Chemistry: Acids & Bases

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The definition of an acid:

A substance that produces hydrogen ions (H⁺) in an aqueous solution

Every acidic compound contains at one hydrogen atom (True/False)

True

All compounds which contain at least two hydrogen atoms are acids (True/False)

False. Not all compounds that contain hydrogen atoms are acids

What does the bascity of an acid refer to?

The number of hydrogen ions that one molecule of that acid can give away when in an aqeuous solution

How could the bascity of an acid be deduced from its formula (4)?

By looking at the subscript of the H in a formula:

1→Monobasic

• 1 molecule produces 1 hydrogen ion upon

  ionisation

2→Dibasic

• 1 molecule produces 2 hydrogen ions upon

  ionisation

3→Tribasic

• 1 molecule produces 3 hydrogen ions upon

  ionisation

Physical properties of acids (6):

• Have a sour taste

• Have a pH less than 7

• Turn blue litmus paper red

• Do not change the colour of red litmus paper

• Turn Universal Indicator from green 🟩 to yellow 🟨 or orange 🟧 or red 🟥

• Are good conductors of electricity in aqueous solutions (mobile ions)

Salt definition:

An ionic compound that is formed when a metal or ammonium ion replaces one or more hydrogen ion of an acid

List a few common metals unreactive with acids:

• Copper (Cu)

• Silver (Ag)

• Gold (Au)

Which metals are too reactive with acids?

Group 1 & Group 2 metals

Base definition:

Metal oxides or metal hydroxides that react with acids to form only salt & water

Why do acids display their acidic properties in water but not organic solvents (…)?

Acids produce hydrogen ions in aqueous solution, which give acids their acidic properties, but do not produce such ions (Ionise) in organic solvents & thus do not display any acidic properties in organic solvents

The chemical properties of acids (equation with a description):

Acids react with reactive metals to form a salt & hydrogen gas

• ACID + METAL → SALT + HYDROGEN

Acids react with bases to form a salt & water only (neutralisation)

• ACID + BASE → SALT + WATER

Acids react with metal carbonates to form a salt, water & carbon dioxide gas

• ACID + METAL CARBONATE → SALT + WATER + CARBON DIOXIDE

Alkali definition:

Any base that dissolves in water & produces hydroxide ions in aqueous solutions

An alkali produces oxide ions in aqueous solutions (True/False)

False. An alkali produces hydroxide ions in aqueous solutions

The similarities and differences between bases and alkalis

Similarities:

• Both neutralise acids to produce only salt & water

Differences:

• All alkalis are bases while not all bases are alkalis

• Bases are either soluble or insoluble while alkalis are strictly soluble

Physical properties of alkalis (7):

• Have a bitter taste

• Have a soapy feel

• Have a pH greater than 7

• Turns damp red litmus paper blue

• Do not change the colour of damp blue litmus paper

• Turn Universal Indicator from green 🟩 to blue 🟦 or violet 🟪

• Are good conductors of electricity in aqueous solutions (mobile ions)

The chemical properties of alkalis (with a description):

Alkalis react with acids to form a salt & water only (neutralisation)

• ALKALI + ACID → SALT + WATER

Alkalis react with ammonium salts to form a salt, water & ammonia gas

• ALKALI + AMMONIUM SALT → SALT + WATER + AMMONIA

The last reaction takes place properly only in the presence of heat

What is the ionic equation of the reaction between hydrochloric acid and sodium hydroxide?

H⁺ (aq) + OH^- (aq) → H₂O (l)

The standard equation for an acid-alkali neutralisation reaction

What is the ionic equation of the reaction between potassium hydroxide and nitric acid?

OH^- (aq) + H⁺ (aq) → H₂O (l)

The standard equation for an alkali-acid neutralisation reaction

「H⁺ (aq) + OH^- (aq) → H₂O (l)」is the standard ionic equation for an acid-oxide reaction (True/False)

False. It is the standard equation for an acid-alkali reaction

What gas is released when lead reacts vigorously with dilute nitric acid?

Nitrogen oxide

How does the solubility of lead nitrate in nitric acid affect the reaction between lead and dilute nitric acid?

Its high solubility allows the lead to dissolve completely

Why does concentrated nitric acid react slower with lead than with dilute nitric acid?

The concentrated form forms an insoluble layer of lead nitrate that prevents the acid from reaching the metal

What insoluble layer forms on the surface of lead when it reacts with dilute hydrochloric acid?

Lead (II) chloride

What gas is produced during the slow reaction of lead with dilute hydrochloric acid?

Hydrogen gas

Lead reacts very slowly with dilute hydrochloric acid because the layer of _____ quickly stops the reaction.

