Acids and bases in daily life

Acids and bases are all around us

You have already learned what acids and bases are, and how the pH scale measures them. In this lesson, we see where they work in our body, in the garden and in the air. Each example uses one idea: an acid and a base neutralise each other.

The pH in our body

Our body works in a narrow range of pH. Blood has a pH of about $7.4$. If it moves much from this, we fall ill. The body has its own ways to keep the pH steady.

The stomach. The stomach makes hydrochloric acid, with a pH of about $1.5$ to $2$. It helps the digestion of food and kills germs that come in with it. The stomach has a coat of mucus that protects it from its own acid. But if the stomach makes too much acid, or the coat is weak, we get indigestion, pain and a burning feeling.

An antacid is a mild base. It neutralises the extra acid. Baking soda, magnesium hydroxide (milk of magnesia) and calcium carbonate are used as antacids. For example:

A strong base such as sodium hydroxide would burn the stomach, so an antacid must be a mild base.

You should find that twice the acid needs twice the tablets, so a stronger dose is not always the answer.

The pH in the mouth

Bacteria in the mouth break down the sugar and the food that is left on the teeth, and they make acids. When the pH falls below $5.5$, the enamel, the hard white coating of the tooth, which is calcium phosphate, starts to dissolve. This is tooth decay. Enamel is the hardest substance in the body, but it cannot grow back.

How to prevent it:

• Brush after meals, to remove food and sugar.
• Use a toothpaste, which is basic. It neutralises the acid.
• Rinse the mouth after eating sweets.
• Eat less sticky sugar.

Stings

• The sting of a bee has methanoic (formic) acid. Rubbing a mild base, such as baking soda, on the place gives relief.
• The sting of a wasp is basic, so a mild acid such as vinegar helps.
• The leaf of the nettle plant has hairs that sting with methanoic acid. Rubbing the leaf of the dock plant, which often grows near it, brings relief. The dock leaf is basic.
• Ants also send out methanoic acid when they bite.

The soil

Plants need a suitable pH in the soil. Most crops grow well at a pH near $6$ to $7$. A farmer tests the soil. Acidic soil is treated with lime (calcium oxide or calcium hydroxide), which is a base. Soil that is too basic is treated with organic matter such as compost, which makes it a little more acidic. The right pH also decides how well the plant takes in minerals.

Acid rain

Rain has a pH of about $5.6$, because it dissolves carbon dioxide from the air and makes weak carbonic acid. Oxides of sulphur and nitrogen from vehicles and factories make the rain more acidic. If the pH falls below $5.6$, it is acid rain. It damages buildings (the marble of old monuments is calcium carbonate, which reacts with acid), plants, and the life of lakes and rivers.

Other examples

• Acidic foods. Lemon, tamarind and curd have acids (citric, tartaric and lactic acids). They taste sour.
• Cleaning. Soap and washing soda are bases, and they remove grease. Vinegar removes the white scale of hard water on a tap, because the scale is a base (calcium carbonate).
• Factory waste. Acidic waste from a factory is neutralised with lime before it goes into a river.
• Swimming pools. The owner tests the pH and adds acid or base to keep it near $7.4$, the pH of our eyes.

Practise with new numbers

Try it in Python

This program finds how much stomach acid a tablet of magnesium hydroxide can neutralise.

Common mistakes

• Using a strong base as an antacid. An antacid must be a mild base. A strong base would harm the stomach.
• Thinking that all acids are harmful. Stomach acid is useful, and so is the acid in a lemon.
• Using an acid on an acid sting. A bee sting is acidic, so a base is needed.
• Mixing up the pH of rain and acid rain. Normal rain is already a little acidic, at about $5.6$.

Remember these

• Stomach acid has a pH of about $1.5$. Antacids are mild bases that neutralise extra acid.
• Tooth enamel dissolves below pH $5.5$. Toothpaste is basic.
• A bee sting is acidic, so use a base. Soil of the wrong pH is corrected with lime.
• Acid rain has a pH below $5.6$.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why is the stomach not burnt by its own hydrochloric acid?

2. A wasp sting is basic. What should be put on it?

3. Why is acidic waste from a factory treated with lime before it goes into a river?

4. Why does vinegar remove the white scale from a tap?