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Acids, Bases, and Salts

Chapter 12: Acids, Bases, and Salts

Welcome, budding chemists! Today, we\'re going to explore the world of acids, bases, and salts. These chemical compounds are everywhere, playing crucial roles in nature and in our daily lives.

Have you ever heard of Lake Natron in northern Tanzania? It\'s an incredible place, but its water is so corrosive that it can burn your skin! Why is it so harsh? Because it\'s highly alkaline, meaning it contains strong bases. This high alkalinity is due to the presence of salts like sodium carbonate and sodium hydrogen carbonate. Despite its harsh conditions, Lake Natron is a vital breeding ground for the lesser flamingos. The alkalinity even helps keep most predators away. This real-world example shows us just how powerful and important these substances can be!

Let\'s start by breaking down each of these important categories.

1. Acids: The Sour and Corrosive

Many naturally occurring substances, like citric acid found in oranges, lemons, and limes, are acids. These are what give citrus fruits their sour taste. We also have man-made acids like nitric acid and sulfuric acid.

What are Acids?

Aqueous solutions of acids contain hydrogen ions (H⁺). This means when an acid dissolves in water, it releases H⁺ ions.
Acids are proton (H⁺) donors. They donate their H⁺ ions to other substances.

Common Acids and Ions Produced in Aqueous Solution

Acid	Formula	Ions produced in aqueous solution
Ethanoic acid	CH₃COOH	H⁺(aq), CH₃COO⁻(aq)
Hydrochloric acid	HCl	H⁺(aq), Cl⁻(aq)
Nitric acid	HNO₃	H⁺(aq), NO₃⁻(aq)
Sulfuric acid	H₂SO₄	H⁺(aq), SO₄²⁻(aq)

Notice how all these acids produce H⁺ ions in water. These H⁺ ions are what give acids their special properties.

Properties of Acids

Acids are very reactive substances and participate in several important chemical reactions:

Reaction with Reactive Metals

When acids react with reactive metals, they form a salt and hydrogen gas.

metal + acid → salt + hydrogen

Example: If you add a piece of magnesium ribbon to dilute sulfuric acid, you\'ll see bubbles of hydrogen gas. The equation is:

Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)

(Magnesium sulfate is the salt formed here).

How do we test for hydrogen gas? When you place a lighted splint (a small burning piece of wood) at the mouth of the test tube, hydrogen gas extinguishes the splint with a characteristic \'pop\' sound. This "pop" tells you hydrogen gas is present!

Test for Hydrogen Gas: The \'pop\' sound.

Important notes on metal reactions:

Unreactive metals like copper or silver do not react with dilute acids.
Lead reacts with dilute hydrochloric acid or sulfuric acid, but it forms an insoluble layer of lead(II) chloride or lead(II) sulfate on its surface. This layer prevents further reaction, protecting the metal.

Reaction with Metal Oxides (Bases)

Metal oxides are a type of base. When acids react with metal oxides, they form a salt and water.

metal oxide + acid → salt + water

Example: When zinc oxide reacts with dilute sulfuric acid, it forms zinc sulfate and water.

ZnO(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l)

Reaction with Bases (Metal Hydroxides)

Metal hydroxides are also bases. Similar to metal oxides, when acids react with metal hydroxides, they form a salt and water.

metal hydroxide + acid → salt + water

Example: Zinc hydroxide reacting with dilute nitric acid produces zinc nitrate and water.

Zn(OH)₂(s) + 2HNO₃(aq) → Zn(NO₃)₂(aq) + 2H₂O(l)

Reaction with Carbonates

Acids react with carbonates to form a salt, water, and carbon dioxide gas.

carbonate + acid → salt + water + carbon dioxide

Example: The reaction between calcium carbonate (like chalk or limestone) and dilute hydrochloric acid produces calcium chloride, water, and carbon dioxide.

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

How do we test for carbon dioxide gas? You can bubble the gas produced through limewater (calcium hydroxide solution). If carbon dioxide is present, the limewater will turn milky (or form a white precipitate).

