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CHEMISTRY · Stage 7 · Chapter 5: Acids and Alkalis

Chemistry 5.7 Chapter 5 Review: Acids and Alkalis

5 min read 7 parts Lesson 7 of 7

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5.7 Chapter 5 Review: Acids and Alkalis — Alhussan International Academy — Chemistry (Stages 7–9) notes: Revise acids, alkalis, pH, indicators, neutralisation, acid rain and gas tests from Chapter 5.Acids — they have pH below 7, taste sour, turn litmus red and contain hydrogen io…

  • Universal indicator
  • Measuring pH
  • Not all acids are dangerous
  • Making a cabbage indicator
  • Salt families
  • Dilution and neutralisation

Part 1 of 7Acids, alkalis and the pH scale

  • Acidsthey have pH below 7, taste sour, turn litmus red and contain hydrogen ions.
  • Alkalisthey have pH above 7, feel soapy, turn litmus blue and contain hydroxide ions.
  • Neutrala neutral solution has pH 7.
  • Universal indicatorits colour changes across the pH range: red, orange, yellow, green, blue, purple from acid to alkali.
  • Measuring pHa pH meter is more precise, and litmus only shows whether something is acidic or alkaline.
  • Safetyconcentrated or strong acids and alkalis are corrosive and need careful handling with eye protection.
  • Not all acids are dangerousmany food acids are safe.

Part 2 of 7Indicators and everyday pH

  • Making a cabbage indicatorheat a mixture of pure water and chopped cabbage in a beaker, filter the mixture and add the indicator to small amounts of acid and alkali.
  • Litmusit is red in acid, and adding enough alkali turns it purple at neutral and then blue.
  • Bloodit is slightly alkaline.
  • Teeththey are damaged when the pH of saliva falls to 5 or below.
  • Farminga farmer who wants to grow bananas on acidic soil should add an alkali.

Part 3 of 7Neutralisation and salts

  • Equation — acid + alkali → salt + water, and ionically H+ + OH− → H2O.
  • Salt families — hydrochloric acid gives chlorides, sulfuric acid sulfates and nitric acid nitrates.
  • Uses — antacids for indigestion, treating acid soils with lime, and making salts such as copper sulfate.
    • Copper sulfate is made by adding excess copper oxide to sulfuric acid, filtering and crystallising.
  • Titration — it finds how much acid neutralises a measured volume of alkali, using a burette and an indicator, and repeated runs give concordant results.
  • Dilution and neutralisation — dilution raises the pH only towards 7, whereas neutralisation can pass 7 if excess alkali is added.

Part 4 of 7Gases and tests

ReactionProductsTest
Acid + metalSalt + hydrogenLighted splint: squeaky pop
Acid + carbonateSalt + water + carbon dioxideLimewater turns cloudy
Acid + alkaliSalt + waterTemperature rise, pH to 7
  • Oxygen — it relights a glowing splint.

Part 5 of 7Acid rain and the environment

  • Formationsulfur dioxide and nitrogen oxides dissolve in rainwater, and cars as well as factories produce them.
  • Damageit harms lakes, trees and limestone buildings.
  • Reducing itremove sulfur from fuels, desulfurise flue gases, use catalytic converters and switch to cleaner energy.
  • Limingit neutralises damaged lakes and soils but treats the symptom rather than the cause.

Part 6 of 7The key idea

  • Acids contain hydrogen ions and alkalis contain hydroxide ions.
  • This explains the pH scale, indicator colours, reactions with metals and carbonates, and why neutralisation produces water.

Part 7 of 7Practical skills

  • Plan a fair test with independent, dependent and control variables.
  • Use correct units such as cm3 and write a gas test with its result.

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