Groups 1 and 2: reactions and solubility

Why reactivity increases down the groups

Atoms react by losing their outer s electrons. Down a group, the outer electrons are further from the nucleus and more shielded, so ionisation energies fall and the electrons are lost more easily. Group 2 atoms must lose two electrons, so they are less reactive than the Group 1 metal in the same period.

Group 2 reactions

• With oxygen: 2Mg + O2 -> 2MgO (bright white flame).
• With chlorine: Ca + Cl2 -> CaCl2.
• With water: Mg reacts very slowly with cold water but quickly with steam: Mg + H2O(g) -> MgO + H2. From Ca down: Ca + 2H2O -> Ca(OH)2 + H2, faster down the group.
• Oxides and hydroxides are bases: MgO + 2HCl -> MgCl2 + H2O; Ca(OH)2 + 2HCl -> CaCl2 + 2H2O. Group 1 metals react with water to give MOH + H2, and their oxides and hydroxides neutralise acids too.

Solubility trends in Group 2

• Hydroxides become more soluble down the group: Mg(OH)2 is sparingly soluble (milk of magnesia, an antacid); Ba(OH)2 is quite soluble, giving a strongly alkaline solution.
• Sulfates become less soluble down the group: MgSO4 is soluble; BaSO4 is insoluble. This is used in the test for sulfate ions, and BaSO4 is used in barium meals for X-rays because it is insoluble and so not absorbed (soluble barium compounds are toxic).

Worked example

Write an equation for the reaction of strontium with water, and predict the pH of the solution.

1. Group 2 metals form a hydroxide and hydrogen.
2. Sr(OH)2 is fairly soluble.

Answer: Sr + 2H2O -> Sr(OH)2 + H2; the solution is alkaline, about pH 13

Check yourself

1. Which Group 2 metal reacts most vigorously with water?
   • Magnesium
   • Barium
   • Calcium
   • Strontium
   Show answer

   Barium. Reactivity increases down the group as ionisation energies fall.

2. What forms when magnesium reacts with steam?
   • Mg(OH)2 and H2
   • MgO and O2
   • Mg(OH)2 and O2
   • MgO and H2
   Show answer

   MgO and H2. With steam the oxide forms: Mg + H2O -> MgO + H2.

3. Which sulfate is least soluble in water?
   • MgSO4
   • BaSO4
   • CaSO4
   • SrSO4
   Show answer

   BaSO4. Sulfate solubility decreases down Group 2.

4. Which hydroxide is most soluble in water?
   • Mg(OH)2
   • Ca(OH)2
   • Ba(OH)2
   • Sr(OH)2
   Show answer

   Ba(OH)2. Hydroxide solubility increases down Group 2.

5. Why does reactivity increase down Group 2?
   • The outer electrons are further from the nucleus and more shielded, so they are lost more easily
   • The nuclear charge decreases
   • The atoms gain electrons more easily
   • The metals become less dense
   Show answer

   The outer electrons are further from the nucleus and more shielded, so they are lost more easily. Lower ionisation energies down the group.

6. Which equation is correct for calcium oxide with hydrochloric acid?
   • CaO + HCl -> CaCl + H2O
   • CaO + 2HCl -> CaCl2 + H2
   • CaO + 2HCl -> CaCl2 + H2O
   • CaO + 2HCl -> Ca(OH)2 + Cl2
   Show answer

   CaO + 2HCl -> CaCl2 + H2O. A basic oxide and an acid give a salt and water.

7. Why can barium sulfate be swallowed safely for an X-ray, when soluble barium compounds are toxic?
   • It is insoluble, so barium ions are not absorbed into the body
   • Barium sulfate is not a barium compound
   • It reacts with stomach acid to form water
   • It is a covalent molecule
   Show answer

   It is insoluble, so barium ions are not absorbed into the body. Insoluble BaSO4 passes through without releasing Ba2+ ions.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.