Lime Water Formula: Chemical Name, Properties & Complete Formula Guide for Students — Grade 8 to 12

What Is Lime Water?

Lime water is a clear, colourless aqueous solution made by dissolving calcium hydroxide [Ca(OH)₂] in water. It is prepared by adding excess calcium hydroxide to distilled water, filtering out the undissolved solid, and collecting the clear solution.

In simple terms: lime water = water + dissolved Ca(OH)₂.

Point: Lime water is NOT the same as lemon/lime juice. The word “lime” here refers to calcium compounds, not the fruit.

The Chemical Formula of Lime Water

The chemical formula of lime water is Ca(OH)₂ dissolved in H₂O.

When written in solution form: Ca(OH)₂ (aq)

This means calcium hydroxide is the solute, and water (H₂O) is the solvent. The solution itself does not have a single molecular formula since it is a mixture, but the active compound the one responsible for all chemical properties is:

Name	Formula
Lime Water (solution)	Ca(OH)2 (aq)
Solute (Calcium Hydroxide)	Ca(OH)2
Solvent (Water)	H2O

Breaking Down the Formula: Ca(OH)₂

• Ca = Calcium (atomic number 20, alkaline earth metal)
• O = Oxygen
• H = Hydrogen
• (OH)₂ = Two hydroxide ions, each carrying a -1 charge
• Overall charge of Ca²⁺ + 2(OH⁻) = neutral compound

Chemical Name of Lime Water

The chemical name of lime water (and of the active compound in it) is Calcium Hydroxide.

Other accepted chemical names used in textbooks:

• Calcium Dihydroxide
• Slaked Lime (common/industrial name)
• Hydrated Lime (used in construction)
• Caustic Lime (older terminology)

Tip: If asked “What is the chemical name of lime water?”, always write Calcium Hydroxide. If asked for the formula, write Ca(OH)₂.

Difference between Quicklime vs. Slaked Lime vs. Lime Water

One of the most common sources of confusion in chemistry is distinguishing between quicklime, slaked lime, and lime water. Here is a clear comparison:

Compound	Common Name	Chemical Formula	Appearance	How It Is Made
Calcium Oxide	Quicklime	CaO	White solid powder	By heating limestone (CaCO3) at high temp
Calcium Hydroxide	Slaked Lime	Ca(OH)2	White solid powder	CaO + H2O → Ca(OH)2
Calcium Hydroxide Solution	Lime Water	Ca(OH)2 (aq)	Clear colourless liquid	Dissolving Ca(OH)2 in water, then filtering
Calcium Carbonate	Limestone / Chalk	CaCO3	White solid rock	Natural mineral
Calcium Bicarbonate	Calcium Hydrogen Carbonate	Ca(HCO3)2	Only in solution	Lime water + excess CO2

Formula Table to Related all Lime Formulas

This is the master reference table. It covers every formula related to lime water that appears in school and university exams:

Formula Name	Formula	Explanation	Variables	Example Use Case
Lime Water	Ca(OH)2 (aq)	Aqueous solution of calcium hydroxide	Ca=Calcium, O=Oxygen, H=Hydrogen	Testing for CO2 gas in lab
Slaked Lime Formula	Ca(OH)2	Solid calcium hydroxide powder	Same as above	Used in whitewash, mortar
Quicklime Formula	CaO	Calcium oxide — anhydrous base	Ca=Calcium, O=Oxygen	Industrial cement production
Slaking of Lime	CaO + H2O → Ca(OH)2	Reaction of quicklime with water	Exothermic reaction, releases heat	Making slaked lime from quicklime
Limestone Formula	CaCO3	Calcium carbonate mineral	Ca, C, O atoms	Chalk, marble, limestone rock
Calcination of Limestone	CaCO3 → CaO + CO2	Heating limestone to form quicklime	High temp required (>840°C)	Cement and lime kiln industry
Lime Water + CO2 (milky)	Ca(OH)2 + CO2 → CaCO3 + H2O	Turns lime water milky/cloudy	CaCO3 = white precipitate	Standard CO2 lab test
Lime Water + Excess CO2	CaCO3 + CO2 + H2O → Ca(HCO3)2	Milky solution clears again	Soluble Ca(HCO3)2 forms	Explains clearing of milky lime water
Lime Water + HCl	Ca(OH)2 + 2HCl → CaCl2 + 2H2O	Neutralization reaction	CaCl2 = calcium chloride salt	Acid-base neutralization problems
Lime Water + H2SO4	Ca(OH)2 + H2SO4 → CaSO4 + 2H2O	Reaction with sulfuric acid	CaSO4 = gypsum/calcium sulfate	Desulfurization of flue gas
Lime Water + SO2	Ca(OH)2 + SO2 → CaSO3 + H2O	Reaction with sulfur dioxide	CaSO3 = calcium sulfite	Pollution control / flue gas cleaning
Lime Water + Cl2	2Ca(OH)2 + 2Cl2 → Ca(OCl)2 + CaCl2 + 2H2O	Forms bleaching powder	Ca(OCl)2 = calcium hypochlorite	Bleaching powder preparation
Dissociation in Water	Ca(OH)2 → Ca2+ + 2OH-	Ionization in aqueous solution	Partial dissociation (slightly soluble)	Explains basic nature of lime water
Calcium Bicarbonate	Ca(HCO3)2	Soluble compound in lime water + excess CO2	HCO3- = bicarbonate ion	Temporary hardness of water
Limewater pH Formula	pH = 14 + log[OH-]	Calculating basicity of lime water	[OH-] = hydroxide ion concentration	Calculating pH of 0.02M Ca(OH)2 soln