Test for Carbon Dioxide Gas: Limewater turns milky.

What is the Role of Water in Acids?

This is a really important point!

Acids only show their characteristic properties (like being corrosive or reacting with metals) when they are dissolved in water.
This is because an acid produces hydrogen ions (H⁺) only in water.

For example, hydrogen chloride (HCl) can exist as different types of particles depending on whether it\'s in an aqueous solution (dissolved in water) or an organic solvent.

In water, hydrogen chloride dissociates into hydrogen ions (H⁺) and chloride ions (Cl⁻). These H⁺ ions are what make it acidic and allow it to react with substances like magnesium.

HCl(aq) → H⁺(aq) + Cl⁻(aq)

In an organic solvent, hydrogen chloride exists as covalent molecules. It does not ionise and therefore does not produce hydrogen ions (H⁺). Without H⁺ ions, there is no reaction with magnesium.

This distinction is crucial: water is essential for an acid to behave like an acid!

2. Bases and Alkalis: The Slippery Counterparts

Just like acids, bases are found everywhere, especially in cleaning products like oven cleaners and drain cleaners.

What are Bases?

Bases are oxides or hydroxides of metals.
They contain either the oxide ion (O²⁻) or the hydroxide ion (OH⁻).
Most bases are insoluble in water. For instance, copper(II) oxide is a base but doesn\'t dissolve in water.

Common Bases

Base	Formula
Sodium oxide	Na₂O
Zinc oxide	ZnO
Copper(II) oxide	CuO
Magnesium hydroxide	Mg(OH)₂
Aluminium hydroxide	Al(OH)₃

What are Alkalis?

Alkalis are bases that are soluble in water.
When alkalis dissolve in water, they produce hydroxide ions (OH⁻).
Alkalis are proton (H⁺) acceptors. They accept the H⁺ ions from acids.

Common Alkalis and Ions Produced in Aqueous Solution

Alkali	Formula	Ions produced in aqueous solution
Sodium hydroxide	NaOH	Na⁺(aq), OH⁻(aq)
Potassium hydroxide	KOH	K⁺(aq), OH⁻(aq)
Calcium hydroxide	Ca(OH)₂	Ca²⁺(aq), OH⁻(aq)
Barium hydroxide	Ba(OH)₂	Ba²⁺(aq), OH⁻(aq)
Ammonia	NH₃	NH₄⁺(aq), OH⁻(aq)

It\'s important to note that aqueous ammonia (ammonia dissolved in water) is an alkali, but it\'s unusual because it does not contain a metal. When ammonia dissolves in water, it forms ammonium ions (NH₄⁺) and hydroxide ions (OH⁻).

Properties of Bases and Alkalis

Reaction with Acids (Neutralisation)

This is one of the most important reactions! Bases react with acids to give a salt and water. This type of reaction is called neutralisation because the acid and base cancel each other out.

base + acid → salt + water

Example: Copper(II) oxide (a base) reacts with dilute sulfuric acid to form copper(II) sulfate (a salt) and water.

CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

What happens in neutralisation at the ionic level? The hydrogen ions (H⁺) from the acid and the hydroxide ions (OH⁻) from the alkali react to form water. The ionic equation for neutralisation is:

H⁺(aq) + OH⁻(aq) → H₂O(l)

Reaction with Ammonium Salts

When bases are heated with ammonium salts, they produce a salt, water, and ammonia gas.

base + ammonium salt → salt + water + ammonia

Example: If you heat calcium oxide (a base) with ammonium chloride (an ammonium salt), you get calcium chloride, water, and ammonia gas.

CaO(s) + 2NH₄Cl(s) → CaCl₂(s) + H₂O(l) + 2NH₃(g)

How do we test for ammonia gas? Ammonia gas has a characteristic pungent smell. You can also test for it by holding a piece of moist red litmus paper at the mouth of the test tube. The ammonia gas will turn the red litmus paper blue.

