Chemical Properties of Acids and Bases
Senior 2 · Chemistry
Chemical Properties of Acids and Bases
Lesson 1 of 4
25 min
Sulfuric acid is made in larger quantities than any other chemical in the world. Sodium hydroxide is made in millions of tonnes for soap, paper and water treatment. Both are prepared by reactions you already know from the units on oxygen and oxides. In this lesson you will recall what acids and bases are, and then see how chemists prepare them.
| Acids | Bases |
|---|---|
| taste sour (never taste laboratory chemicals) | taste bitter and feel slippery |
| turn blue litmus red | soluble bases turn red litmus blue |
| pH below 7 | pH above 7 (for soluble bases) |
| produce hydrogen ions, H+ (present as H3O+), in water | neutralise acids; alkalis produce hydroxide ions, OH−, in water |
Indicators show whether a solution is acidic or alkaline:
| Indicator | In acid | In neutral solution | In alkali |
|---|---|---|---|
| Litmus | red | purple | blue |
| Phenolphthalein | colourless | colourless | pink |
| Methyl orange | red | yellow | yellow |
| Universal indicator | red, orange or yellow (pH 1–6) | green (pH 7) | blue to violet (pH 8–14) |
You can make your own indicator by boiling slices of red (purple) cabbage in water: the purple juice turns red-pink in acids and green-yellow in strong alkalis.
1. An acidic oxide with water. Most non-metal oxides dissolve in water to form acids:
Industry makes sulfuric acid in three stages: burn sulfur to sulfur dioxide (S + O2 → SO2); oxidise it further with a catalyst (2SO2 + O2 → 2SO3); then combine the sulfur trioxide with water. (In practice the SO3 is first absorbed into concentrated sulfuric acid, because adding it straight to water produces a corrosive mist.)
2. Combining hydrogen with a non-metal, then dissolving. Hydrogen burns in chlorine to give hydrogen chloride gas, H2(g) + Cl2(g) → 2HCl(g), which dissolves in water to give hydrochloric acid.
3. A less volatile acid with a salt. Warming sodium chloride with concentrated sulfuric acid drives off hydrogen chloride gas, the usual laboratory method: NaCl(s) + H2SO4(l) → NaHSO4(s) + HCl(g).
1. A reactive metal with water (teacher demonstration, with a tiny piece of metal, behind a safety screen):
2. A basic oxide with water: Na2O(s) + H2O(l) → 2NaOH(aq), and CaO(s) + H2O(l) → Ca(OH)2(aq) (slaking lime).
3. Precipitating an insoluble base. Adding sodium hydroxide solution to a solution of a metal salt forms the metal hydroxide as a solid (a precipitate):
4. Industry: sodium hydroxide is made by the electrolysis of concentrated salt solution (brine), and ammonia solution by dissolving ammonia gas in water: NH3 + H2O ⇌ NH4+ + OH−.
How could you prepare (a) a solution of potassium hydroxide from potassium oxide, and (b) solid copper(II) hydroxide from copper(II) sulfate?
Open the pH Scale simulation. On the Macro screen, test everyday liquids such as orange juice, coffee, milk, soap, blood and drain cleaner with the pH probe. Add water to one of them and watch how dilution moves its pH towards 7.
Warning
Safety: always add acid to water
Tip
Oxide + water: the element decides
Non-metal oxide + water gives an acid (SO3 gives H2SO4; CO2 gives H2CO3). Metal oxide + water gives an alkali (Na2O gives NaOH; CaO gives Ca(OH)2), but only for the soluble ones. Insoluble bases are made by precipitation. For dilution, remember "Do what you oughta, add acid to water".
What is the difference between a base and an alkali?
Solution: A base is any substance (usually a metal oxide or hydroxide) that neutralises an acid to give a salt and water. An alkali is a base that dissolves in water to form hydroxide ions. NaOH is both; CuO is a base but not an alkali.
Write the equation for the preparation of sulfuric acid from sulfur trioxide.
Solution: SO3 + H2O → H2SO4.
Balance: __Na + __H2O → __NaOH + H2, and name the gas.
Solution: 2Na + 2H2O → 2NaOH + H2. The gas is hydrogen (a burning splint gives a squeaky pop).
A solution turns phenolphthalein pink. What does this tell you, and what colour would universal indicator show?
Solution: The solution is alkaline (pH above about 8). Universal indicator would be blue to violet.
How would you prepare iron(II) hydroxide? Write the equation.
Solution: Add sodium hydroxide solution to iron(II) sulfate solution, then filter off and wash the green precipitate: FeSO4 + 2NaOH → Fe(OH)2 + Na2SO4.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
