Acids, Bases and pH
Senior 1 · Chemistry
Acids, Bases and pH
Lesson 1 of 5
22 min
Squeeze a lemon onto your tongue and it tastes sharp and sour. Ikivuguto, milk left to ferment, tastes sour too, and so does vinegar. The word acid comes from the Latin acidus, "sour". The sting of some ants is also an acid; its old name, formic acid, comes from formica, Latin for "ant".
Other substances in your home feel different. Soapy water and the grey water left after soaking wood ash feel slippery between your fingers and taste bitter. These are alkalis. Chemists sort all these substances into acids, bases and alkalis, and they test them not by tasting but with indicators.
Warning
Never taste, touch or smell laboratory chemicals to identify them. Concentrated acids and alkalis burn skin and can blind. The tastes above describe foods only.
Definition
Acid: a substance that produces hydrogen ions, , when it dissolves in water. Acids taste sour, turn blue litmus red and have a pH below 7. Example: hydrochloric acid, .
The first three are mineral acids, made from minerals; they are strong and corrosive. The others are organic acids, found in living things; most are weak and many are safe to eat.
Table 1.1: Some common acids
| Acid | Formula | Where you meet it |
|---|---|---|
| Hydrochloric acid | the laboratory; your stomach makes it to help digest food | |
| Sulfuric acid | car and motorcycle batteries | |
| Nitric acid | making fertilisers | |
| Carbonic acid | fizzy drinks (carbon dioxide dissolved in water) | |
| Citric acid | lemons, oranges, passion fruit | |
| Ethanoic (acetic) acid | vinegar | |
| Lactic acid | sour milk and ikivuguto; tired muscles | |
| Methanoic (formic) acid | ant stings and nettle stings |
Definition
Base: a substance that reacts with an acid and cancels out (neutralises) its acidity, forming a salt and water. Most bases are metal oxides or metal hydroxides. Example: copper(II) oxide, .
Definition
Alkali: a base that dissolves in water, producing hydroxide ions, . Alkalis feel soapy, turn red litmus blue and have a pH above 7. Example: sodium hydroxide, .
Figure 1.1: Every alkali is a base, but not every base is an alkali.
Worked example
Problem. Write equations to show what happens when (a) nitric acid, (b) potassium hydroxide dissolve in water. Which ion makes each solution acidic or alkaline?
Solution. In water an acid releases ions and an alkali releases ions:
The hydrogen ion makes nitric acid acidic; the hydroxide ion makes potassium hydroxide alkaline. Check: the charges on each side add up to zero, as they must.
You cannot see an ion, so chemists use an indicator, a dye that has one colour in acids and a different colour in alkalis. The oldest is litmus, made from lichens, small plant-like organisms that grow on rocks and tree bark. It is sold as a solution or soaked into paper strips. Blue litmus turns red in an acid; red litmus turns blue in an alkali. In a neutral substance, such as pure water or salt water, neither colour changes.
Definition
Indicator: a substance that changes colour depending on whether it is in an acid or an alkali. Example: litmus, red in acids and blue in alkalis.
Methyl orange turns red only in fairly strong acid; it is yellow in water and in alkalis. Phenolphthalein turns pink only in an alkali.
Table 1.2: Three laboratory indicators
| Indicator | Colour in acid | Colour in neutral solution | Colour in alkali |
|---|---|---|---|
| Litmus | red | purple | blue |
| Phenolphthalein | colourless | colourless | pink |
| Methyl orange | red | yellow | yellow |
Worked example
Problem. A class tests five liquids with red and blue litmus paper. Classify each as acidic, alkaline or neutral.
| Liquid | Blue litmus | Red litmus |
|---|---|---|
| A: lemon juice | turns red | stays red |
| B: salt solution | stays blue | stays red |
| C: wood-ash water | stays blue | turns blue |
| D: vinegar | turns red | stays red |
| E: soap solution | stays blue | turns blue |
Solution. An acid turns blue litmus red; an alkali turns red litmus blue; a neutral liquid changes neither. So A and D are acidic, C and E are alkaline, and B is neutral. Notice that one strip of litmus is not always enough: a blue strip that stays blue could be in a neutral liquid or an alkali, so test with both colours.
Important
An acid and an alkali are opposites: ions make a solution acidic and ions make it alkaline. When they meet, they cancel each other out (Lesson 2).
Tip
Blue litmus turns red in acid: "Blue to Red, Acid Read". Red litmus turns blue in an alkali, a Base.
Freesciencelessons: Acids and Alkalis (5 min). Watch for the ions that acids and alkalis produce in water, and the difference between a base and an alkali.
Practice
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
