Topic 8 · page 11 of 29

Oxides and hydroxides of Groups 1 and 2

Metal oxides and hydroxides are bases. Oxides react with water to form hydroxides, and both oxides and hydroxides neutralise dilute acids to give a salt and water. No oxidation number changes, so none of these is redox.

11 min read8.148.14(i)8.14(ii)

Read first: Reactions of Group 1 and 2 metals

Oxides with water 8.148.14(i)

The oxide ion, O²⁻, is a very strong base. It takes H⁺ from water, forming hydroxide ions:

\ce{O^2-(s) + H2O(l) -> 2OH-(aq)}

So a metal oxide reacts with water to give the metal hydroxide, and the solution is alkaline.

Oxide	Equation with water
Sodium oxide	$\ce{Na2O(s) + H2O(l) -> 2NaOH(aq)}$
Potassium oxide	$\ce{K2O(s) + H2O(l) -> 2KOH(aq)}$
Calcium oxide	$\ce{CaO(s) + H2O(l) -> Ca(OH)2(aq)}$
Barium oxide	$\ce{BaO(s) + H2O(l) -> Ba(OH)2(aq)}$

Group 1 oxides dissolve fully, giving strongly alkaline solutions. The Group 2 oxides give less soluble hydroxides: calcium hydroxide is only slightly soluble (its solution is limewater) and magnesium oxide barely reacts with cold water, because magnesium hydroxide is almost insoluble. A little of it does react, which is enough to give a faintly alkaline solution.

Oxides and hydroxides with dilute acid 8.148.14(i)8.14(ii)

Metal oxides and hydroxides are bases. They react with dilute acids to give a salt and water.

Oxides with dilute hydrochloric acid:

\ce{Na2O(s) + 2HCl(aq) -> 2NaCl(aq) + H2O(l)}

\ce{MgO(s) + 2HCl(aq) -> MgCl2(aq) + H2O(l)}

The ionic equation is the same for every oxide: the oxide ion takes two H⁺.

\ce{O^2- + 2H+ -> H2O}

Hydroxides with dilute acid:

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

\ce{Ca(OH)2(aq) + 2HCl(aq) -> CaCl2(aq) + 2H2O(l)}

The ionic equation is the same for all of them:

\ce{OH-(aq) + H+(aq) -> H2O(l)}

For an insoluble solid such as Mg(OH)₂ or MgO, write the solid as it is: $\ce{Mg(OH)2(s) + 2H+(aq) -> Mg^2+(aq) + 2H2O(l)}$ .

Worked exampleNaming the salt

Write the equation for calcium hydroxide with dilute nitric acid.

The salt takes its metal from the base and its anion from the acid: calcium + nitrate = Ca(NO₃)₂.
Hydroxide gives water: $\ce{Ca(OH)2 + 2HNO3 -> Ca(NO3)2 + 2H2O}$
The ratio is 1 : 2 because each Ca(OH)₂ supplies two OH⁻ and each HNO₃ one H⁺.

Check yourself

Write the equation for the reaction of calcium oxide with water and state the colour of universal indicator in the solution.
Write the equation for magnesium oxide with dilute sulfuric acid.
Write the ionic equation for any Group 1 hydroxide with dilute hydrochloric acid.
A student writes $\ce{CaO + 2HCl -> CaCl2 + H2}$ . Correct the equation, and explain how oxidation numbers show this is not a redox reaction.
Write the equation for barium hydroxide with dilute nitric acid.
Explain why sodium oxide reacts with water to form a strongly alkaline solution, but MgO gives only a weakly alkaline one.

Answers cover these until you have tried the questions

$\ce{CaO(s) + H2O(l) -> Ca(OH)2(aq)}$ . The solution is alkaline, so universal indicator turns purple or blue.
$\ce{MgO(s) + H2SO4(aq) -> MgSO4(aq) + H2O(l)}$
$\ce{OH-(aq) + H+(aq) -> H2O(l)}$
$\ce{CaO + 2HCl -> CaCl2 + H2O}$ . A base and an acid give a salt and water, not hydrogen. In the corrected equation Ca stays +2, O stays −2, H stays +1 and Cl stays −1: nothing changes, so it is not redox.
$\ce{Ba(OH)2(aq) + 2HNO3(aq) -> Ba(NO3)2(aq) + 2H2O(l)}$
Sodium oxide reacts fully with water to give sodium hydroxide, which is very soluble and so gives a high concentration of OH⁻. Magnesium oxide barely reacts with water, and the magnesium hydroxide that forms is almost insoluble, so the concentration of OH⁻ is low.