What Are Bases?
In everyday life, you encounter bases more often than you might think — from the baking soda in your kitchen to the soap in your bathroom, and even the antacid tablets used to soothe an upset stomach. Bases are a fundamental class of chemical substances with distinctive properties that set them apart from acids.
A base is a substance that accepts protons (H⁺ ions) or produces hydroxide ions (OH⁻) when dissolved in water. Bases have a pH greater than 7.
An alkali is a base that dissolves in water to produce hydroxide ions (OH⁻) in solution. All alkalis are bases, but not all bases are alkalis — some bases (like copper oxide) do not dissolve in water.
Bases are often described as the chemical 'opposites' of acids. While acids donate protons and have a pH below 7, bases accept protons and have a pH above 7. This complementary relationship is central to understanding neutralisation reactions, which you will explore throughout this unit.
The Brønsted-Lowry Model
The modern way of thinking about acids and bases is called the Brønsted-Lowry model, named after the two chemists who developed it in 1923. In this model:
- An acid is a proton (H⁺) donor
- A base is a proton (H⁺) acceptor
This is more powerful than simply saying bases produce OH⁻ ions (the older Arrhenius definition), because it explains why substances like ammonia (NH₃) are bases even though they contain no OH⁻ — they accept a proton from water.
Think of acids and bases like keys and locks — acids donate a proton (H⁺), and bases are perfectly shaped to accept it. Just as a key fits a lock, a base 'fits' an acid's donated proton, forming water and a salt in the process.
Common examples of bases include:
- Sodium hydroxide (NaOH) — used in drain cleaners
- Calcium hydroxide (Ca(OH)₂) — used in cement and agriculture (lime)
- Magnesium hydroxide (Mg(OH)₂) — the active ingredient in milk of magnesia antacids
- Ammonia (NH₃) — used in cleaning products
- Potassium hydroxide (KOH) — used in soap-making
Physical Properties of Bases
Bases have a distinctive set of physical properties — properties that describe how a substance looks, feels, or behaves without changing its chemical identity.
1. Texture and Feel
Bases feel characteristically slippery or soapy to the touch. This is because hydroxide ions (OH⁻) hydrolyse the ester bonds in the lipids (fats and oils) present in the outer layer of your skin, releasing glycerol and fatty acid salts. These fatty acid salts are effectively soaps — which is why the sensation feels slippery. The OH⁻ ions also break down surface proteins through hydrolysis.
Never touch concentrated bases like sodium hydroxide with bare skin to 'test' their slipperiness. Concentrated alkalis are corrosive and can cause severe chemical burns. The soapy feel is only used as a descriptive property, not a safe identification test in the lab.
2. State at Room Temperature
Bases can exist as:
- Solids — e.g. sodium hydroxide pellets, calcium hydroxide powder
- Liquids — e.g. ammonia solution, sodium hydroxide solution
- Gases — e.g. ammonia gas (NH₃)
3. Colour and Appearance
Most pure bases are white or colourless solids or solutions. Some metal hydroxides, however, can be coloured — for example, copper(II) hydroxide is blue, and iron(III) hydroxide is orange-brown.
4. Solubility
Not all bases dissolve in water. Group 1 metal hydroxides (like NaOH and KOH) and ammonia are soluble in water, making them alkalis. Many other metal hydroxides (like Cu(OH)₂ and Fe(OH)₃) are insoluble or only slightly soluble.
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