--- title: "Acids, Alkalis & pH" description: "Explain acidity using hydrogen ion concentration in water-based solutions, read the usual pH scale from 0 to 14 at 25°C, and use indicators to estimate pH" 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: 2026-11-12 --- > **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`. # Acids, Alkalis & pH Explain acidity using hydrogen ion concentration in water-based solutions, read the usual pH scale from 0 to 14 at 25°C, and use indicators to estimate pH 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 at 25°C, acidic water-based solutions have pH below 7, alkaline solutions have pH above 7, and neutral solutions have pH 7 - Explains how hydrogen ion concentration relates to acidity and pH - Describes the changes of red and blue litmus and uses a universal indicator colour chart to estimate pH - Identifies vinegar and lemon juice as acidic, and bicarbonate of soda solution and household bleach as alkaline ## Lesson: Reading pH and testing solutions An ion is a particle with electric charge. Acids increase the concentration of hydrogen ions, written H⁺, in water. Concentration means the amount in a given volume. For dilute solutions at the same temperature, more H⁺ in that volume means lower pH. Alkalis are bases that dissolve in water and increase hydroxide ions, OH⁻. These can react with H⁺ to form water. *(drawing: At 25°C, below 7 is acidic, 7 is neutral and above 7 is alkaline. Some solutions have pH outside this usual range.)* This lesson uses water-based solutions at 25°C. A pH below 7 is acidic, 7 is neutral and above 7 is alkaline. Lemon juice is often around pH 2 and vinegar around pH 3. Bicarbonate of soda solution is mildly alkaline, often around pH 8 to 9. Household bleach is also alkaline. These values vary between samples. Low pH alone does not tell you whether an acid is strong or weak. Universal indicator estimates pH from colour. Use its supplied chart. Acids give red, orange or yellow; neutral is green; alkalis give blue or purple. An acid turns blue litmus red. An alkali turns red litmus blue. A neutral solution changes neither red nor blue litmus. One unchanged strip is not enough to identify a neutral solution. **Example.** Priya's indicator chart matches orange to pH 4 and blue to pH 10. Orange is acidic because 4 is below 7; blue is alkaline because 10 is above 7. Use a fresh strip for each sample. Test only samples supplied by a teacher and wear eye protection. Never taste samples or mix household cleaners, especially bleach with acids or other cleaners. During neutralisation, H⁺ and OH⁻ react to make water, H₂O. An acid reacting with an alkali also forms a salt, which may stay dissolved. Adding alkali can raise an acid's pH. Too little leaves the mixture acidic; excess alkali can make it alkaline. A green indicator reading at pH 7 shows a neutral mixture at 25°C. It does not show that the mixture is pure water. Use subtraction to compare positions on a pH scale. For example, pH 4 is 7 - 4 = 3 units below 7. A point equally far above 7 is 7 + 3 = 10. Equal distances describe positions on the scale. They do not tell you which amounts of acid and alkali will neutralise each other. **Recap.** Use pH and an indicator chart to classify water-based solutions. At 25°C, pH 7 is neutral. Neutralisation depends on the amounts mixed; excess acid or alkali can remain. *Lesson, questions and flashcards written for this dataset about pH, indicator tests and neutralisation.* ## 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)