--- title: "pH, Kw and Strong Acids and Bases" description: "pH is a way of writing a hydrogen ion concentration as a small number using powers of ten. Water ionises a little itself, and that constant links the two ions, which is what makes the pH of an alkali " canonical: https://lightmysky.com/learn/science/ph-kw-and-strong-acids-and-bases-mt_KJ4yc25Z_l source: https://lightmysky.com/learn/science/ph-kw-and-strong-acids-and-bases-mt_KJ4yc25Z_l.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # pH, Kw and Strong Acids and Bases pH is a way of writing a hydrogen ion concentration as a small number using powers of ten. Water ionises a little itself, and that constant links the two ions, which is what makes the pH of an alkali calculable. Subject: Science · Area: Chemistry · Ages 17 to 18 Page: https://lightmysky.com/learn/science/ph-kw-and-strong-acids-and-bases-mt_KJ4yc25Z_l ## Ready when they can - Calculates pH from the concentration of a strong acid and works backwards from a pH to a concentration - Explains why a tenfold dilution changes pH by exactly one unit - Uses the ionic product of water to get the hydrogen ion concentration in a strong alkali - Explains why Kw changes with temperature and what that does to the pH of pure water ## Lesson: pH, Kw and strong alkalis pH is a compact way to write a hydrogen ion concentration: pH = -log[H+]. A 0.01 mol per cubic decimetre HCl solution has pH 2, and working backwards, a pH of 3 means [H+] of 0.001 mol per cubic decimetre. Dilute an acid tenfold and [H+] drops to a tenth of what it was. The log scale turns that tenth into a rise of exactly one pH unit, which makes dilution maths quick. **Example.** Water itself gives Kw = [H+] times [OH-] = 1.0 x 10-14 at 25 C. In 0.10 mol per cubic decimetre NaOH, [OH-] is 0.10, so [H+] is 1.0 x 10-13 and the pH works out as 13. **Tip.** Kw grows as water warms, so pure water at 50 degrees has a pH below 7. It is still neutral, because [H+] and [OH-] stay equal to each other. **Recap.** Logs compress concentrations to pH, Kw links the two ions, and warm pure water sits below pH 7 yet stays neutral. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Kp, Partial Pressures and Gas Equilibria](https://lightmysky.com/learn/science/kp-partial-pressures-and-gas-equilibria-mt_1R8BpA2hfK) - [The Equilibrium Constant Kc](https://lightmysky.com/learn/science/the-equilibrium-constant-kc-mt_3G8b6Hyz3u) - [Standard Form](https://lightmysky.com/learn/mathematics/standard-form-mt_GbZZ2xjBTt) - [Zero and Negative Indices](https://lightmysky.com/learn/mathematics/zero-and-negative-indices-mt_jAtXBSVqkj) - [Logarithms as the Inverse of Exponentials](https://lightmysky.com/learn/mathematics/logarithms-as-the-inverse-of-exponentials-mt_skLQLUan0T) - [Strong and Weak Acids and the pH Scale](https://lightmysky.com/learn/science/strong-and-weak-acids-and-the-ph-scale-mt_Y4nSn5XH56) ## Opens up - [Amines and Amides: Basicity and the Nitrogen Functional Groups](https://lightmysky.com/learn/science/amines-and-amides-basicity-and-the-nitrogen-functional-groups-mt_cGP3ZJ8S6h) - [Weak Acids, Ka and pKa](https://lightmysky.com/learn/science/weak-acids-ka-and-pka-mt_q7fYb7qzMH)