(i) What is efflorescence. Give an example of efflorescent substance.
(ii) Name two compounds that are responsible for hardness of water
Answers
efflorescence (which means "to flower out" in French) is the migration of a salt to the surface of a porous material, where it forms a coating. The essential process involves the dissolving of an internally held salt in water, or occasionally in another solvent. The water, with the salt now held in solution, migrates to the surface, then evaporates, leaving a coating of the salt.
In what has been described as "primary efflorescence," the water is the invader and the salt was already present internally. Some people describe a reverse process, where the salt is originally present externally and is then carried inside in solution, as "secondary efflorescence." However, others would give this latter phenomenon another name entirely.
Efflorescences can occur in natural and built environments. On porous construction materials it may present a cosmetic outer problem only (primary efflorescence causing staining), but can sometimes indicate internal structural weakness (migration/degradation of component materials). Efflorescence may clog the pores of porous materials, resulting in the destruction of those materials by internal water pressure, as seen in the spalling of brick.
Efflorescence and hardness of water
i) • Efflorescence is the property of hydrated salts to lose their water of crystallisation on being exposed to atmosphere.
• Substances with this property are called efflorescent substances.
• Washing soda or sodium carbonate decahydrate (Na₂CO₃.10H₂O) is an example of an efflorescent substance.
• Washing soda crystals on being exposed to atmosphere, lose their water of crystallisation through evaporation and turn into a white powder.
ii) • Salts of calcium and magnesium are responsible for causing hardness in water.
• Hard water is the water which does not form lather with soap.
• Two compounds responsible for causing temporary hardness in water are Ca(HCO₃)₂ and Mg(HCO₃)₂.
• Temporary hardness can be removed by boiling the water.
• Two compounds responsible for causing permanent hardness in water are CaSO₄ and MgSO₄.
• Permanent hardness cannot be removed by simply boiling the water.