Science - Combined (0653) | Acids, bases and salts

IGCSE Combined Science (0653) Chemistry Notes: C7 Acids, Bases and Salts

Welcome to one of the most fundamental topics in Chemistry! Acids, bases, and salts are everywhere—from the batteries in your remote control to the soap you use to wash your hands, and even the chemical reactions happening in your stomach. Understanding these concepts is key to mastering chemistry reactions!

Quick Reminder: What is Neutralisation?

The core concept underlying this chapter is neutralisation. Neutralisation is the reaction between an acid and a base (or alkali) to form a salt and water.

Memory Aid: Acid + Base = Salt + Water (ABSW)

C7.1 The Characteristic Properties of Acids and Bases

What is pH and the Role of Indicators?

The pH scale is a numerical scale used to specify how acidic or basic (alkaline) a substance is. The scale ranges from 0 to 14.

• pH < 7: Acidic solutions (The lower the number, the stronger the acid, meaning 1 is strongly acidic).
• pH = 7: Neutral solutions (Pure water).
• pH > 7: Basic/Alkaline solutions (The higher the number, the stronger the alkali, meaning 14 is strongly alkaline).

We use substances called indicators to tell us whether a solution is acidic, basic, or neutral.

Universal Indicator (UI)

The most useful indicator for comparing relative acidity and alkalinity is Universal Indicator. It shows a whole range of colours corresponding to specific pH values.

Comparing Neutrality, Relative Acidity and Alkalinity:

• Neutral (pH 7): Green
• Strongly Acidic (pH 1–3): Red/Orange
• Weakly Acidic (pH 4–6): Yellow
• Weakly Alkaline (pH 8–10): Blue
• Strongly Alkaline (pH 11–14): Purple/Violet

Specific Indicators

You must know the effect of acids and alkalis on two common indicators:

Indicator	In Acid (Acidic solution)	In Alkali (Basic solution)
Litmus (paper or solution)	Turns Red	Turns Blue
Methyl Orange	Turns Red	Turns Yellow

C7.1 (Continued) Characteristic Properties of Acids

Acids react in specific ways. You need to know three major reactions for common acids (like $\text{HCl}$ or $\text{H}_2\text{SO}_4$).

1. Acid + Metal

When an acid reacts with a reactive metal (like magnesium or zinc), it produces a salt and hydrogen gas.

Reaction Rule:

\( \text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen} \)

Example:

\( \text{Sulfuric acid} + \text{Zinc} \rightarrow \text{Zinc sulfate} + \text{Hydrogen} \)

Observation: You will see fizzing/effervescence (due to the hydrogen gas produced) and the metal will slowly disappear.

Testing for Hydrogen: Collect the gas and place a lighted splint near it. It will make a loud 'pop' sound.

2. Acid + Base (Neutralisation)

A base is defined as an oxide or hydroxide of a metal. Bases react with acids to form salt and water only.

Reaction Rule:

\( \text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water} \)

Example:

\( \text{Hydrochloric acid} + \text{Copper(II) oxide} \rightarrow \text{Copper(II) chloride} + \text{Water} \)

Observation: If the base is insoluble (like $\text{CuO}$), the solid will disappear as it reacts, and the solution will change colour (e.g., from colourless acid to blue $\text{CuCl}_2$).

3. Acid + Carbonate

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

Reaction Rule:

\( \text{Acid} + \text{Carbonate} \rightarrow \text{Salt} + \text{Water} + \text{Carbon dioxide} \)

Example:

\( \text{Nitric acid} + \text{Sodium carbonate} \rightarrow \text{Sodium nitrate} + \text{Water} + \text{Carbon dioxide} \)

Observation: You will see vigorous fizzing/effervescence (due to the $\text{CO}_2$ produced).

Testing for Carbon Dioxide: Bubble the gas produced through limewater ($\text{aqueous calcium hydroxide}$). If $\text{CO}_2$ is present, the limewater turns milky/cloudy.

C7.1 (Continued) Characteristic Properties of Bases and Alkalis

Bases and Alkalis Defined

• A Base is an oxide or hydroxide of a metal.
• An Alkali is a soluble base. All alkalis are bases, but not all bases are alkalis (e.g., $\text{CuO}$ is a base but not an alkali because it is insoluble in water).

The main characteristic reaction of a base/alkali is neutralisation with an acid (Reaction 2 above).

Key Takeaway: Alkalis dissolve in water and produce hydroxide ions ($\text{OH}^-$), which gives them their strong cleaning properties and high pH.

You must know the effect of alkalis (soluble bases) on indicators:

• Litmus: Turns Blue.
• Methyl Orange: Turns Yellow.

Common Mistake to Avoid: Make sure you know the difference between a base (can be soluble or insoluble) and an alkali (must be soluble).