Lead (II) chloride

Why is lead able to dissolve in highly concentrated hydrochloric acid but not in the dilute form?

Concentrated acid dissolves the lead chloride layer into soluble complex ions

What specific substance forms a protective coating on lead when it is exposed to dilute sulfuric acid?

Lead sulfate

How does the lead sulfate coating affect the reaction between lead and dilute sulfuric acid?

It isolates the metal, causing the reaction to practically stop

Lead has historically been used to store _____ because of the protective coating it forms upon contact.

Sulfuric acid

Under what conditions will concentrated sulfuric acid react with lead to strip the metal?

When the acid is hot

What gaseous byproduct is released during the reaction of lead with hot, concentrated sulfuric acid?

Sulfur dioxide

Identify the three chemical products formed when lead reacts with hot, concentrated sulfuric acid:

Sulfur dioxide (SO₂), lead sulfate (PbSO₄) & water (H₂O)

In the context of lead reactions, which acid produces nitrogen oxide rather than hydrogen gas when dilute?

Nitric acid

How does the physical state of lead nitrate differ when formed in dilute versus highly concentrated nitric acid?

It is highly soluble in dilute acid but forms an insoluble layer in concentrated acid

Passivation definition: (in lead reactions)

The formation of an insoluble layer (like lead sulfate or lead chloride) that protects the underlying metal from further acid attack

Concentration of an acid definition:

A measure of the amount (mol) of acid solute dissolved in a specific volume of solution

This ‘specific volume’ is usually 1 dm³

How could the concentration of an acid or an alkali in a solution be changed (4)?

Variables:

• Solvent volume

• Solute volume

Change:

• Solvent volume ∝ 1/concentration

• Solute volume ∝ concentration

What does the strength of an acid refer to?

A measure of the extent of ionisation (dissociation) of an acid or an alkali, when dissolved in water, to produce H⁺ or OH^- ions

How could the strength of an acid or an alklai in a solution be changed?

It cannot be changed

As per the current syllabus (Chemistry, 2026), it cannot be easily changed & the methods to are not taught anyway

Describing strong or weak acids or alkali (0):

A [strong/weak]₁ [acid/alkali]₂ is an [ ]₂ that is [completely v partially]₁ ionised in an aqueous solution

Self notation:

/ calls for a decision made regardless of preceding clauses (if any), while v calls for a decision which considers preceding clauses, in this case a ‘former or latter’ format

A list of common strong and weak acids, with their formulas:

Strong:

• Hydrochloric acid (HCl)

• Sulfuric acid (H₂SO₄)

• Nitric acid (HNO₃)

Weak:

• Carbonic acid (H₂CO₃)

• Citric acid (C₆H₈O₇)

• Ethanoic acid (CH₃COOH)

A list of common strong and weak alkalis, with their formulas:

Strong:

• Potassium hydroxide (KOH)

• Sodium hydroxide (NaOH)

Weak

• (Aqueous) ammonia/Ammonium hydroxide (NH₄OH)

How is strength affected by concentration?

It is not. Every individual molecule will dissociate (and release a certain amount of ions) according to its strength, its concentration notwithstanding

Strength is a fixed property, while concentration can be changed

What is pH?

A measure of the concentration of hydrogen ions in an aqueous solution

What do the numbers (1, 7 and 14) on a pH scale indicate (3)?

• 1 indicates the lowest pH and the most acidic environment (largest concentration of hydrogen ions)

• 7 indicates the middle pH & a neutral environment (same concentration of hydrogen & hydroxide ions)

• 14 indicates the highest pH & the most alkaline environment (lowest concentration of hydrogen ions)

What can be used to measure pH?

• pH indicator (different colours)

• pH meter

• pH sensor (connected to a data logger)

pH indicator defnition:

Coloured dyes which change colour when added to an acid or an alkali

How can pH be used to compare the strength (not concentration) of different acids (4)?

1. Prepare the desired acids for comparison

2. Ensure that all are of the same concentration

3. Dissolve each acid in the same volume of water

4. Measure the pH (Indicator, meter, sensor)

The lower the pH, the stronger the acid (greater extent of ionisation), and vice versa

How does the Universal indicator work?

It contains a mixture of dyes that each change colour depending on the concentration of hydrogen ions in a solution

The different colours that the universal indicator changes to with each pH range:

• Red 🟥→pH <3

• Orange/Yellow 🟧/🟨→pH 3-6

• Green 🟩→pH 7 (neutral)

• Blue 🟦→pH 8-11

• Violet/Purple 🟪→pH 11-14

What is the color of methyl orange when placed in an acidic solution?