3. Strength of Acids: How Much They Dissociate

When we talk about the "strength" of an acid or an alkali, we\'re referring to how easily it dissociates (breaks apart into ions) when dissolved in water. This is different from concentration!

What is a Strong Acid?

A strong acid is an acid that completely dissociates (or ionises) in an aqueous solution. This means almost all of its molecules break apart to release H⁺ ions.

Example: Hydrochloric acid (HCl) is a strong acid. When it dissolves in water, nearly all the HCl molecules separate into H⁺ and Cl⁻ ions.

HCl(aq) → H⁺(aq) + Cl⁻(aq)

The arrow pointing in one direction (→) shows complete dissociation.

What is a Weak Acid?

A weak acid is an acid that only partially dissociates in an aqueous solution. This means only a small fraction of its molecules release H⁺ ions, while most remain as intact molecules.

Example: Ethanoic acid (CH₃COOH), found in vinegar, is a weak acid. When it dissolves in water, only some of the CH₃COOH molecules dissociate into CH₃COO⁻ and H⁺ ions. The reaction is reversible, meaning the ions can recombine to form the original molecules.

CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)

The double arrow (⇌) shows partial dissociation and a reversible reaction.

Is Strength Related to Concentration?

No! It\'s important not to confuse the strength of an acid with its concentration.
Strength refers to the extent of dissociation (how much it breaks apart into ions).
Concentration refers to how much solute (the acid) is dissolved in a given amount of solvent (water).

A concentrated acid means there\'s a large amount of acid dissolved. A dilute acid means there\'s a small amount of acid dissolved.

The strength of an acid is not affected by its concentration. A dilute solution of a strong acid is still a strong acid because it still fully dissociates, even though there are fewer acid molecules overall. Similarly, a concentrated solution of a weak acid is still a weak acid because it only partially dissociates, even though there are many acid molecules.

4. The pH Scale: Measuring Acidity and Alkalinity

The pH scale is a numerical scale, usually ranging from 0 to 14, that tells us whether a solution is acidic, neutral, or alkaline.

What Does pH Mean?

The pH value of a solution is directly related to the concentration of hydrogen ions (H⁺) or hydroxide ions (OH⁻) present in the solution.

Acidic solutions have pH values less than 7. The lower the pH value, the higher the concentration of H⁺ ions, and the more acidic the solution.
Alkaline solutions have pH values greater than 7. The higher the pH value, the higher the concentration of OH⁻ ions, and the more alkaline the solution.
A neutral solution has a pH value of exactly 7. Pure water, for example, is neutral.

pH Values of Common Substances

The pH Scale with common substances.

0-1 (Very Acidic): Liquid in car battery, gastric juice (in stomach)
2-3 (Acidic): Lemon juice, vinegar
4-6 (Weakly Acidic): Tomato juice, black coffee, milk
7 (Neutral): Pure water
8-9 (Weakly Alkaline): Toothpaste, baking soda
10-11 (Alkaline): Soap, aqueous ammonia
12-14 (Very Alkaline): Bleach, oven cleaner, drain cleaner

How is the pH of a Solution Determined?

The pH value of a solution can be determined using indicators.

Universal Indicator: This is a mixture of different dyes that changes to a variety of colours depending on the pH value of the solution. You can compare the colour obtained with a colour chart to determine the pH. This is very useful because it gives you a range of pH values, not just if it\'s acid or base.

Other Common Indicators

Indicator	Colour in acidic solution	Colour in alkaline solution
Litmus	Red	Blue
Thymolphthalein	Colourless	Blue
Methyl orange	Red	Yellow

5. Oxides: Compounds with Oxygen

An oxide is a chemical compound that contains at least one oxygen atom and one other element. Many acids and alkalis are formed by dissolving oxides in water. Oxides can be broadly classified into three categories: acidic oxides, basic oxides, and amphoteric oxides.

What are Acidic Oxides?

