Oxides and Hydroxides in Groups 1 and 2

The Elements of Group 1 and 2

What are basic oxides?

Metal oxides that react with water to form hydroxides and react with acids to form salts and water.

How are Group 1 and Group 2 oxides classified?

They are basic oxides.

What happens when Group 1 and Group 2 oxides react with water?

They form alkaline solutions containing hydroxide ions, OH⁻.

What is the general equation for a Group 1 oxide reacting with water?

M₂O(s) + H₂O(l) → 2MOH(aq)

What is the general equation for a Group 2 oxide reacting with water?

MO(s) + H₂O(l) → M(OH)₂(aq)

What is the ionic equation for the reaction of Group 1 and Group 2 oxides with water?

O²⁻ + H₂O → 2OH⁻

Why are solutions formed when Group 1 and Group 2 oxides react with water alkaline?

Hydroxide ions, OH⁻, are formed.

What happens to the solubility of Group 2 hydroxides down the group?

Solubility increases down Group 2.

What happens to the alkalinity of Group 2 hydroxide solutions down the group?

Alkalinity increases down the group.

Why does the alkalinity of Group 2 hydroxide solutions increase down the group?

The hydroxides become more soluble, producing a greater concentration of OH⁻ ions.

Why does magnesium oxide produce a solution with relatively low alkalinity?

Magnesium hydroxide has very low solubility in water.

What is limewater?

A saturated aqueous solution of calcium hydroxide, Ca(OH)₂.

What is limewater used to test for?

Carbon dioxide.

What is the positive test for carbon dioxide using limewater?

The limewater turns cloudy or milky because a white precipitate forms.

What white precipitate forms when carbon dioxide is bubbled through limewater?

Calcium carbonate, CaCO₃.

What is the equation for carbon dioxide reacting with limewater?

CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

Why does limewater turn cloudy when carbon dioxide is added?

Insoluble calcium carbonate forms as a white precipitate.

What happens as more carbon dioxide is initially bubbled through limewater?

The amount of calcium carbonate precipitate increases.

Why should limewater not be left exposed to air before being used to test for carbon dioxide?

Carbon dioxide in the air can react with the limewater and form calcium carbonate, making the test unreliable.

What is milk of magnesia?

A suspension of magnesium hydroxide in water used as an antacid.

How does milk of magnesia relieve indigestion?

Magnesium hydroxide neutralises excess hydrochloric acid in the stomach.

What is the equation for magnesium hydroxide reacting with hydrochloric acid?

Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

Why is magnesium hydroxide suitable for use as an antacid?

Its very low solubility means the concentration of OH⁻ ions is low, reducing the risk of damage to human tissue.

How do Group 1 and Group 2 oxides and hydroxides react with acids?

They undergo neutralisation reactions to form a salt and water.

What type of reaction occurs between Group 1 or Group 2 oxides/hydroxides and acids?

Neutralisation.

What are the products when a metal oxide reacts with an acid?

A salt and water.

What are the products when a metal hydroxide reacts with an acid?

A salt and water.

What is the equation for sodium oxide reacting with sulfuric acid?

Na₂O + H₂SO₄ → Na₂SO₄ + H₂O

What is the equation for calcium oxide reacting with nitric acid?

CaO + 2HNO₃ → Ca(NO₃)₂ + H₂O

What is the equation for potassium hydroxide reacting with hydrochloric acid?

KOH + HCl → KCl + H₂O

What is the equation for barium hydroxide reacting with hydrochloric acid?

Ba(OH)₂ + 2HCl → BaCl₂ + 2H₂O

What observations are usually made when Group 1 or Group 2 oxides/hydroxides react with acids?

A white solid reacts to form a colourless solution.

Are reactions between Group 1 or Group 2 oxides/hydroxides and acids exothermic or endothermic?

Exothermic.

Why is lime used in agriculture?

To neutralise excess acidity in soil and improve crop yield.

What is agricultural lime mainly composed of?

Calcium hydroxide, obtained from limestone containing calcium carbonate.

What is the equation for calcium hydroxide neutralising nitric acid?

Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O

What is the solubility trend of Group 2 sulfates down the group?

Solubility decreases down Group 2.

Is magnesium sulfate soluble in water?

Yes, magnesium sulfate is soluble.

Is calcium sulfate soluble in water?

It is slightly soluble.

Are strontium sulfate and barium sulfate soluble in water?

No, they are insoluble.

What is the order of Group 2 sulfate solubility down the group?

MgSO₄ > CaSO₄ > SrSO₄ > BaSO₄.

Are Group 2 nitrates soluble?

Yes, all Group 2 nitrates are soluble.

Are Group 2 chlorides soluble?

Yes, all Group 2 chlorides are soluble.

How can sulfate ions in an aqueous solution be tested for?

Acidify the solution, then add a solution containing Ba²⁺ ions such as barium nitrate; a white precipitate indicates sulfate ions.

What is the positive result for the sulfate ion test?

A white precipitate of barium sulfate forms.

What is the ionic equation for the sulfate ion test?

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Why does barium sulfate form a precipitate in the sulfate test?

Barium sulfate is insoluble in water.

Which barium compounds can be used to test for sulfate ions?

Barium nitrate or barium chloride solution.

Why is acid added before testing for sulfate ions with Ba²⁺?

To remove interfering ions such as carbonate ions that could also form a white precipitate with Ba²⁺.

Which acid can be used when testing for sulfate ions using barium nitrate?

Dilute nitric acid.

How can sodium sulfate solution be tested for sulfate ions?

Add dilute nitric acid followed by barium nitrate solution; a white precipitate of BaSO₄ forms.

What is the equation for sodium sulfate reacting with barium nitrate?

Ba(NO₃)₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaNO₃(aq)

Why are soluble barium compounds dangerous?

Ba²⁺ ions are poisonous to humans.

What is a barium meal?

A suspension containing insoluble barium sulfate that can be used during X-ray imaging.

Why is barium sulfate safe to use in a barium meal despite barium ions being poisonous?

Barium sulfate is insoluble, so Ba²⁺ ions are not free to move or readily absorbed.

Why is barium sulfate useful for X-ray imaging?

It is a dense solid that shows up clearly on X-rays, allowing soft tissues to be seen more clearly.