What Makes a Substance Alkaline?

Introduction

If hydrogen ions make a solution acidic, what makes a solution alkaline? Soap, oven cleaner, limewater and household ammonia feel slippery, turn red litmus blue and have a pH above 7. Once again, one shared particle is responsible: the hydroxide ion , OH⁻. This page explains how alkalis produce hydroxide ions in water and why every alkali is a base but not every base is an alkali — a distinction that trips up many students.

Core explanation

Bases. A base is any substance that can react with an acid and cancel out, or neutralise , its acidity. Bases include metal oxides such as copper(II) oxide (CuO) and magnesium oxide (MgO), metal hydroxides such as sodium hydroxide (NaOH) and calcium hydroxide (Ca(OH)₂), metal carbonates such as calcium carbonate (CaCO₃), and ammonia (NH₃).

Alkalis are soluble bases. Some bases dissolve in water and some do not. A base that dissolves in water is called an alkali . Sodium hydroxide, potassium hydroxide and ammonia are alkalis. Calcium hydroxide is only slightly soluble, but its solution, limewater, is still alkaline. Copper(II) oxide and iron(III) oxide do not dissolve, so although they are bases, they are not alkalis.

Alkalis release OH⁻ in water. When a metal hydroxide dissolves, it separates into a metal ion and hydroxide ions:

NaOH(aq) → Na⁺(aq) + OH⁻(aq)

The hydroxide ion, OH⁻ , is made of one oxygen atom and one hydrogen atom and carries a single negative charge. Ammonia is a little different: it has no hydroxide in its formula, but when it dissolves it takes a hydrogen ion from some water molecules and so produces hydroxide ions:

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

Either way, the solution ends up containing OH⁻, and that is what makes it alkaline.

More OH⁻ means higher pH. The more hydroxide ions in a given volume of solution, the more alkaline it is and the higher its pH. A dilute solution of sodium hydroxide might have pH 12–13, while a solution of baking soda, which produces far fewer hydroxide ions, is only about pH 8–9.

Water itself contains both ions. Even pure water contains a tiny, equal number of H⁺ and OH⁻ ions. When the two are balanced, the solution is neutral. Add an acid and H⁺ is in excess; add an alkali and OH⁻ is in excess. The pH scale is really a measure of which ion is winning and by how much.

Properties of alkaline solutions. Alkalis turn red litmus blue, turn phenolphthalein pink, feel soapy because they react with the oils in skin, and neutralise acids. Concentrated alkalis are corrosive and can damage the eyes even faster than many acids.

Step-by-step reasoning

To decide whether a substance is an alkali:

1. Ask whether it is a base — does it neutralise acids? 2. Ask whether it dissolves in water. 3. If it is a base and soluble, it is an alkali. 4. In water it produces OH⁻ ions, giving a pH above 7. 5. More OH⁻ in the same volume means a higher pH.

Visual explanation

Draw a large circle labelled "bases". Inside it, draw a smaller circle labelled "alkalis". Sodium hydroxide, potassium hydroxide and ammonia go in the small circle. Copper(II) oxide and iron(III) oxide go in the large circle but outside the small one. The picture shows at a glance that all alkalis are bases, but only some bases are alkalis.

Real-world analogy

Bases and alkalis are like athletes and swimmers. Every swimmer is an athlete, but not every athlete can swim — some only run or jump. An alkali is a base that can also "swim", meaning it dissolves in water. Insoluble bases such as copper(II) oxide are still bases, but they are not alkalis.

Real-world example

Farmers sometimes spread slaked lime, which is calcium hydroxide, to reduce soil acidity. The calcium hydroxide dissolves a little in the soil water, releasing hydroxide ions that neutralise hydrogen ions in the soil and raise its pH to a level where crops grow well.

Why?

Why do soluble bases matter so much? A base can only produce hydroxide ions in solution if it dissolves. Insoluble bases can still neutralise acids, but only where the solid touches the acid. Alkalis spread OH⁻ throughout the whole solution, so they react quickly and change indicator colours.

Common misconception

"Base and alkali mean exactly the same thing." They do not. An alkali is a special kind of base — one that dissolves in water to give hydroxide ions. Copper(II) oxide is a base but not an alkali.

Worked example

Question: Potassium hydroxide, KOH, dissolves in water. Name the ions formed and predict the colour of red litmus paper dipped into the solution.

Reasoning: Like sodium hydroxide, KOH separates into a metal ion, K⁺, and a hydroxide ion, OH⁻. Hydroxide ions make the solution alkaline, and alkalis turn red litmus blue.

Answer: Potassium ions (K⁺) and hydroxide ions (OH⁻); the red litmus turns blue.

Quick check

1. Which ion is present in excess in every alkaline solution? Answer: The hydroxide ion, OH⁻.

Exam focus

Learn two linked definitions: a base neutralises an acid; an alkali is a base that dissolves in water to produce hydroxide ions, OH⁻. Be ready to give an example of a base that is not an alkali, such as copper(II) oxide, and to state that higher OH⁻ concentration means higher pH.

Advanced insight

In pure water at 25 °C, the concentrations of H⁺ and OH⁻ are each 1 × 10⁻⁷ mol/dm³, which is where pH 7 comes from. Their product stays constant, so whenever OH⁻ rises, H⁺ falls. An alkaline solution still contains some hydrogen ions — just far fewer than hydroxide ions.

Summary

A base is a substance that neutralises an acid. An alkali is a base that dissolves in water, producing hydroxide ions, OH⁻. Metal hydroxides release OH⁻ directly; ammonia produces it by reacting with water. The more hydroxide ions present, the higher the pH. All alkalis are bases, but insoluble bases such as copper(II) oxide are not alkalis.

Practice questions

1. Define an alkali. Answer: An alkali is a base that dissolves in water to form a solution containing hydroxide ions, OH⁻. 2. Name one base that is not an alkali and explain why. Answer: Copper(II) oxide; it is a base because it neutralises acids, but it is insoluble in water, so it is not an alkali. 3. Write the ions formed when sodium hydroxide dissolves in water. Answer: Sodium ions, Na⁺, and hydroxide ions, OH⁻. 4. Two alkaline solutions have pH 9 and pH 12. Which contains more hydroxide ions per litre? Answer: The solution with pH 12, because a higher pH means a higher concentration of hydroxide ions.