Lime water is a dilute aqueous solution of calcium hydroxide, Ca(OH)₂. The chemical formula of the dissolved solute is Ca(OH)₂. When lime water reacts with carbon dioxide (CO₂), it turns milky white due to the formation of calcium carbonate (CaCO₃) precipitate — this is the standard laboratory test for CO₂. The terms 'lime', 'slaked lime', and 'lime water' all refer to different forms of calcium compounds.
Lime water formula: Ca(OH)₂ (dilute aqueous solution of calcium hydroxide).
Lime water turns milky with CO₂: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O.
CaCO₃ (white precipitate) causes the milky appearance — standard CO₂ test.
Quicklime = CaO; Slaked lime = Ca(OH)₂; Lime water = dilute Ca(OH)₂ solution.
Ca(OH)₂ is a strong base with pH ~12–13.
Excess CO₂ clears the milky solution: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ (soluble).
Used for whitewashing, water treatment, soil treatment, and making mortar.
Bleaching powder is made from Ca(OH)₂ + Cl₂.
Lime water: • Definition: Dilute aqueous solution of calcium hydroxide • Chemical formula of solute: Ca(OH)₂ • Chemical name: Calcium hydroxide • Common name: Lime water (also: limewater) • Appearance: Clear, colourless solution • pH: approximately 12–13 (strongly basic)
Related calcium compounds (often confused): • Quicklime (CaO) — calcium oxide; formed by heating limestone CaCO₃ → CaO + CO₂ (at ~900°C)
• Slaked lime [Ca(OH)₂] — formed by adding water to quicklime CaO + H₂O → Ca(OH)₂ (slaking reaction — exothermic)
• Lime water — dilute solution of Ca(OH)₂ in water (Clear solution; excess Ca(OH)₂ settles as 'milk of lime')
The CO₂ test: CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l) Carbon dioxide + Lime water → Calcium carbonate + Water
Observation: Lime water turns milky/white → confirms presence of CO₂.
CaCO₃ is insoluble in water — it forms a white precipitate, making the solution appear milky.
If excess CO₂ is passed: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ Calcium carbonate dissolves (as soluble calcium bicarbonate) and the solution clears again.
Note: The milky white colour reappears if the cleared solution is boiled: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂↑
This explains why hard water (containing Ca(HCO₃)₂) leaves white deposits when boiled.
Properties of Ca(OH)₂: • White powder (slaked lime) or clear solution (lime water) • Strong base (alkali) • Slightly soluble in water (solubility decreases with temperature — unusual) • Reacts with CO₂: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O • Reacts with acids: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O
Uses of lime water / Ca(OH)₂:
Lime water is a dilute aqueous solution of calcium hydroxide. The chemical formula of the solute is Ca(OH)₂. Lime water is a clear, colourless, strongly alkaline solution.
When CO₂ is passed through lime water, it reacts with calcium hydroxide to form calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is insoluble and forms a white precipitate, making the solution appear milky. This is the standard test for CO₂.
Quicklime = CaO (calcium oxide, made by heating limestone). Slaked lime = Ca(OH)₂ (made by adding water to quicklime: CaO + H₂O → Ca(OH)₂). Lime water = dilute clear solution of Ca(OH)₂ in water. All three are calcium-based compounds used in construction and chemistry.
Initially lime water turns milky (CaCO₃ precipitate). But with excess CO₂, the milkiness clears: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ (soluble calcium bicarbonate). If this solution is boiled, it turns milky again as Ca(HCO₃)₂ decomposes back to CaCO₃.
Lime water (Ca(OH)₂ solution) is used for: (1) laboratory test for CO₂ (turns milky), (2) whitewashing walls, (3) making mortar (hardens by reaction with CO₂), (4) neutralising acidic soil in agriculture, (5) water treatment, and (6) manufacture of bleaching powder.
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