Lss acids and alkalis

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Lower Secondary Science Chemistry - Acids & Alkalis

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Lss acids and alkalis

  1. 1. Acids and Alkalis
  2. 2. Lesson Objectives By the end of the lesson, students sbat: •Describe acids and alkalis – Meaning of an acid and examples – Meaning of an alkali and examples •State the properties of acids and alkalis •State the reaction of acid with metal to produce hydrogen gas •State the reaction of acid with carbonate to produce carbon dioxide gas.
  3. 3. Lesson Objectives • State the reaction of acid with alkali to produce salt and water. • State the uses of acids and alkalis in daily life • Describe indicators – Litmus indicator and simple test for acids and alkalis – Universal indicator (idea of pH) – pH meter and pH sensor
  4. 4. Lesson Objectives • Show an understanding that indicators are substances that change colour when an acid or alkali is added to them.
  5. 5. Recap • Mixtures can be classified as solutions and suspensions. • The differences between a solution and a suspension are: Properties Formation Appearance Homgeneous When left to stand When filtered Examples Solutions A solute dissolves in the solvent Suspensions Insoluble solids mixed with a liquid / gas Clear, transparent Cloudy, opaque Yes No Solute particles do not separate from Solid particles sink to solvent. bottom No residue is left on filter paper Residue is left on filter paper Salt dissolved in water Chalk in water
  6. 6. Acids • An acid is an example of a solution. • Common acids Name Chemical formula Hydrochloric acid Nitric acid HCl HNO3 Sulfuric acid H2SO4 Explanation Hydro- from hydrogen, -chlor from chlorine Nitr- from nitrogen (contains nitrogen) Sulfur- contains sulfur
  7. 7. Acids • What is common in the formula? – They all have H (hydrogen) in them. • Hence, an acid is a substance which produces hydrogen ions in water.
  8. 8. Properties of Acids corrosive sour taste good conductors of electricity
  9. 9. Chemical Properties of Acids • Acids turn blue litmus red
  10. 10. Chemical Properties of Acids • Acids react with metals to produce salt and hydrogen gas. • When a piece of metal is placed into an acid, a gas, hydrogen, is formed. • Effervescence in the solution will be observed.
  11. 11. EXAMPLES • Hydrochloric acid + Magnesium -> Magnesium chloride + Hydrogen Gas • Hydrochloric acid + Calcium -> Calcium chloride + Hydrogen Gas • Hydrochloric acid + Barium -> Barium chloride + Hydrogen Gas
  12. 12. EXAMPLES • Sulfuric acid + Magnesium -> Magnesium sulfate + Hydrogen Gas • Nitric acid + Magnesium -> Magnesium nitrate + Hydrogen Gas • Hydrochloric acid + Magnesium -> Magnesium chloride + Hydrogen Gas
  13. 13. Hydrogen is a colourless and odourless gas. acid + metal → salt + hydrogen • To confirm that gas produced is hydrogen, Test: Place a lighted splint in the gas. Observation: The splint will be extinguished with a ‘pop’ sound.
  14. 14. Think • Why is vinegar often sold in glass bottles rather than tin cans? Vinegar is an acid. The acid will react with the metal containers, it will corrode the metal containers.
  15. 15. Chemical Properties of Acids • Acids react with carbonates to produce salt, carbon dioxide and water. acid + carbonate → salt + carbon + water dioxide • Observation: – Effervescence is observed
  16. 16. EXAMPLES • Hydrochloric acid + Potassium carbonate -> Potassium chloride + Water + Carbon dioxide gas • Hydrochloric acid + magnesium carbonate -> Magnesium chloride + Water + Carbon dioxide gas • Hydrochloric acid + Sodium carbonate -> _________ + Water + Carbon dioxide gas
  17. 17. EXAMPLES • Sulfuric acid + sodium carbonate -> sodium sulfate + water + carbon dioxide gas • Nitric acid + sodium carbonate -> sodium nitrate + water + carbon dioxide gas • Magnesium carbonate + nitric acid -> magnesium nitrate + water + carbon dioxide gas
  18. 18. Test for Carbon Dioxide • How do we know that the gas is carbon dioxide? • Test: delivery tube – Bubble the gas into limewater. limewater • Observation: – White precipitate is observed in limewater. acid + carbonate
  19. 19. Think! Indigestion pills often contain the substances, sodium bicarbonate, calcium carbonate and magnesium carbonate, to react with the excess acid in the stomach. Why is it that when these pills are consumed, may result in frequent burping? When the stomach acid reacts with the carbonates, carbon dioxide is formed.
  20. 20. Alkalis • Common alkalis: – Sodium hydroxide – Potassium hydroxide – Calcium hydroxide NaOH KOH Ca(OH)2 – Ammonium hydroxide NH4OH • What do you see in common? – Hydroxide ions An alkali is a substance that produces hydroxide ions in water.
  21. 21. Physical Properties of Alkalis corrosive soapy feel bitter taste good conductors of electricity
  22. 22. Chemical Properties of Alkalis • Alkalis turn red litmus blue
  23. 23. Chemical Properties of Alkalis • Alkalis react with acids to form salt and water (neutralisation)
  24. 24. EXAMPLES • Hydrochloric acid + sodium hydroxide -> sodium chloride + water • Hydrochloric acid + potassium hydroxide -> potassium chloride + water • Hydrochloric acid + calcium hydroxide -> calcium chloride + water • Hydrochloric acid + ammonium hydroxide -> ammonium chloride + water
  25. 25. PRACTICE • Aqueous ammonia (ammonium hydroxide) + nitric acid -> ammonium nitrate + water • Sulfuric acid + lithium carbonate -> lithium sulfate + water + carbon dioxide • Hydrochloric acid + zinc -> zinc chloride + hydrogen gas
  26. 26. pH If can burn skin, can this Why? burn skin as well?
  27. 27. pH • pH is a measure of acidity an alkalinity • pH range is from 0-14
  28. 28. pH of common substances
  29. 29. pH • The smaller the pH value, the more acidic the substance. • The bigger the pH value, the more alkaline the substance. • pH 7 indicates that the substance is neither acidic nor alkaline (neutral)
  30. 30. More Properties • Acids have a pH of less than 7. • Alkalis have a pH of more than 7. • When an acid (pH <7) reacts with an alkali (pH >7) the resulting mixture is neutral. • The reaction is known as neutralisation. acid + alkali → salt + water
  31. 31. Neutralisation Applications Brushing teeth with toothpaste Explanation Toothpaste is slightly alkaline. It neutralises the acid left when bacteria feeds on the food left in the teeth. Control of pH of soil Lime (calcium oxide) is added to soil to neutralise the acidic soil. Some plants grow better in slightly alkaline soil. E.g. Turnips, cabbage, chrysanthemum. Using conditioner after Shampoo is alkaline. It makes hair feel dry shampooing hair and coarse. Conditioners are slightly acidic, it neutralises the shampoo left in the hair. Taking indigestion pills Indigestion is usually a result of excessive for indigestion acid in the stomach. Taking indigestion tablets which contain sodium bicarbonate neutralises the excessive acid.
  32. 32. Indicators • Indicators are substances which indicate if a substance is acidic or alkaline. • Examples of indicators: – Litmus • In an acid, blue litmus turns red • In an alkali, red litmus turns blue
  33. 33. Indicators – Universal Indicator • measures pH • green in neutral solutions. • Red/orange in acidic solutions. • Blue/purple in alkaline solutions.

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