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Acids, Bases and Salts
Introduction
Understanding the Chemical Properties of Acids and Bases
Practice: Reactions of Acids and Bases
What do All Acids and All Bases Have in Common?
Practice: Ions in Solution
How Strong are Acid or Base Solutions?
Practice: The pH Scale
More About Salts
Practice: Salts and Their Uses
Chapter Review: Mixed Practice
Summary
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Acids, Bases and SaltsUnderstanding the Chemical Properties of Acids and Bases

Reading time: ~30 min

Indicators tell us whether a liquid is an acid or a base. In this section we let acids and bases loose on indicators, on metals, on carbonates, on metal oxides, and on each other — and look for patterns. Every reaction here belongs to one of a few families, and by the end of the section you will be able to predict the products of each one.

Caution

Real acids and bases can burn skin and eyes. Never taste or touch them, and do Activities 2.3 and 2.4 only with your teacher's help. The virtual versions on this page are completely safe.

Acids and Bases in the Laboratory

Activity 2.1 uses nine solutions from the school laboratory. Four are acids: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃) and acetic acid (CH₃COOH). Five are bases: sodium hydroxide (NaOH), calcium hydroxide [Ca(OH)₂], potassium hydroxide (KOH), magnesium hydroxide [Mg(OH)₂] and ammonium hydroxide (NH₄OH). A drop of each goes on a watch-glass and is tested with four indicators: red litmus, blue litmus, phenolphthalein and methyl orange.

Run the activity: tap any ? in Table 2.1 to test that solution with that indicator, and complete both missions.

The table shows a clear pattern. In every acid, blue litmus turns , methyl orange turns red, and phenolphthalein stays . In every base, red litmus turns , phenolphthalein turns , and methyl orange turns yellow.

Tip

One line to remember: acid — blue litmus turns red; base — red litmus turns blue and phenolphthalein turns pink. Methyl orange is red in an acid and yellow in a base.

Indicators You Can Smell

Some substances change their smell instead of their colour in an acid or a base. These are called olfactory indicators. In Activity 2.2, strips of clean cloth are kept overnight in a closed bag of chopped onions, so they smell of onion. One strip gets a few drops of dilute HCl and the other a few drops of dilute NaOH; both are rinsed with water and smelt again. Dilute vanilla essence and clove oil are tested in the same way, in test tubes of dilute HCl and dilute NaOH.

The onion strip that got dilute NaOH is smelt after rinsing. What do you expect?

It still smells strongly of onion
The onion smell has gone
It smells sour, like vinegar

A base wipes out the smell of onion and of vanilla essence, while an acid leaves both smells as they were. So onion and vanilla can tell an acid from a base by smell alone — they are olfactory indicators. Test clove oil in your laboratory and decide for yourself whether it is one too.

How do Acids and Bases React with Metals?

In Activity 2.3, a few zinc granules are dropped into about 5 mL of dilute sulphuric acid. Bubbles of gas appear on the zinc. The gas is led through a delivery tube into soap solution, where it blows soap bubbles, and a burning candle is brought near a bubble. Activity 2.4 repeats the test with warm sodium hydroxide solution instead of an acid.

Run both activities. Choose the liquid, work through the steps with the red button, and complete all three missions.

Every time, the gas burns with a pop: it is hydrogen. The metal pushes hydrogen atoms out of the acid as hydrogen gas and forms a compound called a salt. So the reaction of a metal with an acid is:

Acid + Metal → Salt + Hydrogen gas

With sulphuric acid, the salt is zinc sulphate:

\begin{aligned}&\mathrm{Zn(s) + H_2SO_4(aq)} \\ &\mathrm{\rightarrow ZnSO_4(aq) + H_2(g)}\end{aligned}

With hydrochloric acid, the salt is , ZnCl₂.

Zinc reacts with sodium hydroxide too, giving hydrogen and a salt called sodium zincate:

\begin{aligned}&\mathrm{2NaOH(aq) + Zn(s)} \\ &\mathrm{\rightarrow Na_2ZnO_2(s) + H_2(g)}\end{aligned}

Such reactions with bases are not possible for every metal, though.

Note

Activity 2.3 also asks you to repeat the test with HNO₃ and CH₃COOH. Nitric acid is the odd one out: it is a strong oxidising agent and usually does not let hydrogen escape. You will see why in the next chapter, Metals and Non-metals.

How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?

