Preparation and Classification of Oxides
Senior 2 · Chemistry
Preparation and Classification of Oxides
Lesson 2 of 4
20 min
When the Nyiragongo volcano, just across the border from Rubavu, is active, it releases large amounts of sulfur dioxide. That gas dissolves in rain to make it acidic. Farmers, by contrast, spread lime, a metal oxide, to make acidic soil less acidic. The two effects are opposite because oxides fall into chemical families, and the family decides what an oxide does in water, with acids and with alkalis.
A basic oxide is an oxide that reacts with an acid to form a salt and water only. Most metal oxides are basic, for example Na2O, K2O, CaO, MgO, CuO and FeO.
With acids:
With water: only the oxides of Group 1 metals and of calcium, strontium and barium dissolve and react to form alkalis, which are soluble bases:
The solutions turn red litmus blue and have a pH above 7. Copper(II) oxide and iron(III) oxide are insoluble in water, but they are still basic because they react with acids.
An acidic oxide is an oxide that reacts with an alkali to form a salt and water. Most non-metal oxides are acidic. Those that dissolve in water form acids:
With alkalis:
Silicon dioxide (sand) is acidic even though it is insoluble in water: it reacts with hot concentrated sodium hydroxide, SiO2 + 2NaOH → Na2SiO3 + H2O.
Sulfur dioxide from volcanoes and from burning coal and diesel, and nitrogen dioxide from vehicle engines, dissolve in rainwater to form acids. Acid rain corrodes iron roofing sheets, damages stone buildings and makes lakes and soils more acidic.
Keza has two white powders, X and Y, each an oxide. She shakes each with water and adds universal indicator. X gives pH 12. Y does not dissolve, so she tests it with acid and with alkali: Y dissolves in warm dilute hydrochloric acid but not in sodium hydroxide solution.
An oxide that gave an acidic solution, or that reacted only with the alkali, would be acidic.
Tip
Metals make bases, non-metals make acids
As a first guess, metal oxide = basic and non-metal oxide = acidic. Then learn the exceptions in the next lesson (amphoteric and neutral oxides). The test that always works: a basic oxide reacts with an acid to give salt + water; an acidic oxide reacts with an alkali to give salt + water.
Write a balanced equation for copper(II) oxide reacting with dilute sulfuric acid. What do you observe?
Solution: CuO + H2SO4 → CuSO4 + H2O. The black solid dissolves (faster when warmed) and the solution turns blue.
Complete: SO3 + H2O → ___, and name the product.
Solution: H2SO4, sulfuric acid.
Why does the pH of rain near an active volcano fall?
Solution: The volcano releases sulfur dioxide, an acidic oxide. It dissolves in rainwater to form sulfurous acid (SO2 + H2O → H2SO3), so the rain becomes acidic.
Write the equation for carbon dioxide reacting with sodium hydroxide solution, and name the salt.
Solution: CO2 + 2NaOH → Na2CO3 + H2O. The salt is sodium carbonate.
Iron(III) oxide does not dissolve in water. How can you show that it is a basic oxide?
Solution: Warm it with dilute hydrochloric acid. It dissolves to give a yellow-brown solution of iron(III) chloride and water: Fe2O3 + 6HCl → 2FeCl3 + 3H2O. Reacting with an acid to give a salt and water shows that it is basic.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