Step by Step Formulas How Lime Water Is Prepared 

Step 1: Obtain Calcium Hydroxide (Slaked Lime)

Start with quicklime (CaO). Add water to slake it:

CaO + H2O → Ca(OH)2 [Slaking Reaction — exothermic]

Step 2: Dissolve in Water

Add the calcium hydroxide powder to distilled water. Ca(OH)₂ is only slightly soluble (~1.5 g/L at 25°C). Only a small amount dissolves.

Ca(OH)2 (s) ⇌ Ca2+ (aq) + 2OH- (aq) [Dissolution equilibrium]

Step 3: Filter

Filter the mixture to remove the undissolved Ca(OH)₂. The clear filtrate is lime water.

Step 4: Verify

Bubble CO₂ gas through the clear solution. If it turns milky, lime water is confirmed.

Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l) [Milky = CaCO3 precipitate]

Is Lime Water Acidic or Basic?

Lime water is BASIC (alkaline) in nature. This is one of the most commonly tested exam facts.

Why Lime Water Is Basic – The Scientific Explanation

When calcium hydroxide dissolves in water, it dissociates to release hydroxide ions (OH⁻):

Ca(OH)2 → Ca2+ + 2OH-

The presence of OH⁻ ions makes the solution basic. The pH of lime water is approximately 12.4 (for a saturated solution at 25°C), well above 7.

Property	Value / Description
Nature of Lime Water	Basic (Alkaline)
pH of Lime Water (saturated)	~12.4 at 25°C
pH of Dilute Lime Water	Between 8 and 12, depending on concentration
Ions Responsible for Basicity	OH- (Hydroxide ions)
Indicator Colour (Litmus)	Turns red litmus paper BLUE
Indicator Colour (Phenolphthalein)	Turns PINK/MAGENTA
Reaction with Acids	Neutralisation forms salt and water
Reaction with CO2	Turns milky forms CaCO3 precipitate

Quick Summary of Acid/Base

• Lime water + acid → salt + water (neutralisation)
• Lime water + CO₂ → milky/white precipitate (CaCO₃)
• Lime water turns red litmus blue (confirms alkaline nature)
• Lime water does NOT turn blue litmus red (it is not acidic)

Reactions of Lime Water — Important Chemical Equations

Reaction 1: With Carbon Dioxide (CO₂) — The Most Important Test

Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)Result: Lime water turns MILKY (white precipitate of CaCO3 forms)

Reaction 2: With Excess CO₂ — Clearing of Milky Solution

CaCO3 (s) + CO2 (g) + H2O (l) → Ca(HCO3)2 (aq)Result: Milky solution turns CLEAR again (soluble calcium bicarbonate forms)

Reaction 3: With Hydrochloric Acid (HCl)

Ca(OH)2 (aq) + 2HCl (aq) → CaCl2 (aq) + 2H2O (l)

Reaction 4: With Sulfuric Acid (H₂SO₄)

Ca(OH)2 (aq) + H2SO4 (aq) → CaSO4 (s) + 2H2O (l)Note: CaSO4 (gypsum) is slightly soluble — may form a precipitate

