Introduction to Acids and Bases
2. What Are Bases and Alkalis?
Learning outcomes
- I can define a base and an alkali.
- I can distinguish between bases and alkalis.
- I can identify common bases used in everyday life.
- I can describe the properties of bases.
- I can explain how bases differ from acids.
What Are Bases and Alkalis?
Bases are an important group of substances in chemistry. They are found in cleaning products, medicines, construction materials, agriculture, and many other everyday applications.
Bases are often discussed alongside acids because acids and bases have opposite chemical properties and can react with each other in a process called neutralisation.
What Is a Base?
A base is a substance that can neutralise an acid.
When a base reacts with an acid, the products usually include a salt and water.
For example:
acid + base → salt + water
Hydrochloric acid reacting with magnesium oxide:
2HCl + MgO → MgCl₂ + H₂O
Here, magnesium oxide is the base because it neutralises the hydrochloric acid.
Common bases include:
- magnesium oxide (MgO)
- copper(II) oxide (CuO)
- calcium oxide (CaO)
- sodium hydroxide (NaOH)
- potassium hydroxide (KOH)
Some bases dissolve in water, while others do not.
What Is an Alkali?
An alkali is a base that dissolves in water.
When an alkali dissolves in water, it produces hydroxide ions (OH⁻).
For example, sodium hydroxide dissolves readily in water:
NaOH → Na⁺ + OH⁻
The presence of OH⁻ ions gives alkaline solutions many of their characteristic properties.
Examples of alkalis include:
| Alkali | Formula | Common use |
|---|---|---|
| Sodium hydroxide | NaOH | Drain and oven cleaners |
| Potassium hydroxide | KOH | Making some soaps |
| Calcium hydroxide | Ca(OH)₂ | Treating acidic soils |
| Ammonia solution | NH₃(aq) | Household cleaning products |
Bases vs. Alkalis
The terms base and alkali do not mean exactly the same thing.
The key difference is solubility in water.
| Base | Alkali |
|---|---|
| Neutralises acids | Neutralises acids |
| May be soluble or insoluble in water | Soluble in water |
| Does not necessarily produce OH⁻ in solution | Produces OH⁻ ions in water |
| Examples include CuO and MgO | Examples include NaOH and KOH |
A useful rule to remember is:
All alkalis are bases, but not all bases are alkalis.
For example, copper(II) oxide is a base, but it is not an alkali because it does not dissolve in water.
Sodium hydroxide is both a base and an alkali because it neutralises acids and dissolves in water.
Properties of Bases
Bases and alkaline solutions have several characteristic properties.
1. They Neutralise Acids
Bases react with acids to form salts.
For many bases:
acid + base → salt + water
This reaction is called neutralisation.
2. Alkalis Have a pH Above 7
The pH scale measures how acidic or alkaline a solution is.
- pH below 7: acidic
- pH 7: neutral
- pH above 7: alkaline
Strongly alkaline solutions may have a pH close to 14.
3. They Change the Colour of Indicators
Indicators can be used to identify acids and alkalis.
For example:
| Indicator | In acid | In alkali |
|---|---|---|
| Blue litmus | Red | Blue |
| Red litmus | Red | Blue |
| Phenolphthalein | Colourless | Pink |
| Universal indicator | Red–yellow | Blue–purple |
Therefore, one simple test for an alkaline solution is to place red litmus paper into it. The paper will turn blue.
4. Some Feel Slippery
Some alkaline solutions feel slippery or soapy because they interact with oils on the skin.
However, chemicals should never be identified by touching them. Strong alkalis can cause serious chemical burns.
Bases in Everyday Life
Bases are much more common in everyday life than they might first appear.
Baking soda contains sodium hydrogencarbonate, a weak base. It is used in baking and can also help neutralise acids.
Antacid medicines often contain bases such as magnesium hydroxide or calcium carbonate. These substances neutralise excess acid in the stomach.
Cleaning products may contain alkaline substances because they can help break down grease and oils.
Agricultural lime, often containing calcium carbonate, can be added to acidic soil to increase its pH.
Toothpaste is often mildly alkaline and can help neutralise acids produced by bacteria in the mouth.
Acids vs. Bases
Acids and bases have contrasting properties.
| Property | Acids | Bases/Alkalis |
|---|---|---|
| pH | Below 7 | Above 7 for alkaline solutions |
| Important ion in water | H⁺ | OH⁻ for alkalis |
| Litmus test | Blue → red | Red → blue |
| Universal indicator | Red/orange/yellow | Blue/purple |
| Reaction together | Neutralise bases | Neutralise acids |
When an acid and a suitable base react in the correct amounts, their acidic and basic properties can cancel each other out.
This is neutralisation.
Did You Know?
Some of the strongest alkalis are extremely corrosive.
Sodium hydroxide, for example, is useful for breaking down fats and grease, which is why it can be found in some drain cleaners. The same chemical property also means concentrated sodium hydroxide can severely damage skin and eyes.
The word alkali comes historically from an Arabic word associated with the ashes of plants. Early chemists discovered that plant ashes could produce alkaline solutions when mixed with water.
Key Terms
Base: A substance that neutralises an acid.
Alkali: A base that dissolves in water.
Hydroxide ion (OH⁻): An ion produced by alkalis in aqueous solution.
Neutralisation: A reaction in which an acid reacts with a base.
Indicator: A substance that changes colour depending on pH.
pH: A scale used to describe how acidic or alkaline a solution is.
Key Takeaways
- A base is a substance that can neutralise an acid.
- An alkali is a soluble base.
- All alkalis are bases, but not all bases are alkalis.
- Alkalis produce OH⁻ ions in water.
- Alkaline solutions have a pH greater than 7.
- Bases can change indicators such as litmus and universal indicator.
- Common bases are found in antacids, cleaning products, toothpaste, baking products, and agriculture.
- Acids and bases have contrasting properties and react together in neutralisation reactions.