RubricMark · Study Lab

Acids, bases and pH: strong and weak acids explained

Distinguish acid strength from concentration, explore the pH scale and practise neutralisation equations with worked chemistry examples.

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Before you start

This lesson is designed for students in Years 9–11 studying chemical reactions. We recommend setting aside approximately 25 minutes of uninterrupted study time to complete this guide. Before you begin, you should be familiar with basic atomic structure (specifically protons and electrons), the concept of ions, chemical formulas, and how to balance simple chemical equations. You will need a pen, a notebook, and a scientific calculator to work through the calculations.

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What you'll learn

  • Distinguish between strong and weak acids based on their percent dissociation in aqueous solutions.
  • Explain the logarithmic nature of the pH scale and calculate how hydrogen ion concentration changes during dilution.
  • Predict the products of acid-metal and acid-base neutralisation reactions, including writing balanced molecular and net ionic equations.
  • Interpret indicator color changes using a universal indicator color chart pH reference to determine solution acidity.

Core concepts

Acids, Bases, and Alkalis

An acid is a substance that releases hydrogen ions (H+) when dissolved in water. A base is a substance that can neutralise an acid to form a salt and water. If a base can dissolve in water, we call it an alkali. Therefore, all alkalis are bases, but not all bases are alkalis. Soluble alkalis release hydroxide ions (OH-) in aqueous solutions.

Strong vs Weak Acids Difference

The strength of an acid refers to how easily it splits apart into ions when mixed with water. This process is called dissociation. The strong vs weak acids difference lies entirely in this dissociation percentage:

  • Strong Acids: Dissociate completely (100% dissociation) in water. For example, hydrochloric acid (HCl) splits entirely into hydrogen ions and chloride ions: HCl(aq) → H+(aq) + Cl-(aq).
  • Weak Acids: Dissociate only partially (typically 1% to 5% dissociation) in water. For example, ethanoic acid (CH3COOH), found in vinegar, exists mostly as intact molecules, with only a tiny fraction releasing H+ ions: CH3COOH(aq) ↔ CH3COO-(aq) + H+(aq).

It is crucial not to confuse acid strength with concentration. Concentration refers to how many moles of acid are dissolved per litre of solution, whereas strength refers to the proportion of those molecules that have dissociated into ions.

The pH Scale Logarithmic Explanation

The pH scale is a measure of the concentration of hydrogen ions [H+] in a solution. The "p" stands for potential and "H" stands for hydrogen. The pH scale logarithmic explanation reveals that pH is not a linear scale. Every single unit shift on the pH scale represents a 10-fold (factor of 10) change in hydrogen ion concentration.

For example, a solution with a pH of 3 has 10 times more H+ ions than a solution with a pH of 4, and 100 times (10 x 10) more H+ ions than a solution with a pH of 5. If you dilute an acid to reduce its H+ concentration by a factor of 10, the pH will increase by exactly 1 unit.

Neutralisation Reaction Ionic Equation

When an acid reacts with a base, they undergo a neutralisation reaction. The general word equation is: Acid + Base → Salt + Water. To write a neutralisation reaction ionic equation, we focus only on the species that actually change state or bond during the reaction. Ions that do not participate are called spectator ions.

For the reaction between hydrochloric acid and sodium hydroxide: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l). Written as separate ions: H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) → Na+(aq) + Cl-(aq) + H2O(l). By crossing out the spectator ions (Na+ and Cl-), we get the net ionic equation for any strong acid-strong base neutralisation: H+(aq) + OH-(aq) → H2O(l).

Acid-Metal Reactions and Exceptions

Acids react with reactive metals to produce a salt and hydrogen gas: Acid + Metal → Salt + Hydrogen gas. However, there are important acid metal reaction exceptions. Metals that are less reactive than hydrogen in the reactivity series—such as copper, silver, and gold—will not react with dilute acids like hydrochloric acid because they cannot displace the hydrogen ions.

To understand how these concepts fit into your broader science curriculum, you can review the Australian Curriculum Science standards which outline the expectations for chemical reactions at this level.

Worked examples

Example 1: The Dilution Dilemma

A student dilutes 10 mL of a 0.1 M HCl solution (which has a pH of 1) by adding 990 mL of distilled water. Calculate the final volume, the dilution factor, and the new pH of the solution.

  1. Find the final volume: Add the initial volume of the acid to the volume of water added. Final Volume = 10 mL + 990 mL = 1000 mL.
  2. Calculate the dilution factor: Divide the final volume by the initial volume. Dilution Factor = 1000 mL / 10 mL = 100. The solution has been diluted by a factor of 100.
  3. Determine the shift in pH: Since the pH scale is logarithmic, a dilution factor of 100 (which is 10 to the power of 2) reduces the hydrogen ion concentration by 100 times. This causes the pH to increase by 2 units.
  4. Calculate the final pH: New pH = Initial pH + 2 = 1 + 2 = 3. The final pH of the diluted solution is 3.