In Activity 2.5, test tube A holds about 0.5 g of sodium carbonate (Na₂CO₃) and test tube B the same amount of sodium hydrogencarbonate (NaHCO₃). About 2 mL of dilute HCl goes into each, and the gas that comes off is bubbled through lime water, which is a solution of calcium hydroxide.

Pick a test tube, add the acid, and bubble the gas through the lime water — again and again. Complete all three missions.

Both test tubes fizz and give the same gas — carbon dioxide, the gas that turns lime water milky.

Test tube A: \begin{aligned}&\mathrm{Na_2CO_3(s) + 2HCl(aq)} \\ &\mathrm{\rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g)}\end{aligned}

Test tube B: \begin{aligned}&\mathrm{NaHCO_3(s) + HCl(aq)} \\ &\mathrm{\rightarrow NaCl(aq) + H_2O(l) + CO_2(g)}\end{aligned}

The milkiness is a white precipitate of calcium carbonate:

\begin{aligned}&\mathrm{Ca(OH)_2(aq) + CO_2(g)} \\ &\mathrm{\rightarrow CaCO_3(s) + H_2O(l)}\end{aligned}

With excess carbon dioxide it becomes calcium hydrogencarbonate, which is soluble in water — so the milkiness disappears:

\begin{aligned}&\mathrm{CaCO_3(s) + H_2O(l) + CO_2(g)} \\ &\mathrm{\rightarrow Ca(HCO_3)_2(aq)}\end{aligned}

Limestone, chalk and marble are all different forms of calcium carbonate. All metal carbonates and hydrogencarbonates react with acids in the same way:

Metal carbonate (or hydrogencarbonate) + Acid → Salt + Carbon dioxide + Water

So marble chips dropped into dilute HCl give calcium chloride, water and .

How do Acids and Bases React with each other?

In Activity 2.6, two drops of phenolphthalein go into about 2 mL of dilute sodium hydroxide solution, turning it pink. Then dilute hydrochloric acid is added, drop by drop. The widget also shows the pH, a number from 0 to 14 that you will meet properly in Section 2.3: below 7 is acidic, 7 is neutral, above 7 is basic.

Be the chemist. Complete both missions, and keep an eye on the pH.

The acid cancelled the base, and then the extra base cancelled the extra acid. The effect of a base is nullified by an acid, and vice versa:

\begin{aligned}&\mathrm{NaOH(aq) + HCl(aq)} \\ &\mathrm{\rightarrow NaCl(aq) + H_2O(l)}\end{aligned}

A reaction between an acid and a base that gives a salt and water is called a neutralisation reaction. In general,

Base + Acid → Salt + Water

Phenolphthalein lost its colour at the very drop where the base ran out, so it signalled the moment of neutralisation. The salt formed in Activity 2.6 is — common salt.

Reaction of Metallic Oxides with Acids

In Activity 2.7, a little black copper(II) oxide is placed in a beaker, and dilute hydrochloric acid is added slowly while stirring. Predict first: what will you see?

A colourless solution, with the black powder still at the bottom
The black powder dissolves and the solution turns blue-green
A pink solution
A milky white solution

The copper oxide dissolves, and the blue-green colour comes from copper(II) chloride formed in the reaction. Balance it: CuO + HCl → CuCl₂ + H₂O.

\begin{aligned}&\mathrm{CuO(s) + 2HCl(aq)} \\ &\mathrm{\rightarrow CuCl_2(aq) + H_2O(l)}\end{aligned}

The general reaction between a metal oxide and an acid is

Metal oxide + Acid → Salt + Water

That is exactly what a base does with an acid. So metallic oxides behave like bases, and they are called basic oxides.

Reaction of a Non-metallic Oxide with Base

Look back at the lime-water test of Activity 2.5. Calcium hydroxide is a base, and carbon dioxide is the oxide of a non-metal, carbon. Together they gave calcium carbonate and water — a salt and water:

\mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O}

That is just what an acid does with a base. So carbon dioxide behaves like an acid here, and non-metallic oxides are in nature.

Five reactions to know by heart

  1. Acid + Metal → Salt + Hydrogen gas
  2. Metal carbonate (or hydrogencarbonate) + Acid → Salt + Carbon dioxide + Water
  3. Base + Acid → Salt + Water (neutralisation)
  4. Metal oxide + Acid → Salt + Water
  5. Non-metal oxide + Base → Salt + Water

Speed Round: Which Gas?

Hydrogen, carbon dioxide, or no gas at all? Use the five reactions to decide; after each answer you will see what forms.

Get five right to move on.

Sina