Reaction 5: With Sulfur Dioxide (SO₂)

Ca(OH)2 (aq) + SO2 (g) → CaSO3 (s) + H2O (l)Application: Removing SO2 from industrial flue gases to reduce acid rain

Reaction 6: With Chlorine Gas (Cl₂) — Making Bleaching Powder

2Ca(OH)2 + 2Cl2 → Ca(OCl)2 + CaCl2 + 2H2OProduct: Bleaching powder [Ca(OCl)2 + CaCl2 mixture]

Reaction 7: With Ammonium Salts

Ca(OH)2 + 2NH4Cl → CaCl2 + 2NH3 + 2H2OApplication: Lab preparation of ammonia gas

Solved Examples

Question: What is the chemical formula of lime water?

Solution:

• Lime water is an aqueous solution of calcium hydroxide.
• Calcium is in Group 2 of the periodic table → valency = 2 (Ca²⁺)
• Hydroxide ion = OH⁻ (valency = 1)
• To balance: Ca²⁺ needs two OH⁻ ions → Ca(OH)₂

Answer: Ca(OH)₂ (aq) Chemical Name: Calcium Hydroxide

Question: When CO₂ is passed through lime water, what happens? Write the equation.

Solution:

Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)Observation: Lime water turns milky/white because insoluble CaCO3 precipitate forms.

Question: If excess CO₂ is passed through milky lime water, what do you observe?

Solution:

CaCO3 (s) + CO2 (g) + H2O (l) → Ca(HCO3)2 (aq)Observation: The milky solution turns CLEAR again because soluble Ca(HCO3)2 is formed.

Question: The concentration of OH⁻ in lime water is 0.02 M. Calculate the pH.

Solution:

• pOH = -log[OH⁻] = -log(0.02) = -log(2 × 10⁻²) = 2 – log2 = 2 – 0.301 = 1.699
• pH = 14 – pOH = 14 – 1.699 = 12.3

Answer: pH ≈ 12.3 (strongly basic)

Question: Write and balance the reaction between lime water and dilute HCl.

Solution:

Ca(OH)2 + 2HCl → CaCl2 + 2H2OCheck: Ca: 1=1 ✓ O: 2=2 ✓ H: 4=4 ✓ Cl: 2=2 ✓ (Balanced)

Common Student Mistakes

Mistake	Incorrect Version	Correct Version	Why Students Make This Error
Confusing quicklime and slaked lime	Ca(OH)2 = quicklime	CaO = quicklime; Ca(OH)2 = slaked lime	Both are ‘lime’ compounds
Writing formula without brackets	CaOH2	Ca(OH)2	Brackets show two OH groups as a unit
Thinking lime water is acidic	Lime water is acidic	Lime water is BASIC (pH ~12.4)	Confusing ‘lime’ fruit with lime the mineral
Forgetting (aq) in solution formula	Ca(OH)2	Ca(OH)2 (aq) for lime water	Not distinguishing solid from solution
CO2 test — wrong product stated	Lime water + CO2 → CaO + H2O	Lime water + CO2 → CaCO3 + H2O	Mixing up reactants and products
Not writing 2 in 2HCl	Ca(OH)2 + HCl → CaCl + H2O	Ca(OH)2 + 2HCl → CaCl2 + 2H2O	Forgetting to balance the equation

Memory Tricks & Tips

Trick 1: The Lime Ladder

Remember the progression from rock to solution:

• CaCO₃ (Limestone) — ROCK (calcium carbonate)
• CaO (Quicklime) — HEAT IT — calcium oxide
• Ca(OH)₂ (Slaked Lime) — ADD WATER — calcium hydroxide (solid)
• Ca(OH)₂ (aq) (Lime Water) — DISSOLVE IN WATER — aqueous solution

Trick 2: The CO₂ Test Mnemonic

“Lime goes MILKY with CO₂… then CLEARS with excess.”

• Milky = CaCO₃ forms (white precipitate)
• Clears = Ca(HCO₃)₂ forms (soluble — dissolves the precipitate)

Trick 3: Remembering the Formula Ca(OH)₂

“Calcium Always Holds 2 OHs” — Ca always pairs with two hydroxide groups because its valency is +2.

Trick 4: Basic or Acidic?

“Lime = Alkaline” — both words have the letter ‘A’. Lime water is always Alkaline.

