--- title: "Calcium Hydroxide (Ca(OH)2) — Gen Chem II Guide" description: "Calcium hydroxide is a strong base that dissociates in water to release OH- ions, raising pH. Learn its Kb behavior, solubility limits, and role in neutralization." canonical: "https://fiveable.me/general-chemistry-ii/key-terms/calcium-hydroxide-caoh2" type: "key-term" subject: "General Chemistry II" unit: "Unit 3" --- # Calcium Hydroxide (Ca(OH)2) — Gen Chem II Guide ## Definition Calcium hydroxide, Ca(OH)2 (slaked lime), is a strong base that dissociates fully in water to release Ca2+ and two OH- ions per formula unit, raising the pH and neutralizing acids, though its low solubility limits how concentrated a solution you can actually make. ## What It Is Calcium hydroxide, with the formula Ca(OH)2, is an inorganic compound also called slaked lime. You make it by reacting calcium oxide (CaO) with water. In aqueous solution it behaves as a **strong base**: whatever portion does dissolve dissociates completely into Ca2+ and two OH- ions, which is why it pushes the pH well above 7. Here's the catch that makes Ca(OH)2 interesting in Gen Chem II: it's only sparingly soluble, around 1.73 g per liter of water. So even though it's a *strong* base (complete dissociation), it's not a *concentrated* one. The amount that dissolves is governed by a solubility equilibrium, but the small portion in solution still acts as a strong electrolyte. Notice each formula unit releases two hydroxide ions, so [OH-] is roughly twice the molar concentration of dissolved Ca(OH)2. ## Why It Matters This term lives in Topic 3.2, acid and base strength with Ka and Kb. Ca(OH)2 is a clean example of a strong base, useful for showing that strength means *complete dissociation*, not high concentration. That distinction trips up a lot of people, and Ca(OH)2 is the perfect counterexample because it dissociates fully yet has low solubility. It also bridges into the solubility and equilibrium units later in the course. When you calculate the pH of a Ca(OH)2 solution, you combine strong-base behavior (full dissociation) with the fact that you must account for two OH- ions per formula unit. That two-for-one stoichiometry is a common spot to lose points. ## Connections ### Hydroxide Ion (OH-) (Unit 3) Ca(OH)2 is a source of OH-, and because each formula unit releases two of them, the hydroxide concentration is double the dissolved Ca(OH)2 concentration when you set up pH math. ### Neutralization (Unit 3) The OH- from calcium hydroxide reacts with H+ from acids to form water, which is why slaked lime is used to neutralize acidic spills and adjust pH in water treatment. ### Sodium Hydroxide (Unit 3) NaOH is another strong base, but it's far more soluble than Ca(OH)2. Comparing them shows that 'strong' (full dissociation) and 'concentrated' (lots of base in solution) are two different ideas. ### pH (Unit 3) Because Ca(OH)2 fully dissociates, you find [OH-], convert to pOH, then use pH = 14 - pOH to get the basic pH of the solution. ## On the AP Exam On problem sets and exams you'll most often calculate the pH or pOH of a Ca(OH)2 solution. The skill being tested is remembering to multiply the molar concentration by two before finding [OH-], then converting to pOH and pH. You may also see conceptual multiple-choice questions asking why Ca(OH)2 counts as a strong base despite low solubility, or short-answer prompts comparing it to NaOH or KOH. In lab, it shows up in titrations and in observing carbonation when it reacts with CO2 to form solid calcium carbonate. ## calcium hydroxide (Ca(OH)2) vs Sodium Hydroxide Both are strong bases that fully dissociate, but Ca(OH)2 releases two OH- per formula unit and is barely soluble (~1.73 g/L), while NaOH releases one OH- per formula unit and is extremely soluble. So NaOH solutions can reach much higher [OH-] and pH even though both are equally 'strong' by the dissociation definition. ## Key Takeaways - Calcium hydroxide, Ca(OH)2, is a strong base because the portion that dissolves dissociates completely into Ca2+ and OH-. - Each formula unit produces two OH- ions, so [OH-] is about twice the dissolved molar concentration of Ca(OH)2. - Strong does not mean concentrated: Ca(OH)2 has a low solubility of around 1.73 g/L, so its solutions are limited in how basic they can get. - To find pH, calculate [OH-], convert to pOH, then use pH = 14 - pOH. - Ca(OH)2 reacts with CO2 in air to form calcium carbonate, and it neutralizes acids by supplying hydroxide ions. ## FAQs ### What is calcium hydroxide and why is it a strong base? Calcium hydroxide, Ca(OH)2 (slaked lime), is