The Characteristic Properties of Acids and Bases (Chapter 7.1)

Hello future chemists! This chapter is incredibly practical because acids and bases are everywhere—from the lemons you eat (citric acid) to the soap you use (alkaline solution). Understanding them is key to mastering many other topics in IGCSE Chemistry, especially when we look at making salts.
We are going to explore what makes an acid an acid, what makes a base a base, and how they react together. Don't worry if this seems tricky at first; we'll use simple definitions and handy tricks!

Part 1: Defining Acids – The \(H^+\) Story

The most important thing to remember about an acid is what it releases when dissolved in water.

Core Definition of Acids
  • Aqueous solutions of acids always contain hydrogen ions, \(H^+\) (aq).
  • It is these \(H^+\) ions that are responsible for the characteristic properties of acids (like tasting sour or reacting with metals).
Extended Definition: Acids as Proton Donors (Supplement)

In chemistry, a hydrogen ion (\(H^+\)) is often referred to as a proton because a hydrogen atom usually consists of one proton and one electron; losing the electron leaves only the proton behind.

  • Acids are defined as proton donors. They "donate" or give away the \(H^+\) ion during a reaction.

Did you know? Concentrated acids are dangerous! They are highly corrosive because the high concentration of \(H^+\) ions readily react with (and destroy) materials, including your skin.

Identifying Acids using Indicators (Core)

Indicators are substances that change colour depending on whether they are in an acidic or alkaline solution.

  • Litmus Paper: Turns Blue $\rightarrow$ Red in acid.
  • Methyl Orange: Turns Yellow $\rightarrow$ Red/Pink in acid.
  • Thymolphthalein: Remains Colourless in acid.

Quick Tip (Mnemonics):

Acid Reds Litmus. (Acid $\rightarrow$ Red)
Bases Blue Litmus. (Base $\rightarrow$ Blue)

Key Takeaway for Acids

Acids contain \(H^+\) ions (protons) and turn blue litmus paper red.

Part 2: Characteristic Reactions of Acids (Core)

Acids react characteristically with metals, bases, and carbonates. Remember the M-B-C rule!

1. Reaction with Metals (M)

Acids react with reactive metals (those above hydrogen in the reactivity series) to produce a salt and hydrogen gas.

Acid + Metal $\rightarrow$ Salt + Hydrogen

Example:
Sulphuric acid + Zinc $\rightarrow$ Zinc sulphate + Hydrogen
\(\mathrm{H_2SO_4(aq) + Zn(s) \rightarrow ZnSO_4(aq) + H_2(g)}\)

The hydrogen gas produced can be tested using the squeaky pop test (using a lighted splint).

2. Reaction with Bases (B) - Neutralisation

Bases are substances that neutralise acids. The reaction produces a salt and water only.

Acid + Base $\rightarrow$ Salt + Water

Example:
Hydrochloric acid + Copper oxide $\rightarrow$ Copper(II) chloride + Water
\(\mathrm{2HCl(aq) + CuO(s) \rightarrow CuCl_2(aq) + H_2O(l)}\)

3. Reaction with Carbonates (C)

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

Acid + Carbonate $\rightarrow$ Salt + Water + Carbon Dioxide

Example:
Nitric acid + Calcium carbonate $\rightarrow$ Calcium nitrate + Water + Carbon Dioxide
\(\mathrm{2HNO_3(aq) + CaCO_3(s) \rightarrow Ca(NO_3)_2(aq) + H_2O(l) + CO_2(g)}\)

The carbon dioxide gas produced is tested by bubbling it through limewater (aqueous calcium hydroxide), which turns milky (white precipitate formed).

Quick Review: The Three Acid Products

If the acid reacts with a:
- Metal: You get Hydrogen gas.
- Base: You get Water (Neutralisation).
- Carbonate: You get Carbon Dioxide gas and Water.

Part 3: Bases and Alkalis – The \(OH^-\) Story

Bases are the chemical opposites of acids.

Core Definitions of Bases and Alkalis
  • A Base is typically a metal oxide (like CuO, CaO) or a metal hydroxide (like NaOH).
  • An Alkali is a soluble base—a base that dissolves in water.
  • Aqueous solutions of alkalis always contain hydroxide ions, \(OH^-\) (aq).

Key Difference: All alkalis are bases, but not all bases are alkalis.
Example: Sodium hydroxide (NaOH) is an alkali (soluble), but Copper(II) oxide (CuO) is a base (insoluble).

Extended Definition: Bases as Proton Acceptors (Supplement)
  • Bases (including alkalis) are defined as proton acceptors. They accept the \(H^+\) proton donated by the acid.
  • The hydroxide ion in an alkali (\(OH^-\)) is a very strong proton acceptor: \(OH^-(aq) + H^+(aq) \rightarrow H_2O(l)\).
Identifying Alkalis using Indicators (Core)
  • Litmus Paper: Turns Red $\rightarrow$ Blue in alkali.
  • Methyl Orange: Remains Yellow in alkali.
  • Thymolphthalein: Turns Colourless $\rightarrow$ Blue in alkali.
Key Takeaway for Bases and Alkalis

Bases are metal oxides/hydroxides. Alkalis are soluble bases containing \(OH^-\) ions (proton acceptors) and turn red litmus blue.

