--- title: "Acids, Alkalis & pH" description: "Define acids and alkalis in terms of hydrogen ion concentration, describe the pH scale (0–14), and explain how indicators are used to identify and measure acidity or alkalinity" canonical: https://lightmysky.com/learn/science/acids-alkalis-and-ph-mt_AN2kJE6I0s source: https://lightmysky.com/learn/science/acids-alkalis-and-ph-mt_AN2kJE6I0s.md retrieved: 2026-10-19 --- > **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`. # Acids, Alkalis & pH Define acids and alkalis in terms of hydrogen ion concentration, describe the pH scale (0–14), and explain how indicators are used to identify and measure acidity or alkalinity Subject: Science · Area: Matter & Materials · Ages 12 to 13 Page: https://lightmysky.com/learn/science/acids-alkalis-and-ph-mt_AN2kJE6I0s ## Ready when they can - States that acids have pH below 7, alkalis have pH above 7, and neutral is pH 7 - Explains what hydrogen ions (H⁺) have to do with acidity - Names common indicators (litmus, universal indicator) and describes colour changes - Identifies everyday acids (vinegar, lemon juice) and alkalis (bleach, bicarbonate of soda) ## Lesson: Acids, Alkalis, and the pH Scale Chemists sort liquids into three groups: acids, alkalis, and neutral. What decides the group is how many hydrogen ions, written H⁺, are floating around in the liquid. A hydrogen atom that has lost its one electron becomes an H⁺ ion. Acids are substances that release H⁺ ions when they dissolve in water, and the more H⁺ ions crowded in, the more acidic the liquid is. Alkalis work the other way. They release hydroxide ions, OH⁻, which soak up H⁺ ions. *(drawing: The pH scale runs from 0 to 14. Below 7 is acid, above 7 is alkali, and 7 sits right in the middle as neutral, like pure water.)* The pH number tells you how strong an acid or alkali is. Low numbers mean lots of H⁺ ions: lemon juice sits around pH 2 and vinegar around pH 3. High numbers mean lots of OH⁻ ions: a solution of bicarbonate of soda is around pH 9, and bleach is higher still. Pure water, at exactly 7, is neither acid nor alkali. **Example.** Priya tests two kitchen liquids with universal indicator paper. She dips a strip into vinegar, and it turns red-orange, showing a pH of about 3, an acid. She dips a fresh strip into a solution of bicarbonate of soda, and it turns blue-green, showing a pH of about 9, an alkali. Mix an acid with an alkali and they cancel each other out. The H⁺ ions from the acid join the OH⁻ ions from the alkali to make plain water. Keep adding alkali to an acid and the mixture climbs the pH scale until it reaches green, pH 7: neutral. This balancing act is called neutralisation, and it is how indigestion tablets calm an over-acidic stomach. **Tip.** Indicators are dyes that change colour depending on the number of H⁺ ions around them. Litmus gives a simple acid-or-alkali answer: red for acid, blue for alkali. Universal indicator goes further and gives a different colour for every number on the pH scale. **Recap.** pH tells you how many H⁺ ions are in a liquid: below 7 is acid, above 7 is alkali, 7 is neutral. Indicators show you which by changing colour, and mixing acid with alkali can balance both out. *Adapted from OpenStax Chemistry (CC-BY 4.0), openstax.org* · [license](https://creativecommons.org/licenses/by/4.0/) ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Atoms, Elements & Compounds](https://lightmysky.com/learn/science/atoms-elements-and-compounds-mt_Z_Wu_77ybI) ## Opens up - [Coral Bleaching & Acidification](https://lightmysky.com/learn/science/coral-bleaching-and-acidification-mt_EXO1bJ3G_v) - [Acid Reactions & Salts](https://lightmysky.com/learn/science/acid-reactions-and-salts-mt_QKRYLwaOU8) - [Strong and Weak Acids and the pH Scale](https://lightmysky.com/learn/science/strong-and-weak-acids-and-the-ph-scale-mt_Y4nSn5XH56)