7 = alkali.
pH is a logarithmic measure of H⁺ concentration: a change of 1 pH unit = ×10 change in [H⁺].
Strong acids have pH 0–2; weak acids pH 3–6; weak alkalis pH 8–11; strong alkalis pH 12–14.

Proton Transfer (Extended Tier)

Acids are **proton donors** (H⁺ = proton).
Bases are **proton acceptors**.
In water, HCl donates a proton to H₂O, forming H₃O⁺ and Cl⁻.

Strong & Weak Acids (Extended Tier)

**Strong acids** **completely dissociate** in water (e.g., HCl, H₂SO₄).
**Weak acids** **partially dissociate**; an equilibrium exists (e.g., CH₃CH₂COOH ⇌ H⁺ + CH₃CH₂COO⁻).
Concentration ≠ strength: a dilute strong acid can be more acidic than a concentrated weak acid.
Weak acids have pH closer to 7 (e.g., propanoic acid pH ≈ 5).

Classifying Oxides

**Acidic oxides** (non‑metal + oxygen): react with bases to form salt + water; produce acidic solutions (e.g., CO₂, SO₂).
**Basic oxides** (metal + oxygen): react with acids to form salt + water; produce alkaline solutions (e.g., CuO, CaO).
**Amphoteric oxides** (e.g., ZnO, Al₂O₃) react with both acids and bases to give salt + water.
**Neutral oxides** (e.g., N₂O, NO, CO) do not react with acids or bases.
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O (as base); Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O (as acid).

Particle arrangement in solids, liquids and gases – useful for understanding dissolution of acids and bases.

Practice questions

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1.Which of the following is a property of acids?
Easy
- AThey have a pH greater than 7
- BThey turn blue litmus red
- CThey are proton acceptors
- DThey release hydroxide ions in water
2.Which ion is present in aqueous solutions of alkalis?
Easy
- AH⁺
- BOH⁻
- CCl⁻
- DNa⁺
3.What is the colour of methyl orange in an alkaline solution?
Easy
- ARed
- BYellow
- CBlue
- DColourless
4.Which of the following statements about strong and weak acids is correct?
Medium
- AA strong acid has a higher pH than a weak acid of the same concentration
- BA weak acid partially dissociates in water
- CA strong acid has a lower concentration of hydrogen ions than a weak acid
- DA weak acid completely dissociates in water
5.Which type of oxide is formed when a non-metal combines with oxygen?
Medium
- ABasic oxide
- BAcidic oxide
- CAmphoteric oxide
- DNeutral oxide
6.Which substance could be used to neutralise an acid?
Medium
- ALemon juice (pH 2)
- BBattery acid (pH 1)
- CToothpaste (pH 8)
- DVinegar (pH 3)
7.Which gas is produced when an acid reacts with a metal carbonate?
Medium
- AHydrogen
- BOxygen
- CCarbon dioxide
- DAmmonia
8.The pH of acid X is 6 and the pH of acid Y is 3. Which statement is correct?
Hard
- AThe hydrogen ion concentration of acid X is 1000 times greater than acid Y
- BThe hydrogen ion concentration of acid Y is 3 times greater than acid X
- CThe hydrogen ion concentration of acid Y is 1000 times greater than acid X
- DThe hydrogen ion concentration of acid X is 100 times greater than acid Y

Slides

Properties of Acids

Acids have pH values below 7, taste sour (when edible) and are corrosive.
Acids neutralise bases to form a salt and water.
In water, acids produce hydrogen ions (H⁺), which make the solution acidic.
Example: HCl(aq) → H⁺(aq) + Cl⁻(aq)

Which ion is responsible for the acidic properties of a solution?

OH⁻
H⁺
Cl⁻
Na⁺

Reactions of Acids with Metals

Only metals above hydrogen in the reactivity series react with dilute acids.
The reaction produces a salt and hydrogen gas: acid + metal → salt + hydrogen.
Example: Mg + 2HCl → MgCl₂ + H₂.

When an acid reacts with a metal, the products are a ____ and ____ gas.

Reactions of Acids with Bases and Carbonates

Acids neutralise bases (metal oxides/hydroxides) to form salt and water: acid + base → salt + water.
Acids react with metal carbonates to form salt, carbon dioxide and water: acid + carbonate → salt + CO₂ + H₂O.
Example: MgCO₃ + 2HCl → MgCl₂ + CO₂ + H₂O.

Which gas is produced when an acid reacts with a metal carbonate?

Hydrogen
Oxygen
Carbon dioxide
Ammonia

Indicators

Indicators change colour in acidic or alkaline solutions.
Litmus: red in acid, blue in alkali.
Methyl orange: red in acid, yellow in alkali.
Thymolphthalein: colourless in acid, blue in alkali.
Universal indicator gives a range of colours to estimate pH.

Which of these are true about indicators?

Litmus turns blue in acid.
Methyl orange turns yellow in alkali.
Thymolphthalein is colourless in acid.
Universal indicator is a single dye.

Properties of Bases and Alkalis

Bases have pH above 7; water-soluble bases are called alkalis.
Bases neutralise acids to form salt and water.
In water, alkalis produce hydroxide ions (OH⁻).
Example: NaOH(s) → Na⁺(aq) + OH⁻(aq).

Which ion is produced when an alkali dissolves in water?

H⁺
OH⁻
Cl⁻
Na⁺

Neutralisation and the pH Scale

Neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l).
pH scale ranges from 0 (strong acid) to 14 (strong alkali); pH 7 is neutral.
Lower pH = more acidic (higher H⁺ concentration).
The pH scale is logarithmic: a change of 1 in pH means a 10-fold change in H⁺ concentration.

A solution with pH 3 has ____ times the H⁺ concentration of a solution with pH 4.

Strong and Weak Acids (Extended)

Strong acids (e.g., HCl, H₂SO₄) dissociate completely in water: HCl → H⁺ + Cl⁻.
Weak acids (e.g., ethanoic acid) dissociate partially: CH₃COOH ⇌ H⁺ + CH₃COO⁻.
A concentrated weak acid can have a higher pH than a dilute strong acid.

Which statement about a weak acid is correct?

It dissociates completely in water.
It has a pH close to 7 but still below 7.
It always has a lower pH than a strong acid.
It produces no hydrogen ions.

Proton Transfer (Extended)

Acids are proton donors (they donate H⁺).
Bases are proton acceptors (they accept H⁺).
In the reaction HCl + H₂O → H₃O⁺ + Cl⁻, water acts as a base by accepting a proton.

Which of these are true about proton transfer?

Acids donate protons.
Bases accept protons.
Water can act as a base.
Protons are the same as electrons.

Classifying Oxides

Acidic oxides: formed from non-metals (e.g., CO₂, SO₂); react with bases to form salt and water.
Basic oxides: formed from metals (e.g., CuO, CaO); react with acids to form salt and water.
Amphoteric oxides (e.g., ZnO, Al₂O₃): react with both acids and bases.
Neutral oxides (e.g., CO, N₂O): do not react with acids or bases.

Which type of oxide is formed when a metal reacts with oxygen?

Acidic oxide
Basic oxide
Amphoteric oxide
Neutral oxide

Reaction of Alkalis with Ammonium Salts

Ammonium salts react with alkalis when warmed to produce ammonia gas, water and a salt.
Example: NH₄Cl + NaOH → NaCl + H₂O + NH₃.
Ammonia turns damp red litmus paper blue.

When an ammonium salt is warmed with an alkali, the gas produced is ____.

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