Unit 9: Properties & Reactions of s-block Metal Compounds

Exploring the chemistry of essential compounds formed from sodium and calcium.

9.25 Compounds of Sodium (NaCl, NaNO₃, Na₂SO₄, NaOH)

Compounds of s-block metals like sodium are typically white crystalline solids that form colorless solutions when dissolved in water. This is because the Na⁺ ion, like other s- and p-block metal ions, does not absorb visible light. All common sodium compounds are soluble in water.

  • Sodium Chloride (NaCl): Commonly known as table salt, this is a neutral salt formed from a strong acid (HCl) and a strong base (NaOH). It is essential for life and is used extensively as a food preservative and flavoring. Because it is cheap and non-toxic, it is also spread on roads in winter to lower the freezing point of water and prevent ice formation.
  • Sodium Nitrate (NaNO₃): Also known as Chile saltpeter, this neutral salt is a vital source of nitrate ions for plants. It is a key component of many agricultural fertilizers.
  • Sodium Sulfate (Na₂SO₄): A neutral salt primarily used as a filler and processing aid in powdered laundry detergents.
  • Sodium Hydroxide (NaOH): Commonly known as caustic soda or lye, this is a very strong base (alkali). It is highly corrosive and is used in many industrial processes, including the manufacture of soaps (through the saponification of fats), paper, and rayon.
Solved Examples:
  1. Why is sodium hydroxide considered a strong base?
    Solution: When dissolved in water, sodium hydroxide (NaOH) fully dissociates into its ions, Na⁺(aq) and OH⁻(aq). This complete dissociation releases a high concentration of hydroxide ions (OH⁻), which is the definition of a strong base.
  2. Explain how spreading sodium chloride on an icy road helps to melt the ice.
    Solution: Dissolving a solute like NaCl in water lowers its freezing point (a colligative property called freezing point depression). The salt dissolves in the thin layer of liquid water on the surface of the ice, creating a saltwater solution with a freezing point below 0 °C. This causes the surrounding ice to melt.
  3. What is the primary role of sodium nitrate in agriculture?
    Solution: Its primary role is to act as a fertilizer. It provides a soluble source of nitrate ions, which are essential for plants to synthesize proteins and DNA.
  4. What does the term "caustic" in "caustic soda" refer to?
    Solution: "Caustic" refers to the ability of the substance (sodium hydroxide) to burn or corrode organic tissue by chemical action. It highlights its highly corrosive nature.
  5. A student has a white, crystalline solid that dissolves in water to form a neutral solution. Could this solid be sodium hydroxide?
    Solution: No. While sodium hydroxide is a white, crystalline solid, it dissolves to form a strongly alkaline (basic) solution, not a neutral one. The solid is more likely to be a salt like NaCl or Na₂SO₄.

9.26 Compounds of Calcium (CaCl₂, CaSO₄, CaO, Ca(OH)₂)

Like sodium compounds, calcium compounds are typically white solids containing the colorless Ca²⁺ ion. However, their solubilities vary more widely.

  • Calcium Chloride (CaCl₂): A neutral salt that is highly soluble in water. It is even more effective than NaCl as a de-icer because it is highly hygroscopic (absorbs moisture from the air) and releases heat when it dissolves. It is also used as a drying agent in laboratories and to control dust on unpaved roads.
  • Calcium Sulfate (CaSO₄): This salt is only sparingly soluble in water. Its most common hydrated form is gypsum ($CaSO_4 \cdot 2H_2O$). When gypsum is heated, it loses some water to form plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$). When water is added back to plaster of Paris, it forms a paste that hardens back into solid gypsum, making it ideal for casts for broken bones and in construction as drywall.
  • Calcium Oxide (CaO): Commonly known as quicklime, this is a basic oxide. It is produced by the thermal decomposition of limestone ($CaCO_3$). It is used in huge quantities in industry to remove acidic impurities in steel manufacturing and to produce cement. It is also hygroscopic and can be used as a drying agent.
  • Calcium Hydroxide (Ca(OH)₂): Known as slaked lime, this is a base that is only slightly soluble in water. A saturated aqueous solution of calcium hydroxide is called limewater, which is famously used as the chemical test for carbon dioxide (it turns cloudy/milky as insoluble $CaCO_3$ is formed). It is also used in water treatment and to raise the pH of acidic soil.
Solved Examples:
  1. Write the balanced chemical equation for the reaction that occurs when limewater is used to test for carbon dioxide.
    Solution: $Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$. The formation of the white precipitate of calcium carbonate ($CaCO_3$) causes the cloudy appearance.
  2. What is the difference between quicklime and slaked lime?
    Solution: Quicklime is calcium oxide (CaO). Slaked lime is calcium hydroxide ($Ca(OH)_2$), which is formed when water is added to quicklime ($CaO + H_2O \rightarrow Ca(OH)_2$).
  3. Explain how plaster of Paris works to make a hard cast.
    Solution: Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$) is mixed with water to form a paste. The paste then undergoes a chemical reaction, re-hydrating to form an interlocking crystal structure of solid gypsum ($CaSO_4 \cdot 2H_2O$), which is hard and rigid.
  4. How is calcium oxide used in steel manufacturing?
    Solution: Calcium oxide is a basic oxide. It is added to the molten iron to react with and remove acidic impurities like silicon dioxide ($SiO_2$), forming molten slag ($CaSiO_3$), which can be separated.
  5. A farmer has acidic soil. Which of the compounds discussed would be most suitable to add to the soil to fix this problem?
    Solution: Calcium hydroxide (slaked lime) or calcium carbonate would be suitable. They are bases that will neutralize the excess acid in the soil, raising its pH to a level more suitable for crop growth.

Knowledge Check (20 Questions)

Answer: Table salt.

Answer: Sodium nitrate ($NaNO_3$).

Answer: CaO (Calcium Oxide).

Answer: Limewater is a saturated solution of calcium hydroxide ($Ca(OH)_2$), used to test for carbon dioxide.

Answer: Sodium hydroxide (NaOH).

Answer: The hydrated form of calcium sulfate, $CaSO_4 \cdot 2H_2O$.

Answer: As a drying agent or to control dust on roads.

Answer: Because the Na⁺ ion does not have electrons that can be excited by visible light.

Answer: The soap or paper industry.

Answer: It re-hydrates with water to form gypsum ($CaSO_4 \cdot 2H_2O$).

Answer: Neutral.

Answer: By the thermal decomposition of limestone ($CaCO_3$) in a kiln.

Answer: Calcium carbonate ($CaCO_3$).

Answer: Sodium compounds.

Answer: As a filler in powdered detergents.

Answer: It is highly hygroscopic and releases heat (exothermic dissolution) when it dissolves in water.

Answer: $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(s)$

Answer: Very high (typically > 13), as it is a strong base.

Answer: It is sparingly soluble in water and would not effectively lower the freezing point.

Answer: To neutralize acidic soil or in water treatment to adjust pH.
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