Definition and Properties of Bases
This topic covers the definition of bases and the crucial terminology used to describe them. Understanding the difference between strength (dissociation) and concentration (moles per volume) is key for ESAT chemistry questions.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- A base can be defined in two ways: as a proton (H⁺) acceptor, or as a substance that produces hydroxide ions (OH⁻) when dissolved in water.
- Bases that are soluble in water are called alkalis. They form alkaline solutions, which are characterised by the presence of aqueous OH⁻ ions.
- The strength of a base (strong vs weak) describes how completely it dissociates into ions in solution. This is an intrinsic property of the substance.
- The concentration of a base (concentrated vs dilute) describes how much of the substance is dissolved in a given volume of solvent. This is a variable property of the solution.
- It is possible to have a dilute solution of a strong base (e.g., 0.1 M NaOH) or a concentrated solution of a weak base (e.g., 10 M NH₃).
- Many metal oxides and hydroxides from Group 1 and 2 (e.g., Na₂O, Ca(OH)₂) react with or dissolve in water to form strongly alkaline solutions.
Formulae
NaOH(aq) → Na+(aq) + OH-(aq) To represent the complete dissociation of a strong base like sodium hydroxide in water.
NH3(aq) + H2O(l) ≤> NH4+(aq) + OH-(aq) To show the partial dissociation (equilibrium) of a weak base like ammonia in water. Note ammonia accepts a proton from water to form OH⁻.
CaO(s) + H2O(l) → Ca(OH)2(aq) To illustrate a basic metal oxide reacting with water to form a metal hydroxide, which then creates an alkaline solution.
Definitions
- Base
- A substance that accepts a proton (H⁺) or produces hydroxide ions (OH⁻) in an aqueous solution.
- Alkali
- A type of base that is soluble in water, forming an alkaline solution.
- Strong Base
- A base that fully ionises or dissociates in aqueous solution. The reaction is shown with a one-way arrow (→).
- Weak Base
- A base that only partially ionises in aqueous solution. The reaction is a reversible equilibrium, shown with a two-way arrow (⇌).
- Concentrated
- A relative term for a solution containing a large amount of solute per unit volume.
- Dilute
- A relative term for a solution containing a small amount of solute per unit volume.
Worked example
A chemist prepares four aqueous solutions. Which of the following correctly describes a concentrated solution of a weak base?
- 1
Analyse the request:
We need a solution that is both 'concentrated' and contains a 'weak base'.
- 2
Define the terms:
'Concentrated' implies a high molarity (e.g., > 1 mol/dm³).
'Weak base' implies a substance that only partially dissociates, such as ammonia (NH₃).
Strong bases include Group 1 hydroxides like NaOH and KOH.
- 3
Evaluate Option A (0.05 mol/dm³ NaOH):
This is a strong base, and it is dilute.
- 4
Evaluate Option B (8.0 mol/dm³ NH₃):
Ammonia (NH₃) is a classic weak base.
A concentration of 8.0 mol/dm³ is high, so this is a concentrated solution.
- 5
Evaluate Option C (8.0 mol/dm³ KOH):
Potassium hydroxide (KOH) is a strong base.
This solution is concentrated, but the base is not weak.
- 6
Evaluate Option D (0.05 mol/dm³ NH₃):
This is a weak base, but the solution is dilute.
- 7
Conclusion:
Option B is the only one that meets both criteria:
a concentrated solution of a weak base.
Answer: B) 8.0 mol/dm³ NH₃(aq)
Common mistakes
- ×Confusing strength with concentration. Remember that a strong base is always strong regardless of dilution, and a weak base is always weak even when concentrated.
- ×Assuming all metal hydroxides are alkalis. Some, like copper(II) hydroxide (Cu(OH)₂), are bases but are insoluble in water, so they do not form alkaline solutions.
- ×Forgetting that ammonia (NH₃) is the most common example of a weak base you will encounter. It acts as a base by accepting a proton from water.
No-calculator tips
- ✓Focus on the definitions. Questions on this topic test your understanding of the terms 'strong', 'weak', 'dilute', and 'concentrated', not your ability to calculate pH.
- ✓Memorise key examples: NaOH and KOH are your go-to strong bases. NH₃ is your key weak base.
- ✓For concentration, think relatively. Without a calculator, you won't be asked to decide if 1.5 M is dilute or concentrated in isolation. You will compare it to other values, like 0.1 M (clearly more dilute) or 10 M (clearly more concentrated).