Acids, Bases and Salts

Lesson

Part 1 — what makes a substance an acid or a base, how indicators reveal them, how they react, and what happens to them in water.

In this lesson you will

Define acids and bases by the ions they give in water
Identify acids and bases with litmus, natural and olfactory indicators
Write equations for reactions of acids with metals, carbonates, hydrogencarbonates and bases
Explain why metal oxides are basic and non-metal oxides are acidic
Explain why acid solutions conduct electricity and why acid must be added to water

\ce{NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l)}

An acid and a base cancel each other out and leave a salt behind.

Concept

Sour or soapy: two families of substances

Acids taste sour. Bases taste bitter and feel soapy. In the laboratory we never taste or touch chemicals to identify them: many acids and bases are corrosive.

Table sorting common household items into acids and bases with the main substance in each
Found at home	Acid or base?	Main substance
Lemon, orange	Acid	Citric acid
Tamarind (imli)	Acid	Tartaric acid
Curd, buttermilk	Acid	Lactic acid
Vinegar in pickles	Acid	Acetic (ethanoic) acid
Soap solution, toothpaste	Base	Basic substances
Milk of magnesia (antacid)	Base	Magnesium hydroxide

Diagram

Litmus: the classic indicator

Litmus colours are written as words; pure water changes neither strip.

An indicator is a substance that shows, by a change of colour or smell, whether a solution is acidic or basic. Litmus, a purple dye from lichens, is the best known: acids turn blue litmus red; bases turn red litmus blue.

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