Calculate The Solubility Of Ca Oh 2 In Water

Use this interactive calcium hydroxide solubility calculator to estimate molar solubility, grams per liter, grams dissolved in your sample volume, and equilibrium ion concentrations from Ksp. It supports pure water and common-ion conditions.

Expert Guide: How to Calculate the Solubility of Ca(OH)2 in Water

Calcium hydroxide, written as Ca(OH)2 and often called slaked lime, is a classic example used in equilibrium chemistry because it is only sparingly soluble in water. Even though it is not highly soluble, the small amount that does dissolve strongly affects solution chemistry by producing hydroxide ions, which makes the solution basic. If you want to calculate the solubility of Ca(OH)2 in water correctly, you need to understand the dissolution equation, the meaning of Ksp, and how common ions change the final answer.

When solid calcium hydroxide dissolves, it separates into calcium ions and hydroxide ions according to the balanced equilibrium:

Ca(OH)2(s) ⇌ Ca2+(aq) + 2OH-(aq)

This equation tells you the stoichiometric relationship: every 1 mole of dissolved Ca(OH)2 produces 1 mole of Ca2+ and 2 moles of OH-. That 1:1:2 ratio is the foundation of all the calculations. The system is governed by the solubility product constant, Ksp, which for this salt is written as:

Ksp = [Ca2+][OH-]^2

Because the pure solid does not appear in the equilibrium expression, Ksp depends only on the dissolved ion concentrations at equilibrium. In introductory chemistry, a commonly used room-temperature value is around 5.5 × 10^-6 at 25 C, though tabulated values vary somewhat by source, ionic strength, and measurement method. That variation is normal in real chemical data, and your instructor or laboratory manual may specify a preferred value.

What “solubility” means here

In this context, solubility usually means the molar solubility, often symbolized by s. Molar solubility is the number of moles of Ca(OH)2 that dissolve per liter of solution at equilibrium. Once you know molar solubility, you can convert it to grams per liter using the molar mass of calcium hydroxide, which is approximately 74.092 g/mol.

• Molar solubility: mol/L of Ca(OH)2 that dissolves
• Mass solubility: grams of Ca(OH)2 dissolved per liter
• Equilibrium [Ca2+]: often equal to s in pure water
• Equilibrium [OH-]: often equal to 2s in pure water

Pure water calculation: the standard shortcut

If calcium hydroxide dissolves in pure water with no initial calcium or hydroxide already present, let the molar solubility be s. Then at equilibrium:

• [Ca2+] = s
• [OH-] = 2s

Substitute those into the Ksp expression:

Ksp = (s)(2s)^2 = 4s^3

So the pure-water solubility formula becomes:

s = (Ksp / 4)^(1/3)

Using Ksp = 5.5 × 10^-6:

s = (5.5 × 10^-6 / 4)^(1/3) ≈ 0.0111 mol/L

Now convert that to grams per liter:

0.0111 mol/L × 74.092 g/mol ≈ 0.82 g/L

This means that at about 25 C, a saturated solution of calcium hydroxide in pure water contains roughly 0.82 grams per liter dissolved, based on the selected Ksp value. That is why limewater is only mildly concentrated in terms of dissolved solid, even though it is quite basic because hydroxide ions are produced efficiently.

Key idea: calcium hydroxide has low mass solubility, but because each dissolved formula unit creates two hydroxide ions, even a small dissolved amount can create a strongly basic solution.

Exact equilibrium when common ions are present

The simple formula above only works for pure water. In many practical situations, the water already contains calcium ions, hydroxide ions, or both. For example, if NaOH is present, the initial hydroxide concentration is not zero. If CaCl2 is present, the initial calcium concentration is not zero. In these cases, the common ion effect suppresses the solubility of Ca(OH)2, sometimes dramatically.

Let the initial concentrations be:

• Initial [Ca2+] = C
• Initial [OH-] = H

If an additional amount s of Ca(OH)2 dissolves, then:

• [Ca2+] = C + s
• [OH-] = H + 2s

The equilibrium condition becomes:

Ksp = (C + s)(H + 2s)^2

This is no longer a simple 4s^3 shortcut. It is an exact cubic-type equilibrium relation, and in real calculator tools it is usually solved numerically. That is what the calculator above does in auto mode. This approach is especially useful when the common-ion concentrations are not tiny relative to the final solubility.

