Acid

From the moment we wake up to the moment we sleep, we handle many different materials. The fruit and vegetables we eat, the toothpaste we brush with, the soap, the shampoo, the hand wash and sanitizer, the liquid used to clean a mirror or a bathroom. Every one of these has some acid, some base or some salt mixed into it. Which of them is sour and which is not is something our tongue often tells us straight away.

Lemon, orange, lime, tomato, curd, pickle, vinegar and fizzy soda drinks are things we eat and drink often. What they share is sourness, and that sourness comes from the acid inside them. The sour materials around us are acids.

Not everything in the house is sour, though. The salt kept in the kitchen is not sour at all; it is a salt. Soap and toothpaste behave in a basic way instead. So the first skill to build here is telling which material is acidic, which is basic and which is a salt.

An acid dissolves in water. Once it has dissolved, it releases hydrogen ions into that water. This hydrogen ion is the real mark of an acid. A sour taste is only the outward hint; the inner reason is the hydrogen ion.

Not every acid is fit to eat. The acids found in plants and animals are generally edible, while the acids made in a laboratory are not. Lemon juice can be tasted safely, but tasting laboratory hydrochloric acid or sulphuric acid is extremely dangerous.

Acids can be divided into two kinds according to where they come from. They are organic acids and inorganic acids.

One point here needs care. Hydrochloric acid is found in the human stomach, yet it is not an organic acid. The kind an acid belongs to is decided by the source it is built from, not by the place it happens to be found. Hydrochloric acid is built from mineral substances, so it stays an inorganic acid even when it is found inside the body.

Acids can also be divided into two kinds according to how many hydrogen ions they give when they dissolve in water. Not all acids give the same amount. Some give many, some give only a few.

Take one lemon and carry out the steps below, and the physical properties of an acid show themselves. Write down what you see at each step.

• Taste a little lemon juice and write down what the taste is like.
• Dissolve the lemon juice in water and see whether it mixes or not.
• Dip red litmus into the lemon juice.
• Dip blue litmus into the lemon juice.
• Mix a little methyl orange into the lemon juice.
• Mix a little phenolphthalein into the lemon juice.

Putting those observations together, the physical properties of an acid come out like this.

• An acid tastes sour.
• An acid dissolves in water.
• An acid turns blue litmus red, but it does not change the colour of red litmus.
• An acid turns methyl orange red.
• An acid makes no change at all to the colour of phenolphthalein.

How an acid behaves with other substances is what we call its chemical properties. The first of them is ionisation on dissolving in water. Acids give hydrogen ions when they dissolve. Hydrochloric acid breaks into a hydrogen ion and a chloride ion in water.

\[ \mathrm{HCl} \rightleftharpoons \mathrm{H}^+ + \mathrm{Cl}^- \]

What happens between an acid and a metal can be watched using magnesium ribbon and dilute hydrochloric acid. Magnesium ribbon, dilute hydrochloric acid and a matchstick are what the test needs.

• Take a little dilute hydrochloric acid in a test tube.
• Now put a small piece of magnesium ribbon into it.
• Watch carefully what is going on inside the test tube.
• Once bubbles appear, bring a burning matchstick near the mouth of the test tube and listen for whether a pop sound comes.

Bubbles rise in the test tube, and a pop sound comes when the burning matchstick is brought near. That sound is the sign of hydrogen gas burning. It shows that when acids react chemically with metals they produce the salt of the metal and hydrogen gas. Magnesium reacting with hydrochloric acid produces magnesium chloride and hydrogen.

\[ \mathrm{Mg} + 2\mathrm{HCl} \rightarrow \mathrm{MgCl_2} + \mathrm{H_2} \]

To watch the next reaction we need calcium carbonate, dilute hydrochloric acid and lime water.

• Take a few pieces of calcium carbonate in a test tube.
• Now add a little dilute hydrochloric acid to it.
• Observe the bubbles coming out in the test tube.
• Pass the colourless gas that comes out into lime water and see what happens to the lime water.

The lime water turns milky. That is the test which identifies carbon dioxide gas. It shows that when acids react chemically with metal carbonates they produce the salt of the metal and carbon dioxide gas. Calcium carbonate reacting with hydrochloric acid produces calcium chloride, carbon dioxide and water.

\[ \mathrm{CaCO_3} + 2\mathrm{HCl} \rightarrow \mathrm{CaCl_2} + \mathrm{CO_2} + \mathrm{H_2O} \]

The third reaction is between an acid and a base. It needs dilute hydrochloric acid, a few pellets of sodium hydroxide, a test tube, a porcelain basin, a spirit lamp or a bunsen burner, a spatula, a glass rod, a tripod stand and a wire gauze.

• Take a test tube half full of water.
• Put a few pellets of sodium hydroxide into it.
• Stir with the glass rod until the pellets dissolve fully.
• Pour the solution that forms into the porcelain basin.
• Mix dilute hydrochloric acid into it.
• Place the wire gauze on the tripod stand, heat the basin on it, and after some time observe what is left behind.

After the heating, a white solid stays behind in the basin. That solid is sodium chloride, which is common salt, and the water has gone off as vapour. When an acid and a base react chemically, a salt and water are formed.

\[ \mathrm{HCl} + \mathrm{NaOH} \rightarrow \mathrm{NaCl} + \mathrm{H_2O} \]

Acids are useful from the kitchen to the factory, and one easy test shows it. Cut a radish or a carrot into small pieces, divide them into two parts and put them into two bottles. Leave one bottle with nothing added, and pour vinegar into the other until the pieces are covered. Look at both bottles between one week and 15 days later. The pieces with no vinegar begin to rot, while the pieces under vinegar stay good for much longer, because the acetic acid stops the spoiling germs from growing.

The uses of the organic acids are given below.

The uses of the inorganic acids are given below.