Part 4: Characteristic Reactions of Bases and Alkalis (Core)

1. Reaction with Acids (Neutralisation)

As discussed above, this is the most common reaction for both bases and alkalis:

Base/Alkali + Acid $\rightarrow$ Salt + Water

2. Reaction of Alkalis with Ammonium Salts

When an alkali (like NaOH or KOH) is warmed with an ammonium salt (like $\mathrm{NH_4Cl}$), it produces a salt, water, and ammonia gas.

Alkali + Ammonium Salt $\rightarrow$ Salt + Water + Ammonia

Example:
\(\mathrm{NaOH(aq) + NH_4Cl(s) \rightarrow NaCl(aq) + H_2O(l) + NH_3(g)}\)

This reaction is the standard chemical test for the presence of the ammonium ion (\(NH_4^+\)). Ammonia gas (\(NH_3\)) is identified because it turns damp red litmus paper blue (it is alkaline).

Part 5: Measuring Acidity and Alkalinity (Core)

To compare how strong or weak an acid or alkali is, we use the pH scale and Universal Indicator.

The pH Scale

The pH scale is a measure of the concentration of hydrogen ions (\(H^+\)) in a solution. It ranges from 0 to 14.

  • pH 0–6: Acidic (The lower the pH, the higher the concentration of \(H^+\) ions, and the stronger the acid).
  • pH 7: Neutral (Pure water).
  • pH 8–14: Alkaline (The higher the pH, the higher the concentration of \(OH^-\) ions, and the stronger the alkali).
Universal Indicator (UI)

Universal indicator is a mixture of dyes that gives a whole range of colours corresponding to different pH values. This allows us to compare relative acidity and relative alkalinity.

  • Strong Acid (pH 1-2): Red
  • Weak Acid (pH 5-6): Orange/Yellow
  • Neutral (pH 7): Green
  • Weak Alkali (pH 8-9): Blue
  • Strong Alkali (pH 13-14): Purple/Violet
The Neutralisation Reaction (Core)

Neutralisation is the reaction between an acid and an alkali (or base) that cancels out their properties. Since acids provide \(H^+\) ions and alkalis provide \(OH^-\) ions, the essential reaction in neutralisation is simply the formation of water.

Ionic Equation for Neutralisation:

\[ H^+(aq) + OH^-(aq) \rightarrow H_2O(l) \]

This equation is vital! It represents the net ionic change whenever a strong acid reacts with a strong alkali.

Part 6: Strong vs. Weak Acids (Extended/Supplement)

Not all acids (or alkalis) are created equal. We classify them based on how much they break apart (dissociate) in water.

Complete vs. Partial Dissociation

When an acid is put into water, its molecules split up into ions (dissociate).

1. Strong Acids (Completely Dissociated)
  • A strong acid is one that completely dissociates (breaks apart) in aqueous solution.
  • This means that almost all the acid molecules turn into \(H^+\) ions.
  • This high concentration of \(H^+\) ions results in a very low pH (e.g., pH 1).

Example: Hydrochloric Acid (\(HCl\))
The reaction uses a single arrow ($\rightarrow$) because the process is complete:

\[ \mathrm{HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)} \]

Other common strong acids include Sulfuric acid (\(H_2SO_4\)) and Nitric acid (\(HNO_3\)).

2. Weak Acids (Partially Dissociated)
  • A weak acid is one that is only partially dissociated in aqueous solution.
  • Only a small fraction of the molecules break up into \(H^+\) ions; most remain as intact acid molecules.
  • This low concentration of \(H^+\) ions results in a higher pH (e.g., pH 5 or 6).

Example: Ethanoic Acid (\(\mathrm{CH_3COOH}\)), found in vinegar.
The reaction uses a reversible arrow ($\rightleftharpoons$) because dissociation is incomplete:

\[ \mathrm{CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq)} \]

Analogy: Imagine dropping 100 LEGO blocks (acid molecules) into water. If it’s a strong acid, 99 blocks immediately break into ions. If it’s a weak acid, only 5 blocks break into ions; the rest stay whole.

Common Mistakes to Avoid
  • Do not confuse the terms strong/weak (which relate to dissociation) with concentrated/dilute (which relate to the amount of acid dissolved in the volume of solvent).
  • A dilute strong acid (e.g., dilute HCl) can have the same pH as a concentrated weak acid (e.g., concentrated vinegar).
Key Takeaway for Extended Concepts

Strength relates to dissociation. Strong acids ($\rightarrow$) dissociate fully and have a very low pH. Weak acids ($\rightleftharpoons$) dissociate partially and have a moderate pH.