Acids, Bases & pH
Aqueous solutions are classified as acidic, neutral, or alkaline using the pH scale from 0 to 14. In water, acids ionise to release hydrogen ions (H⁺) and alkalis release hydroxide ions (OH⁻); when an acid reacts with a base or alkali, neutralisation occurs to form a salt and water.
- Acid
- A substance that releases hydrogen ions (H+) when dissolved in water.
- Alkali
- A soluble base that produces hydroxide ions (OH-) in aqueous solution.
- Base
- A substance that can neutralise an acid to form a salt.
Acids, Bases & pH: Reaction Profile (ΔH & E_a) + Le Chatelier Yield Simulator
Toggle a catalyst on the enthalpy profile diagram or adjust temperature and pressure to see how dynamic equilibrium shifts.
Modelled on N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (ΔH = −92 kJ/mol) (4 moles gas → 2 moles gas):
Higher pressure shifts equilibrium to the side with fewer gas moles (right, 4 → 2). Lower temperature favours the exothermic forward reaction.
Key Concepts & Exam Definitions (5)
- Acid
- A substance that releases hydrogen ions (H+) when dissolved in water.
- Alkali
- A soluble base that produces hydroxide ions (OH-) in aqueous solution.
- Base
- A substance that can neutralise an acid to form a salt.
- Strong Acid
- An acid that completely ionises in an aqueous solution (e.g., hydrochloric acid).
- Weak Acid
- An acid that only partially ionises in an aqueous solution (e.g., ethanoic acid).
Quick-Fire Flashcards (5)
Click any card to check the answer, then mark it as Known to track your progress.
What colour does universal indicator turn in a strong acid?
What is the general word equation for an acid reacting with a metal oxide?
What ion makes a solution alkaline?
What is a strong acid?
What gas is produced when an acid reacts with a metal carbonate?
Exam-Style Questions & Mark Schemes (3)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Write the word equation for the reaction between hydrochloric acid and sodium hydroxide.
Click “Check Mark Scheme” to compare your answer, Hydrochloric acid + Sodium hydroxide -> Sodium chloride + Water.
- Q2
Explain the difference between a strong acid and a concentrated acid.
Click “Check Mark Scheme” to compare your answer, A strong acid fully ionises in water. A concentrated acid has a large amount of acid molecules relative to the volume of water, regardless of whether it is strong or weak.
- Q3
How can you measure the pH of a solution?
Click “Check Mark Scheme” to compare your answer, Using universal indicator or a pH probe.
Common Exam Questions (3)
What is the pH scale?
The pH scale runs from 0 to 14 and measures the concentration of H⁺ ions in an aqueous solution: pH 0 is strongly acidic, pH 7 is neutral, and pH 14 is strongly alkaline.
What defines an acid?
An acid is a substance that dissolves in water to produce aqueous hydrogen ions (H⁺), giving a solution with a pH less than 7.
What happens in a neutralisation reaction?
In a neutralisation reaction, an acid reacts with a base or alkali to form a salt and water. In terms of ions, hydrogen ions from the acid react with hydroxide ions from the alkali according to the ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l).
Need more questions on Acids, Bases & pH, or want to organise your own notes?
Type a specific exam question you are stuck on, pick one of the prompts below, or paste your class notes to build a custom set of flashcards and practice questions on this page.
Related study guides
Rates of Reaction & Catalysts
The rate of a chemical reaction measures how fast reactants are converted into products. Under collision theory, chemical reactions only occur when particles collide with energy greater than or equal to the activation energy; changing temperature, concentration, gas pressure, surface area, or adding a catalyst alters collision frequency or energy.
Moles, Relative Formula Mass & Calculations
Quantitative chemistry uses relative formula masses (Mr) and the mole concept to calculate reacting masses, gas volumes, and solution concentrations. Applying the core relationship moles = mass ÷ Mr allows you to determine limiting reactants, percentage yields, and titration concentrations.