Most acidic oxides are formed from non-metals.
They often dissolve in water to form acids.

Example: Sulfur dioxide (a non-metal oxide) reacts with water to form sulfurous acid.

SO₂(g) + H₂O(l) → H₂SO₃(aq)

Common Acidic Oxides:

Carbon dioxide (CO₂): Forms carbonic acid (H₂CO₃) in water. It\'s the fizz in carbonated drinks!
Sulfur trioxide (SO₃): Forms sulfuric acid (H₂SO₄) in water.
Phosphorus(V) oxide (P₂O₅): Forms phosphoric acid (H₃PO₄) in water.

Properties of Acidic Oxides:

They react with alkalis (bases) to form a salt and water.

Example: Carbon dioxide reacts with sodium hydroxide to form sodium carbonate and water.

CO₂(g) + 2NaOH(aq) → Na₂CO₃(aq) + H₂O(l)

Silicon dioxide (SiO₂) is an acidic oxide, but it\'s a solid at room temperature and does not dissolve in water. However, it reacts with hot concentrated sodium hydroxide to form sodium silicate.

What are Basic Oxides?

Most basic oxides are formed from metals.
Many basic oxides are insoluble in water. However, some, like sodium oxide (Na₂O) and potassium oxide (K₂O), dissolve in water to form alkalis.

Common Basic Oxides:

Copper oxide (CuO)
Calcium oxide (CaO)

Properties of Basic Oxides:

They react with acids to form a salt and water.

Example: Magnesium oxide reacts with nitric acid to form magnesium nitrate and water.

MgO(s) + 2HNO₃(aq) → Mg(NO₃)₂(aq) + H₂O(l)

What are Amphoteric Oxides?

Amphoteric oxides are a special type of metallic oxide that can react with both acids and bases to form salts and water. They can behave as a basic oxide or an acidic oxide depending on what they react with.

Common Amphoteric Oxides:

Zinc oxide (ZnO)
Aluminium oxide (Al₂O₃)
Lead(II) oxide (PbO)

Examples of Amphoteric Behaviour (using Zinc oxide and Aluminium oxide):

Reacting with an acid (behaving as a basic oxide):

ZnO(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l)

Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)

Reacting with a base/alkali (behaving as an acidic oxide):

ZnO(s) + 2NaOH(aq) + H₂O(l) → Na₂ZnO₂(aq) + 2H₂O(l)

(forms sodium zincate)

Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAlO₂(aq) + 4H₂O(l)

(forms sodium aluminate)

How Can We Classify an Unknown Oxide?

You can determine if an oxide is basic, acidic, or amphoteric by testing its reactions with acids and alkalis. Here’s a helpful flowchart:

Flowchart for Classifying an Unknown Oxide.

Does the oxide react with an acid?
- Yes: It\'s either a Basic oxide or an Amphoteric oxide. Go to step 2.
- No: It\'s an Acidic oxide. (It doesn\'t react with acid, so it must react with an alkali to show its acidic nature.)
If it reacted with acid (Basic or Amphoteric), now test: Does it react with an alkali?
- Yes: It\'s an Amphoteric oxide (reacts with both acid and alkali).
- No: It\'s a Basic oxide (reacts only with acid, not alkali).

This systematic approach helps identify the type of oxide you have!

6. Salts: The Products of Reactions

Salts are very common compounds that result from the reaction of acids with bases, metals, or carbonates.

What are Salts?

Salts are ionic compounds. This means they are made up of a cation (positive ion) and an anion (negative ion).

For example, in potassium chloride (KCl), K⁺ is the cation (from a base like KOH) and Cl⁻ is the anion (from an acid like HCl).

How are Salts Formed?

Reactants	Products
Acid + alkali	Salt + water
Acid + metal	Salt + hydrogen gas
Acid + insoluble base	Salt + water
Acid + insoluble carbonate	Salt + water + carbon dioxide gas

These are the fundamental reactions that produce salts. The specific type of salt formed depends on the acid and base/metal/carbonate used.