Red 🟥

What color does methyl orange exhibit in an alkaline solution?

Yellow 🟨

Methyl orange transitions between colors within the pH range of _____ to _____.

3, 5

What color does screened (combined with green/blue dye) methyl orange exhibit in an acidic solution?

Violet 🟪

What is the color of screened methyl orange in an alkaline solution?

Green 🟩

The indicator screened methyl orange changes color within the pH range of _____ to _____.

3, 5

What color is litmus when it is in an acidic solution?

Red 🟥

What color does litmus exhibit in an alkaline solution?

Blue 🟦

Litmus transitions between colors within the pH range of _____ to _____.

5, 8

What color is thymolphthalein in an acidic solution?

Colourless

What color does thymolphthalein exhibit in an alkaline solution?

Blue 🟦

Thymolphthalein transitions between colors within the pH range of _____ to _____.

9, 10.5

What is the color of bromothymol blue in an acidic solution?

Yellow 🟨

What color does bromothymol blue exhibit in an alkaline solution?

Blue 🟦

Bromothymol blue transitions between colors within the pH range of _____ to _____.

6, 8

What color is phenolphthalein in an acidic solution?

Colourless

What color does phenolphthalein exhibit in an alkaline solution?

PInk 🩷

Phenolphthalein transitions between colors within the pH range of _____ to _____.

8, 10

Which indicator transitions from colorless to blue in the pH range of 9 to 10.5?

Thymolphthalein

Which indicator transitions from violet to green in the pH range of 3 to 5?

Screened methyl orange

In acidic solutions, both phenolphthalein and thymolphthalein are _____.

Colourless

Which indicator transitions from yellow to blue between pH 6 and pH 8?

Bromothymol blue

Which specific indicator listed turns pink in an alkaline solution?

Phenolphthalein

Which two indicators in the table share the same pH color change range of 3 to 5?

Methyl orange & Screened methyl orange

Why is it important to take note of soil pH?

The pH of soil affects the growth & development of plants by determining how easily plants can absorb nitrogen

Which factors affect the pH of soils?

• Type of soil

• Fertilisers used

The pH of soils range from 3-5 (True/False)

False. It ranges from pH 4-8

Do plants have an optimum soil pH (?)?

Yes. It ranges from slightly acidic to neutral; pH 5-7

What makes soil unsuitable for plant growth?

• Acid rain

• Excessive use of fertilisers

They abnormally alter soil pH

What could be done when soil is too acidic?

Liming the soil by adding bases [calcium (hydr)oxide]

Why must the liming of soil always be done together with ammonium-based fertiliser?

They should never be done together. Such bases would react with ammonium salts to form ammonia gas which escapes into the atmosphere. Thus, the plants cannot absorb the ammonium ion which provides the nitrogen it needs. The calcium oxide/hydroxide required to neutralise the soil acidity would also be lost

Oxide definition:

Compounds of oxygen and another element

What are the five classifications of oxides?

There are only four oxides:

• Acidic

• Basic

• Amphoteric

• Neutral

Most oxides formed by metals belong to which chemical category?

Basic oxides

What products are generated when a basic oxide reacts with an acid?

Salt & water only

What is the typical physical state of basic oxides at room temperature?

Solids

Most basic oxides are insoluble in water, but sodium oxide and potassium oxide dissolve to form _____.

Alkalis

Which type of metallic oxide is defined by its ability to react with both acids and bases?

Amphoteric oxides

What products are formed when an amphoteric oxide reacts with either an acid or a base?

Salt & water only

In the mnemonic 'Z.A.P.', which three metallic oxides are identified as amphoteric?

Zinc oxide (ZnO), aluminium oxide (Al₂O₃), & lead(II) oxide (PbO)

Describe the water solubility of these amphoteric oxides: Zinc oxide, Aluminum oxide, and Lead (II)oxide

They are all insoluble in water

Most non-metallic oxides fall into which category?

Acidic oxides

What products result from the reaction between an acidic oxide and an alkali?

Salt & water only

What is the common physical state of acidic oxides at room temperature?

Gases

Most acidic oxides react with water to produce _____.

Acids

Which specific acid is formed when carbon dioxide ($CO_{2}$) dissolves in water?

Carbonic acid (H₂CO₃)

Which acidic oxide is the precursor to sulfuric acid ($H_{2}SO_{4}$)?

Sulfur trioxide (SO₃)

The reaction between phosphorus (V) oxide (P₄O₁₀) and water (H₂0) yields which acid?

Phosphoric acid (H₃PO₂)

Which group of non-metallic oxides exhibits neither acidic nor basic properties?

Neutral oxides