a strong base because the part of it that dissolves dissociates completely into Ca2+ and two OH- ions. Those hydroxide ions raise the pH and neutralize acids. ### Is calcium hydroxide a strong base even though it barely dissolves? Yes. Strength refers to complete dissociation, not how much dissolves. Ca(OH)2 only dissolves about 1.73 g/L, but every dissolved unit splits fully into ions, so it's still classified as a strong base. ### How is calcium hydroxide different from sodium hydroxide? Both are strong bases, but Ca(OH)2 gives two OH- per formula unit and is poorly soluble, while NaOH gives one OH- and is highly soluble. That means NaOH can produce much more concentrated, higher-pH solutions. ### How do you calculate the pH of a calcium hydroxide solution? Multiply the molar concentration of Ca(OH)2 by two to get [OH-], take the negative log to get pOH, then subtract from 14: pH = 14 - pOH. The factor of two is the step people most often forget. ### What happens when calcium hydroxide reacts with carbon dioxide? It forms solid calcium carbonate (CaCO3) in a process called carbonation. This is why lime-based plaster and mortar harden as they take up CO2 from the air. ## Related Study Guides - [3.2 Acid and base strength, Ka and Kb](/general-chemistry-ii/unit-3/acid-base-strength-ka-kb/study-guide/gUCdGcU2iB8Ww2yF) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/general-chemistry-ii/key-terms/calcium-hydroxide-caoh2#resource","name":"Calcium Hydroxide (Ca(OH)2) — Gen Chem II Guide","url":"https://fiveable.me/general-chemistry-ii/key-terms/calcium-hydroxide-caoh2","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/general-chemistry-ii/key-terms/calcium-hydroxide-caoh2#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-06-29T17:36:59.662Z","isPartOf":{"@type":"Collection","name":"General Chemistry II Key Terms","url":"https://fiveable.me/general-chemistry-ii/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/general-chemistry-ii/key-terms/calcium-hydroxide-caoh2#term","name":"calcium hydroxide (Ca(OH)2)","description":"Calcium hydroxide, Ca(OH)2 (slaked lime), is a strong base that dissociates fully in water to release Ca2+ and two OH- ions per formula unit, raising the pH and neutralizing acids, though its low solubility limits how concentrated a solution you can actually make.","url":"https://fiveable.me/general-chemistry-ii/key-terms/calcium-hydroxide-caoh2","inDefinedTermSet":{"@type":"DefinedTermSet","name":"General Chemistry II Key Terms","url":"https://fiveable.me/general-chemistry-ii/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is calcium hydroxide and why is it a strong base?","acceptedAnswer":{"@type":"Answer","text":"Calcium hydroxide, Ca(OH)2 (slaked lime), is a strong base because the part of it that dissolves dissociates completely into Ca2+ and two OH- ions. Those hydroxide ions raise the pH and neutralize acids."}},{"@type":"Question","name":"Is calcium hydroxide a strong base even though it barely dissolves?","acceptedAnswer":{"@type":"Answer","text":"Yes. Strength refers to complete dissociation, not how much dissolves. Ca(OH)2 only dissolves about 1.73 g/L, but every dissolved unit splits fully into ions, so it's still classified as a strong base."}},{"@type":"Question","name":"How is calcium hydroxide different from sodium hydroxide?","acceptedAnswer":{"@type":"Answer","text":"Both are strong bases, but Ca(OH)2 gives two OH- per formula unit and is poorly soluble, while NaOH gives one OH- and is highly soluble. That means NaOH can produce much more concentrated, higher-pH solutions."}},{"@type":"Question","name":"How do you calculate the pH of a calcium hydroxide solution?","acceptedAnswer":{"@type":"Answer","text":"Multiply the molar concentration of Ca(OH)2 by two to get [OH-], take the negative log to get pOH, then subtract from 14: pH = 14 - pOH. The factor of two is the step people most often forget."}},{"@type":"Question","name":"What happens when calcium hydroxide reacts with carbon dioxide?","acceptedAnswer":{"@type":"Answer","text":"It forms solid calcium carbonate (CaCO3) in a process called carbonation. This is why lime-based plaster and mortar harden as they take up CO2 from the air."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"General Chemistry II","item":"https://fiveable.me/general-chemistry-ii"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/general-chemistry-ii/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 3","item":"https://fiveable.me/general-chemistry-ii/unit-3"},{"@type":"ListItem","position":4,"name":"calcium hydroxide (Ca(OH)2)"}]}]} ```