Acids and Alkalis 2

Year 7 Revision: Topic 3 - Acids

Acids and Alkalis Definitions:

  • Acids: Substances that release hydrogen ions (H^+) when dissolved in water.
    • Have a sour taste.
    • Can corrode metals.
  • Alkalis: Substances that release hydroxide ions (OH^-) when dissolved in water.
    • Have a bitter taste.
    • Have a slippery feel.

pH Scale:

  • Ranges from 0 to 14.
    • pH < 7: Acidic
      • Strong acids (e.g., hydrochloric acid) can have a pH close to 0.
    • pH = 7: Neutral
      • Pure water.
    • pH > 7: Alkaline
      • Strong alkalis (e.g., sodium hydroxide) can have a pH close to 14.

Indicators:

  • Substances that change color in response to pH changes.
    • Examples: litmus paper, universal indicator.

Which Acid is Which?

  • Hydrochloric Acid (HCl):
    • Found in gastric juice; aids in digestion and kills bacteria.
    • Used in cleaning agents and metal processing.
  • Sulfuric Acid (H2SO4):
    • Strong acid used in car batteries and as a dehydrating agent.
    • Important in the manufacture of fertilizers and explosives.
  • Nitric Acid (HNO_3):
    • Used in fertilizers, explosives, and etching metals.
    • Can cause severe burns and is a strong oxidizer.
  • Acetic Acid (CH_3COOH):
    • Found in vinegar; used in food preservation and flavoring.
    • Weak acid with a pH around 2-3.
  • Citric Acid (C6H8O_7):
    • Naturally occurring in citrus fruits; used as a preservative and flavoring agent.
    • Weak acid with a pH around 3-4.

Uses of Acids:

  • Used in various industries, including food, cleaning, and manufacturing.

Litmus Paper Function:

  • Made from natural dyes extracted from lichens.
  • Used to test the acidity or alkalinity of a solution.
    • Red litmus paper: turns blue in alkaline solutions (pH > 7).
    • Blue litmus paper: turns red in acidic solutions (pH < 7).
  • Limitations:
    • Does not provide a specific pH value; only indicates whether a solution is acidic or alkaline.
  • Practical Use:
    • Commonly used in laboratories and classrooms for quick pH testing.
  • Extension:
    • Practice interpreting results from various solutions using litmus paper and understand the implications of the results.

Universal Indicator:

  • Description: A mixture of several indicators that provides a color change across a wide range of pH levels.
  • Color Changes:
    • Red (pH 0-3): Strong acids (e.g., hydrochloric acid).
    • Orange (pH 4-6): Weak acids (e.g., vinegar).
    • Green (pH 7): Neutral (e.g., distilled water).
    • Blue (pH 8-10): Weak alkalis (e.g., baking soda).
    • Purple (pH 11-14): Strong alkalis (e.g., sodium hydroxide).
  • Usage: Can be used to determine the pH of a solution by comparing the color change to a pH scale.
  • Practical Application: Used in laboratories, environmental testing, and educational settings to assess the acidity or alkalinity of various substances.
  • Starter: Predict the color of a universal indicator in various solutions (e.g., lemon juice, soap, distilled water) and explain the reasoning behind your predictions.

Red Cabbage Indicator:

  • Description: Red cabbage contains anthocyanins, which are pigments that change color based on pH.
  • Color Changes:
    • Pink/red in acidic solutions (pH < 7).
    • Purple in neutral solutions (pH ≈ 7).
    • Green/yellow in alkaline solutions (pH > 7).
  • Preparation: Chop red cabbage and boil it in water to extract the pigment. The resulting liquid can be used as a natural pH indicator.
  • Applications:
    • Can be used in experiments to test the pH of various household substances:
      • Lemon juice (acidic)
      • Baking soda (alkaline)
      • Vinegar (acidic)
      • Soap (alkaline)
  • Advantages:
    • It is a natural and safe alternative to synthetic indicators.
    • It provides a visual representation of pH changes, making it useful for educational demonstrations.
  • Limitations:
    • The color changes may not be as distinct as those of synthetic indicators, and the exact pH value cannot be determined without a pH scale for comparison.
  • Starter: Name 4 substances that can be tested with red cabbage juice and predict the color change for each.

Neutralisation:

  • Definition: A chemical reaction between an acid and an alkali that results in the formation of a salt and water.
  • General Equation: Acid + Alkali → Salt + Water
  • Example: HCl (hydrochloric acid) + NaOH (sodium hydroxide) → NaCl (sodium chloride) + H_2O (water).
  • Types of Salts:
    • The type of salt produced depends on the acid and alkali used.
      • Hydrochloric acid + Sodium hydroxide → Sodium chloride (table salt).
      • Sulfuric acid + Potassium hydroxide → Potassium sulfate.
  • Applications:
    • Treating acid spills with bases.
    • Using antacids to relieve heartburn by neutralising stomach acid.
    • Agricultural practices to adjust soil pH for optimal plant growth.
  • Indicators in Neutralisation:
    • Indicators can be used to determine when neutralisation has occurred.
      • For example, phenolphthalein turns pink in alkaline solutions and colorless in acidic solutions.
  • Support: Read page 8 of your textbook for detailed examples of neutralisation reactions, including practical experiments that demonstrate the process, such as titration.

Indigestion Tablets:

  • Function: Indigestion tablets contain alkaline substances (e.g., magnesium hydroxide, calcium carbonate) that neutralise excess stomach acid (hydrochloric acid).
  • Mechanism: When the tablet dissolves in the stomach, it reacts with the acid to form water and a salt, alleviating discomfort caused by acidity.
    • Example reaction: Mg(OH)2 (magnesium hydroxide) + 2HCl (hydrochloric acid) → MgCl2 (magnesium chloride) + 2H_2O (water).
  • Importance:
    • Maintaining the correct pH in the stomach is crucial for proper digestion and to prevent conditions like acid reflux.
    • Overproduction of stomach acid can lead to discomfort, ulcers, and other digestive issues.
  • Types of Indigestion Tablets:
    • Antacids: Over-the-counter medications that provide quick relief from heartburn and indigestion. They work by neutralising stomach acid.
    • Proton Pump Inhibitors: Medications that reduce the production of stomach acid, providing longer-term relief for chronic conditions.
  • Starter: Complete questions related to how indigestion tablets function, including their chemical reactions and the importance of pH balance in digestion.

Testing Gases:

  • Hydrogen (H₂):
    • Produced when acids react with metals (e.g., zinc + hydrochloric acid).
    • Test: Collect the gas in a test tube and bring a lit splint near the opening. A 'pop' sound indicates the presence of hydrogen, confirming the reaction.
  • Carbon Dioxide (CO₂):
    • Produced when acids react with carbonates (e.g., hydrochloric acid + sodium carbonate).
    • Test: Bubble the gas through limewater (calcium hydroxide solution). If carbon dioxide is present, the limewater turns cloudy due to the formation of calcium carbonate.
  • Oxygen (O₂):
    • Produced during decomposition reactions (e.g., decomposition of hydrogen peroxide).
    • Test: Bring a glowing splint near the gas. If it relights, oxygen is present, indicating a successful test.