Example 2: Predicting the Unreactive

Write the expected chemical equations when dilute hydrochloric acid (HCl) is added to (a) magnesium ribbon and (b) copper turnings. Explain why one reaction proceeds vigorously while the other shows no change.

  1. Analyze magnesium: Magnesium is high in the reactivity series (above hydrogen). It will displace hydrogen from the acid. The balanced equation is: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g). This reaction proceeds vigorously, producing bubbles of hydrogen gas.
  2. Analyze copper: Copper is below hydrogen in the reactivity series. It is an exception to the general metal-acid reaction rule. The equation is: Cu(s) + HCl(aq) → No Reaction.
  3. Explain the difference: Magnesium readily loses electrons to hydrogen ions, reducing them to hydrogen gas. Copper is less reactive than hydrogen and cannot transfer electrons to hydrogen ions, resulting in no chemical change.

In tests and exams

  • Classifying chemical species: You will often be asked to identify strong vs weak acids or bases from a list based on their dissociation percentages. Remember that strong acids dissociate 100% while weak acids dissociate only 1-5%.
  • pH scale calculations and dilution: Examiners frequently test your understanding of the logarithmic scale by asking you to calculate the new pH after a 10-fold, 100-fold, or 1000-fold dilution.
  • Predicting reaction products: Be prepared to write balanced molecular and net ionic equations. Always check if the metal provided is unreactive (like copper) before writing a product!

Quick recap

  • Strong acids dissociate 100% in water, while weak acids dissociate only partially (1-5%).
  • The pH scale is logarithmic; each change of 1 pH unit represents a 10-fold change in hydrogen ion concentration.
  • Diluting an acid by a factor of 10 increases its pH by 1 unit.
  • Neutralisation reactions between strong acids and strong bases have the net ionic equation: H+(aq) + OH-(aq) → H2O(l).
  • Unreactive metals below hydrogen in the reactivity series (like copper) do not react with dilute acids.

Practice

  1. Warm-up 1: Identify whether the following acid is strong or weak, and state its approximate dissociation percentage in water: Ethanoic acid (CH3COOH).
  2. Warm-up 2: A solution of hydrochloric acid has a pH of 2. If you dilute this solution by a factor of 10, what will be the new pH?
  3. Standard 1: Write the balanced molecular equation and the net ionic equation for the reaction between dilute sulfuric acid (H2SO4) and sodium hydroxide (NaOH). Include state symbols.
  4. Standard 2: Explain why a 0.1 M solution of hydrochloric acid (HCl) conducts electricity much better than a 0.1 M solution of ethanoic acid (CH3COOH).
  5. Challenge: You are given three unlabelled beakers containing copper, zinc, and calcium carbonate. Explain how you could use dilute hydrochloric acid and a universal indicator color chart pH guide to safely identify the contents of each beaker. Describe the observations you would make.

Answers

  1. Warm-up 1: Ethanoic acid is a weak acid. It dissociates only partially in water, with a dissociation percentage of approximately 1% to 5%.
  2. Warm-up 2: Diluting the acid by a factor of 10 decreases the hydrogen ion concentration by 10 times. On the logarithmic pH scale, a 10-fold decrease in H+ concentration increases the pH by exactly 1 unit. New pH = 2 + 1 = 3.
  3. Standard 1: The balanced molecular equation is: H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l). The net ionic equation, after removing the spectator sodium (Na+) and sulfate (SO4^2-) ions, is: H+(aq) + OH-(aq) → H2O(l).
  4. Standard 2: Electrical conductivity depends on the concentration of free-moving ions. Hydrochloric acid is a strong acid and dissociates 100% into H+ and Cl- ions, providing a high concentration of mobile ions. Ethanoic acid is a weak acid and only dissociates about 5%, leaving most molecules neutral and uncharged, resulting in fewer mobile ions to conduct electricity.
  5. Challenge: Add dilute hydrochloric acid to a small sample of each substance:
    • The beaker containing copper will show no reaction (no bubbles or temperature change) because copper is below hydrogen in the reactivity series.
    • The beaker containing zinc will react to produce hydrogen gas, visible as moderate bubbling: Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g).
    • The beaker containing calcium carbonate will fizz rapidly, producing carbon dioxide gas: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l). To confirm, test the resulting solution with universal indicator; the neutralised salt solution will shift the indicator color toward green (neutral pH ~7), whereas the unreacted acid remains red.

Common mistakes

  • Equating 'strong' with 'concentrated': Students often think a strong acid is always dangerous and concentrated. Fix: Remember that 'strong' refers to 100% dissociation, while 'concentrated' refers to having a large amount of solute per volume. You can have a highly dilute solution of a strong acid.
  • Assuming all metals react with acids: Students often write equations for copper reacting with acid. Fix: Check the reactivity series. Unreactive metals like copper, silver, and gold do not react with dilute acids.
  • Treating the pH scale as linear: Believing that a change from pH 2 to pH 4 means the acid is 'twice' as weak. Fix: Remember the pH scale is logarithmic. A change of 2 pH units represents a 100-fold (10^2) decrease in hydrogen ion concentration.

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