Worked example in pure water

1. Write the dissolution equation: Ca(OH)2(s) ⇌ Ca2+ + 2OH-
2. Write the Ksp expression: Ksp = [Ca2+][OH-]^2
3. Let s = molar solubility in pure water
4. Substitute [Ca2+] = s and [OH-] = 2s
5. Solve Ksp = 4s^3
6. Convert mol/L to g/L if needed

With Ksp = 5.5 × 10^-6:

• s ≈ 0.0111 mol/L
• [Ca2+] ≈ 0.0111 M
• [OH-] ≈ 0.0222 M
• Mass solubility ≈ 0.82 g/L

Worked example with added hydroxide

Suppose the solution already contains 0.010 M OH- from another base. Then:

• [Ca2+] = s
• [OH-] = 0.010 + 2s

Now solve:

5.5 × 10^-6 = s(0.010 + 2s)^2

The solution for s is lower than in pure water because hydroxide is already present. The system needs less additional dissolved Ca(OH)2 to satisfy Ksp. This is the classic common-ion effect. The exact answer depends on solving the equation numerically, which the calculator performs automatically.

Comparison table: stoichiometric relationships and formulas

Scenario	Equilibrium concentrations	Equation to solve	Best method
Pure water	[Ca2+] = s, [OH-] = 2s	Ksp = 4s^3	Direct cube-root formula
Initial Ca2+ present	[Ca2+] = C + s, [OH-] = 2s	Ksp = (C + s)(2s)^2	Numerical or careful approximation
Initial OH- present	[Ca2+] = s, [OH-] = H + 2s	Ksp = s(H + 2s)^2	Numerical or approximation if H is large
Both ions initially present	[Ca2+] = C + s, [OH-] = H + 2s	Ksp = (C + s)(H + 2s)^2	Numerical solution recommended

How temperature affects Ca(OH)2 solubility

One interesting feature of calcium hydroxide is that its solubility generally decreases as temperature rises over common instructional ranges. This behavior differs from many salts that become more soluble at higher temperature. As a result, cold water can hold a bit more dissolved Ca(OH)2 than warm water. If you are comparing calculations at different temperatures, you must use a Ksp value appropriate for the chosen temperature.

Temperature	Representative Ksp	Approximate molar solubility, s	Approximate mass solubility
0 C	9.0 × 10^-6	0.0131 M	0.97 g/L
20 C	6.0 × 10^-6	0.0114 M	0.85 g/L
25 C	5.5 × 10^-6	0.0111 M	0.82 g/L
40 C	4.1 × 10^-6	0.0101 M	0.75 g/L

These values are suitable for educational calculations and align with the trend commonly reported in chemistry references. Exact values can differ slightly across data compilations.

Converting between molar solubility and grams

Students often compute the molar solubility correctly but forget to convert the answer into the units required by the problem. Here are the standard conversions:

• g/L = (mol/L) × 74.092 g/mol
• grams dissolved in sample = (g/L) × sample volume in liters
• [Ca2+]eq = C + s
• [OH-]eq = H + 2s

For a 500 mL sample of saturated limewater at 25 C in pure water, using about 0.82 g/L:

Mass dissolved = 0.82 g/L × 0.500 L ≈ 0.41 g

Common mistakes to avoid

• Forgetting that each mole of Ca(OH)2 gives 2 moles of OH-
• Using Ksp = s^2 or Ksp = s^3 instead of 4s^3 in pure water
• Ignoring initial ion concentrations when common ions are present
• Using the wrong Ksp for the temperature
• Reporting mol/L when the question asks for g/L or total grams

Why Ca(OH)2 matters in real applications

Calcium hydroxide is important in water treatment, environmental chemistry, agriculture, construction materials, and laboratory analysis. In water treatment, it is used to adjust pH, soften water, and help remove impurities. In environmental systems, lime affects alkalinity and can neutralize acidity. In the lab, saturated limewater is a familiar reagent for demonstrating carbon dioxide because CO2 reacts to form insoluble calcium carbonate, causing a milky appearance.

Because solubility limits how much dissolved Ca(OH)2 is available, accurate calculation matters. In process design, using too high a dissolved concentration can overestimate alkalinity delivery. In educational settings, understanding the Ksp-based calculation builds the foundation for more advanced topics such as selective precipitation, ionic strength effects, and pH-coupled equilibria.

Authoritative references for further reading

If you want to verify data or explore related water chemistry concepts, these are strong starting points:

• U.S. Environmental Protection Agency water quality resources
• NIST Chemistry WebBook
• Chemistry LibreTexts educational resources

Final takeaway

To calculate the solubility of Ca(OH)2 in water, begin with the dissolution equation and Ksp expression. In pure water, use the shortcut s = (Ksp/4)^(1/3). If calcium or hydroxide ions are already present, solve the exact equilibrium relation Ksp = (C + s)(H + 2s)^2. Then convert from molar solubility to grams per liter or grams in your sample volume as needed. The calculator above handles both the simple classroom version and the more realistic common-ion case, making it useful for homework, lab prep, and quick chemistry checks.