Base
Generally the substances that have a bitter or astringent taste are bases. Some bases dissolve in water and make alkali. Bases are of a slippery nature. Sodium hydroxide and potassium hydroxide are examples of bases. A mixture of aluminium hydroxide and magnesium hydroxide, sold as antacid, is used as a medicine for gastritis, and these medicines that are used to cure the acidity of the stomach are bases.
The sentence some bases dissolve in water and make alkali goes past in a moment and is worth stopping on, because it defines a word you will meet for years. An alkali is not a different kind of substance from a base. An alkali is simply a base that dissolves in water. Sodium hydroxide dissolves easily, so it is an alkali. Magnesium hydroxide, the white powder in an antacid bottle, hardly dissolves at all, so it is a base and not an alkali, and that is precisely why it can be swallowed safely while sodium hydroxide cannot. So every alkali is a base, but not every base is an alkali. Notice the shape of that sentence, because it turns up all over science: every square is a rectangle, but not every rectangle is a square.
Physical properties of base
| A base changes red litmus paper into blue. |
| Most bases taste bitter and astringent. |
| A base is slippery, like a soap solution. |
| A base changes methyl orange into yellow. |
| A base changes phenolphthalein into pink. |
The book lists slipperiness as a property and leaves it there, but the reason behind it is worth having, because it turns one of the five properties into a safety rule. Your skin is covered with a thin natural layer of oil. A base reacts with oil and fat and turns them into soap, which is in fact how soap has always been made: fat boiled up with a base such as wood ash or lye. So when a base touches your fingers, it starts turning the oil on your skin into soap, and soap between your fingers is what slippery feels like. With hand soap that is the whole idea and it stops there. But with a strong laboratory base such as sodium hydroxide, the reaction does not politely stop at the oil layer, and the slippery feeling means it has begun on your skin itself. So here is the rule: if something you have touched in a laboratory suddenly feels soapy, do not stand there working out what it is. Get it under running water at once, keep it there for at least ten minutes, and tell your teacher.
Ash is a kind of base. It is used for washing utensils and clothes, and for reducing the acidity of soil.
Salt
Salts are generally neutral compounds. A salt is formed when an acid and a base combine. Sodium chloride, which is common salt, and sodium carbonate are examples of salts. Generally salts are of three kinds: neutral, acidic and basic. For instance sodium chloride is a neutral salt, while ammonium chloride is an acidic salt. In the same way sodium carbonate is a basic salt. Baking soda, which is sodium bicarbonate and is used to make roti rise, is also a basic salt.
This is the single most confusing word in the topic, because you have used it all your life for one white powder. In chemistry, salt is the name of a whole family, and there are thousands of members. Sodium chloride is only the one that happens to be edible and famous, which is why it gets called common salt. Every other member is a salt too: sodium carbonate, ammonium chloride, magnesium sulphate, copper sulphate, and on and on. Look at the property list and you will see the confusion written into it. The book says salts are generally tasteless or bitter, and then has to add that common salt is salty, which reads oddly until you realise that common salt is the exception in the family, not the rule. So when an examination question says a salt, read it as any member of the salt family, and read common salt or sodium chloride as the one in the kitchen.
Physical properties of salt
| Generally the taste of a salt is tasteless or bitter, but common salt is salty. |
| Salts are generally soluble in water. |
| Some salts are white, meaning colourless, and some are coloured. |
| Salts need more heat to melt and to boil. |
| A neutral salt shows no effect at all on an indicator. |
One idea that explains five everyday habits
Look at the sentence a salt is formed when an acid and a base combine. That single line is doing more work than any other sentence in this unit, because it means an acid and a base cancel each other out. Put them together and neither survives; what is left is a salt and water, and the salt is usually neutral. Chemists call this neutralisation, and once you have it, a whole row of familiar habits stops being folk wisdom and becomes chemistry you can explain.
| The habit | The problem | Why the cure works |
|---|---|---|
| An antacid tablet for gastritis | Too much hydrochloric acid reaching the stomach wall | Aluminium and magnesium hydroxide are bases and cancel the acid |
| Soap or titepati on a sisnu sting | Formic acid injected under the skin | Soap is a base, so it works against the acid that is doing the burning |
| Soap where an ant or a bee has stung | Formic acid again, from the sting | Same reason. Acid problem, base cure |
| Vinegar where a wasp has stung | A wasp injects an ALKALINE substance, not an acid | So the cure has to flip: base problem, acid cure |
| Ash spread on a field | Soil that has become too acidic for crops | Ash is a base, so it reduces the acidity of the soil |
Four of those five rows use a base to fix an acid, so a student who has merely memorised will answer put soap on it every time and will be right four times out of five. The wasp row is where memorising fails and understanding survives. A wasp injects an alkaline substance, so soap would make it worse, and the cure has to be an acid: vinegar, lemon, anything sour. If you can explain why the wasp is the odd one out, you do not have five facts, you have one rule. And notice how a village that never heard the word neutralisation arrived at exactly the right answer for both, because somebody tried soap on a wasp sting, found it useless, tried vinegar, and told everybody. That is science being done without the vocabulary.
The book says salts are generally neutral compounds and then immediately lists acidic and basic salts, which looks like a contradiction until you think about where a salt comes from. A salt is what is left after an acid and a base have cancelled each other, and a cancellation is only exactly even if the two sides were evenly matched. Common salt comes from a strong acid and a strong base, so neither wins and sodium chloride is neutral. Sodium carbonate comes from a strong base and a weak acid, so the base side is left slightly in charge and the salt behaves as a base, which is why washing soda cleans and why baking soda is listed as a basic salt. Ammonium chloride comes from a strong acid and a weak base, so the acid side is left in charge and the salt is acidic. You can test all of this yourself: dissolve a salt in water and dip both litmus papers. Common salt leaves them alone, and washing soda turns red litmus blue, which is the property doing exactly what this explanation predicts.
One last everyday piece, because you have just been given everything needed to explain it. Baking soda makes roti rise because heat, and anything sour in the dough, break it down and release carbon dioxide gas. Bubbles of that gas are trapped in the dough and push it up as it cooks, and the holes you can see in a finished piece of bread are where those bubbles were. That is also why recipes so often pair baking soda with curd or lemon: the sour thing is an acid, the soda is a basic salt, and putting them together is a neutralisation that releases the gas faster. And it is the same gas from the last topic, the one that dissolves in water to make carbonic acid, turning up in a third place.
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