The Strong Bases: A Complete Guide

Ask chemists to name a strong base and almost all of them will say sodium hydroxide. It’s the benchmark: cheap, soluble, fully dissociated and aggressive enough to dissolve grease, hair and aluminium foil. But it has company. The list of strong bases is short and follows a very clear pattern on the periodic table, which makes it easy to learn.

What makes a base “strong”

A strong base dissociates completely in water, releasing all of its hydroxide ions:

NaOH(s) → Na⁺(aq) + OH⁻(aq)

There’s no equilibrium: in dilute solution, there are no intact NaOH units left. A weak base, such as ammonia, only partly reacts with water, so most of it stays as neutral molecules. (See what is a base? for the general idea, and concentrated vs strong for why “strong” is not the same as “concentrated”.)

The list

Group 1 hydroxides (all strong, all soluble)

Base	Formula	Solubility in water at 20 °C	Notes
Lithium hydroxide	LiOH	about 128 g/dm³	Least soluble of Group 1; used in CO₂ scrubbers and lithium greases
Sodium hydroxide	NaOH	about 1100 g/dm³	“Caustic soda”; the most important industrial base
Potassium hydroxide	KOH	about 1100 g/dm³	“Caustic potash”; used in alkaline batteries and soft soaps
Rubidium hydroxide	RbOH	very high	Rarely used outside research
Caesium hydroxide	CsOH	very high	Among the strongest hydroxide bases; attacks glass

Group 2 hydroxides (strong, but limited by solubility)

Base	Formula	Solubility at 20 °C	Saturated solution pH (approx.)
Calcium hydroxide	Ca(OH)₂	about 1.7 g/dm³	12.4
Strontium hydroxide	Sr(OH)₂	several g/dm³ (sources vary)	about 13
Barium hydroxide	Ba(OH)₂	about 39 g/dm³ (as the octahydrate)	about 13.4

Each Group 2 hydroxide releases two hydroxide ions per formula unit:

Ba(OH)₂ → Ba²⁺ + 2OH⁻

So 0.010 mol/dm³ Ba(OH)₂ gives 0.020 mol/dm³ OH⁻. Forgetting to double is a classic exam error.

The calcium hydroxide question

Is calcium hydroxide strong or weak? Students often think it’s weak because its solution (limewater) isn’t very alkaline compared with sodium hydroxide. But the reason is solubility, not strength. Only about 1.7 g dissolves per dm³ at room temperature, around 0.02 mol/dm³. Every bit that does dissolve is completely dissociated. So calcium hydroxide is a strong base with low solubility.

Its solubility also has an unusual quirk: it decreases as temperature rises. Hot limewater holds less calcium hydroxide than cold. Most solids become more soluble when heated, so this is worth remembering if you ever prepare limewater: make it with cold water and filter it before use.

What about magnesium and beryllium hydroxide?

• Magnesium hydroxide, Mg(OH)₂, is so insoluble (around 0.01 g/dm³) that its saturated solution only reaches about pH 10.4. Whether it’s classed as strong depends on the textbook: what dissolves is largely dissociated, but because so little dissolves, it’s usually treated as a weak base in practice. That gentleness is exactly why it’s used as an antacid (milk of magnesia).
• Beryllium hydroxide, Be(OH)₂, is insoluble and amphoteric. It reacts with strong alkalis as well as acids, so it’s not a strong base at all. See amphoteric substances.

The periodic trend

Base strength and solubility of metal hydroxides both increase down Groups 1 and 2. As the metal ion gets bigger and its charge is spread over a larger volume, it holds hydroxide less tightly, and the ions separate more easily in water. Group 1 ions have a single positive charge, so they hold hydroxide less tightly than Group 2 ions of similar size, which is why every Group 1 hydroxide is strong and soluble.

Transition metal hydroxides like Cu(OH)₂ and Fe(OH)₃ are insoluble and are weak bases (and some are amphoteric). They form as coloured precipitates when you add sodium hydroxide to solutions of their salts, a standard test for identifying metal ions.

Bases stronger than hydroxide

Hydroxide is the strongest base that can exist in water, for the same reason H₃O⁺ is the strongest acid: water levels anything stronger. If you add a base stronger than OH⁻ to water, it simply takes a proton from water and makes OH⁻.

Some compounds are far stronger bases than hydroxide. They’re used in organic synthesis in dry, non-aqueous solvents:

Base	Formula	What happens in water
Sodium amide	NaNH₂	NH₂⁻ + H₂O → NH₃ + OH⁻
Sodium hydride	NaH	H⁻ + H₂O → H₂ + OH⁻ (vigorous, hydrogen gas released)
Butyllithium	C₄H₉Li	Reacts violently, releasing butane
Oxide ion (e.g., in Na₂O)	O²⁻	O²⁻ + H₂O → 2OH⁻

Metal oxides such as sodium oxide and calcium oxide (quicklime) behave the same way: they react with water to make hydroxides, which is why quicklime gets hot when wetted.

Where strong bases are used

• Soap making: sodium hydroxide (hard soaps) and potassium hydroxide (soft and liquid soaps) turn fats into soap.
• Paper and textiles: sodium hydroxide breaks down lignin in wood pulp and treats cotton.
• Aluminium production: hot sodium hydroxide dissolves aluminium oxide from bauxite in the Bayer process.
• Drain and oven cleaners: sodium hydroxide dissolves grease and hair by hydrolysing fats and proteins.
• Batteries: potassium hydroxide is the electrolyte in alkaline batteries.
• Construction and agriculture: calcium hydroxide in mortar, plaster and for liming acidic soils.
• Carbon dioxide removal: lithium hydroxide absorbs CO₂ in spacecraft and submarines, chosen because it’s the lightest.
• Chemistry labs: barium hydroxide solution absorbs CO₂ and is used in some titrations.

Safety

Strong bases are at least as dangerous as strong acids, and often more so for eyes. They react with the fats in cell membranes, turning them into soap and allowing the alkali to penetrate deeper. An alkali splash in the eye is a medical emergency: rinse with running water for at least 15 minutes and get help.

Dissolving solid sodium or potassium hydroxide releases a lot of heat. Always add the solid slowly to cold water, stirring, never the other way round, and wear eye protection and gloves. Barium compounds that dissolve are also toxic.

Quick practice

1. Calculate the pH of 0.020 mol/dm³ KOH.
2. Calculate the pH of 0.0050 mol/dm³ Ba(OH)₂.
3. Explain why limewater has a lower pH than 0.1 mol/dm³ NaOH, even though Ca(OH)₂ is a strong base.

Answers: (1) pOH = 1.70, pH = 12.30. (2) [OH⁻] = 0.010, pOH = 2.00, pH = 12.00. (3) Calcium hydroxide has low solubility, so saturated limewater only contains about 0.04 mol/dm³ OH⁻ at most; the NaOH solution has 0.1 mol/dm³. Strength is the same (complete dissociation); concentration is different.

Key takeaways

• The strong bases are the Group 1 hydroxides (LiOH to CsOH) and the heavier Group 2 hydroxides (Ca, Sr, Ba).
• Group 2 hydroxides release two OH⁻ each; calcium hydroxide is strong but not very soluble.
• Mg(OH)₂ is usually treated as weak in practice because it’s nearly insoluble; Be(OH)₂ is amphoteric.
• Hydroxide is the strongest base possible in water; stronger bases like NaNH₂ and NaH react with water to form OH⁻.
• Work out pH from concentration using pOH and pH or the pH calculator.