Trick 5: Writing the Correct Formula

CaO = no H (no water yet) → quicklime. Ca(OH)₂ = H is present (water was added) → slaked lime / lime water.

Frequently Asked Questions about Lime Water Formula

Q. What is the chemical formula of lime water?

The chemical formula of lime water is Ca(OH)₂. Lime water is an aqueous solution of calcium hydroxide. The solute is Ca(OH)₂ (calcium hydroxide), and the solvent is H₂O (water). In solution notation, it is written as Ca(OH)₂ (aq).

Q. What is the chemical name of lime water?

The chemical name of lime water is Calcium Hydroxide. Its other accepted names include calcium dihydroxide, slaked lime, hydrated lime, and caustic lime. The IUPAC name is calcium dihydroxide. In everyday academic and exam contexts, calcium hydroxide is the standard accepted answer.

Q. Is lime water acidic or basic?

Lime water is basic (alkaline), not acidic. When Ca(OH)₂ dissolves in water, it releases hydroxide ions (OH⁻), which increase the pH above 7. The pH of saturated lime water is approximately 12.4 at 25°C. It turns red litmus paper blue, confirming its basic nature.

Q. What is the formula of slaked lime water?

Slaked lime has the formula Ca(OH)₂. It is a solid white powder formed when quicklime (CaO) reacts with water: CaO + H₂O → Ca(OH)₂. When dissolved in water, this solid becomes lime water. The formula remains Ca(OH)₂ whether in solid (slaked lime) or dissolved (lime water) form.

Q. What is quicklime formula vs. slaked lime formula?

Quicklime is CaO (calcium oxide) — a white solid formed by heating limestone (CaCO₃). Slaked lime is Ca(OH)₂ (calcium hydroxide) formed when water is added to quicklime: CaO + H₂O → Ca(OH)₂. Lime water is simply slaked lime dissolved in water, giving Ca(OH)₂ (aq).

Q. Why does lime water turn milky when CO₂ is passed through it?

Lime water turns milky because CO₂ reacts with Ca(OH)₂ to form calcium carbonate (CaCO₃), which is an insoluble white solid: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The white precipitate of CaCO₃ makes the solution appear cloudy or milky. This reaction is used as the standard lab test for CO₂.

Q. What happens when excess CO₂ is bubbled into milky lime water?

When excess CO₂ is passed, the milky lime water clears again. This is because the insoluble CaCO₃ reacts with more CO₂ and water to form soluble calcium bicarbonate: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. Ca(HCO₃)₂ is soluble, so the white precipitate dissolves and the solution becomes clear again.

Q. What is the molar mass of Ca(OH)₂?

The molar mass of Ca(OH)₂ is 74 g/mol, calculated as: Ca = 40 g/mol, O = 16 g/mol × 2 = 32 g/mol, H = 1 g/mol × 2 = 2 g/mol. Total = 40 + 32 + 2 = 74 g/mol. This is used in stoichiometry calculations involving lime water reactions.

Conclusion

Understanding the lime water formula is not just about memorising Ca(OH)₂. It is about grasping the entire lime chemistry family from limestone (CaCO₃) all the way to lime water (Ca(OH)₂ aq) — and knowing how each compound transforms into the next.

Here is a final summary of the most important points:

Concept	Answer
Chemical formula of lime water	Ca(OH)2 (aq)
Chemical name of lime water	Calcium Hydroxide
Lime water — acidic or basic?	BASIC (alkaline), pH ~12.4
Quicklime formula	CaO
Slaked lime formula	Ca(OH)2
Limestone formula	CaCO3
CO2 test with lime water	Turns MILKY → Ca(OH)2 + CO2 → CaCO3 + H2O
Excess CO2 with lime water	Turns CLEAR again → CaCO3 + CO2 + H2O → Ca(HCO3)2
Molar mass of Ca(OH)2	74 g/mol

Chemistry becomes much less scary when you understand the “why” behind each formula. Lime water is one of the most elegant examples of how a simple compound calcium hydroxide connects geology (limestone), industry (cement), environmental science (CO₂ testing), and everyday chemistry (pH and neutralisation).

You now have every formula, reaction, and concept you need to tackle any exam question on lime water with confidence. Keep practising the equations, and you will find they become second nature.

Remember: Ca(OH)2 is lime water. It is BASIC. It turns milky with CO2. And it forms the bridge between quicklime (CaO) and limestone (CaCO3). Master this family of compounds, and lime water questions will never trip you up again.