Bases and Alkalis Around Us

Introduction

Acids are only half of the story. The soap you wash your hands with, the baking soda that makes a cake rise and the tablet you might take for indigestion all belong to a second family of substances: bases . Bases are the chemical opposites of acids. When the two meet, they cancel each other out. This page introduces the bases and alkalis you meet every day, the properties they share, and the important difference between the words base and alkali .

Core explanation

What is a base? A base is a substance that reacts with an acid and removes its acidity. This cancelling reaction is called neutralisation , and you will study it in detail later in this unit. Many metal oxides, metal hydroxides and metal carbonates are bases.

What is an alkali? Some bases dissolve in water and some do not. A base that dissolves in water is called an alkali . So every alkali is a base, but not every base is an alkali. For example, copper(II) oxide is a base because it reacts with acids, but it does not dissolve in water, so it is not an alkali. Sodium hydroxide dissolves easily, so it is both a base and an alkali.

Bases and alkalis in the home.

Product Base or alkali it contains --- --- Soap and washing powder Sodium or potassium salts that give alkaline solutions Baking soda Sodium hydrogencarbonate Indigestion tablets Magnesium hydroxide, calcium carbonate Glass and window cleaners Ammonia solution Oven and drain cleaners Sodium hydroxide Garden lime Calcium hydroxide or calcium carbonate Toothpaste Mild bases such as calcium carbonate

Shared properties of alkalis. Solutions of alkalis tend to:

- taste bitter (in foods such as tonic water or cocoa; we never taste unknown chemicals); - feel soapy or slippery — partly because strong alkalis react with the oils in your skin and begin to turn them into soap; - turn red litmus blue ; - have a pH above 7 ; - neutralise acids.

Mild and dangerous. Just as with acids, there is a huge range of strength. Baking soda is gentle enough to eat in cakes. Sodium hydroxide in oven cleaner is caustic : it can burn skin and cause blindness if splashed into the eyes. Strong alkalis are often more damaging to eyes than acids of similar strength, because they keep attacking tissue instead of stopping at the surface.

Step-by-step reasoning

To decide whether a household substance is a base, an alkali, or neither:

1. Ask whether it reacts with an acid and cancels its acidity. If yes, it is a base. 2. Ask whether it dissolves in water. If the base dissolves, it is also an alkali. 3. If you have its solution, check it with red litmus: a change to blue confirms an alkaline solution. 4. Use the label and hazard symbols to judge how strong and dangerous it is.

Visual explanation

Picture two overlapping circles as a nesting diagram: a large circle labelled "bases" and a smaller circle inside it labelled "alkalis". Copper(II) oxide sits in the large circle but outside the small one. Sodium hydroxide and ammonia solution sit inside the small circle, because they are bases that dissolve in water.

Real-world analogy

Think of "base" as the word fruit and "alkali" as the word citrus fruit . Every citrus fruit is a fruit, but an apple is a fruit without being a citrus fruit. In the same way, every alkali is a base, but insoluble bases are not alkalis.

Real-world example

Baking soda helps cakes rise. When sodium hydrogencarbonate meets an acid in the mixture, such as the acid in buttermilk or in baking powder, it reacts to release bubbles of carbon dioxide gas. The bubbles get trapped in the batter and make the cake light and airy.

Why?

Why do alkalis feel slippery? Strong alkalis react with the fats and oils in the outer layer of your skin, turning them into soap-like substances. The slippery feeling is partly your own skin being chemically changed, which is exactly why you must never touch strong alkalis.

Common misconception

"Bases and alkalis are safe because they are the opposite of acids." This is false. Strong alkalis such as sodium hydroxide are just as corrosive as strong acids, and they are especially dangerous to the eyes.

Worked example

Question: Magnesium oxide reacts with hydrochloric acid, but only a tiny amount dissolves in water. Sodium hydroxide reacts with acids and dissolves easily. Classify each substance.

Reasoning: Both react with an acid, so both are bases. Only sodium hydroxide dissolves well in water, so only it counts as an alkali.

Answer: Magnesium oxide is a base (it is usually not classed as an alkali); sodium hydroxide is a base and an alkali.

Quick check

1. Why is every alkali a base, but not every base an alkali? Answer: An alkali is simply a base that dissolves in water; bases that do not dissolve are still bases but not alkalis.

Exam focus

Be able to define base and alkali precisely and give examples of each. Learn the properties of alkalis: bitter taste, soapy feel, red litmus turns blue, pH above 7. State that alkalis are soluble bases.

Advanced insight

At a deeper level, alkalis produce hydroxide ions (OH⁻) when they dissolve in water. Ammonia is an interesting case: it contains no hydroxide itself, but it reacts with water to form a small amount of hydroxide ions, making ammonia solution a weak alkali.

Summary

Bases are substances that react with acids and cancel their acidity. Alkalis are bases that dissolve in water. Common examples include soap, baking soda, antacids, ammonia cleaners and oven cleaners. Alkaline solutions taste bitter, feel soapy, turn red litmus blue and have a pH above 7. Strong alkalis are caustic and must be handled with care.

Practice questions

1. Define the term alkali. Answer: An alkali is a base that dissolves in water, forming a solution with a pH above 7. 2. Name two household products that contain a base and name the base in each. Answer: For example, indigestion tablets contain magnesium hydroxide, and baking soda is sodium hydrogencarbonate. 3. What colour change would a solution of sodium hydroxide cause in red litmus paper? Answer: The red litmus paper would turn blue. 4. Explain why strong alkalis feel slippery on the skin and why this is a warning sign. Answer: They react with fats and oils in the skin, turning them into soap-like substances; this means the skin is being damaged.