C7.2 Oxides

Oxides are compounds containing oxygen and one other element. We classify them based on whether they behave like an acid or a base when they react with water or other substances.

1. Basic Oxides

These are oxides formed by metals. They react with acids to form a salt and water (acting as a base).

• Characteristics: Formed by metals. React with acids.
• Examples: Copper(II) oxide ($\text{CuO}$) and Calcium oxide ($\text{CaO}$).
• Analogy: Since metals form basic oxides, they are like the ‘base’ side of the chemical team.

2. Acidic Oxides

These are oxides formed by non-metals. When dissolved in water, they often form an acidic solution.

• Characteristics: Formed by non-metals. React with bases/alkalis.
• Examples: Sulfur dioxide ($\text{SO}_2$) and Carbon dioxide ($\text{CO}_2$).
• Real World Connection: $\text{SO}_2$ and nitrogen oxides are responsible for acid rain because they dissolve in rain water to form dilute acids.

C7.3 Preparation of Salts

A salt is an ionic compound formed when the hydrogen atom in an acid is replaced by a metal ion or an ammonium ion ($\text{NH}_4^+$).

Since we are in the Core section, we focus only on preparing soluble salts. Soluble salts are usually prepared by reacting an acid with an excess of an insoluble solid (metal, base, or carbonate) OR by titrating an acid with a soluble alkali.

Method 1: Preparation using Excess Insoluble Reactants (Acid + Excess Metal/Insoluble Base/Insoluble Carbonate)

This method works because the metal, insoluble base, or insoluble carbonate can be easily removed by filtration, leaving a pure salt solution.

You can use:

(b) Excess metal: (e.g., $\text{Zn}$ with $\text{H}_2\text{SO}_4$ to make $\text{ZnSO}_4$)

(c) Excess insoluble base: (e.g., $\text{CuO}$ with $\text{HCl}$ to make $\text{CuCl}_2$)

(d) Excess insoluble carbonate: (e.g., $\text{CaCO}_3$ with $\text{HNO}_3$ to make $\text{Ca}(\text{NO}_3)_2$)

Step-by-step for Insoluble Reactants:

1. Add Excess: Add the insoluble reactant (metal, base, or carbonate) to the warm dilute acid until the solid is in excess (meaning some solid remains unreacted at the bottom). This ensures all the acid is used up.
2. Filter: Filter the mixture to remove the excess unreacted insoluble solid (the residue). This leaves a pure solution of the salt (the filtrate).
3. Heat/Evaporate: Gently heat the salt solution (filtrate) to evaporate some of the water until the solution is saturated (point of crystallisation).
4. Crystallise: Allow the saturated solution to cool slowly. Crystals of the pure salt will form.
5. Dry: Remove the crystals and dry them (e.g., using filter paper or a warm oven).

Note: We use excess solid reactant because we do not know the exact concentration of the acid, and we want to ensure the final solution only contains the salt and water, not leftover acid.

Method 2: Preparation using Titration (Acid + Alkali)

This method must be used when both the acid and the base (alkali) are soluble (e.g., making $\text{NaCl}$ from $\text{NaOH}$ and $\text{HCl}$). Since both reactants are dissolved, we cannot simply use excess and filter out the unreacted solid.

(a) An alkali by titration: (e.g., $\text{NaOH}$ with $\text{HCl}$ to make $\text{NaCl}$)

Step-by-step using Titration:

1. Titrate: Carefully mix the known volumes and concentrations of acid and alkali using a burette and pipette, along with an indicator (like methyl orange or phenolphthalein).
2. Find the Endpoint: Stop adding the alkali exactly when the indicator changes colour, showing the solution is perfectly neutral. Record the volume used.
3. Repeat without Indicator: Repeat the exact procedure using the recorded volumes, but without the indicator. This prevents the indicator from contaminating the salt product.
4. Crystallise and Dry: Gently heat the resulting neutral salt solution to evaporate water until saturated, allow it to cool and crystallise, and then dry the pure salt crystals.

C7.3 (Supplement content - for Extended students only)

Although the Core section focuses on soluble salts, Extended students must also know how to prepare insoluble salts.

Preparation of Insoluble Salts by Precipitation

Precipitation is the formation of a solid (precipitate) from a solution during a chemical reaction. This happens when two soluble salts react to form an insoluble salt.

Method:

1. Mix Soluble Solutions: Mix two different aqueous solutions, each containing one of the ions needed for the insoluble salt.
2. Filter: The insoluble salt forms immediately as a precipitate. This precipitate is then separated from the liquid by filtration.
3. Wash: The precipitate is washed thoroughly with distilled water to remove any remaining soluble ions.
4. Dry: The clean solid precipitate is then dried.

(Candidates do not need to know general solubility rules for salts.)