ch.9

• Acids found at home and in the laboratory


  • Home: Soft drinks, Vinegar, Yoghurt, Lemon (juice), Body lotion, Hair conditioner, grapes, spinach, Toilet cleaners
  • Laboratories: Hydrochloric acid, Sulphuric acid, Nitric acid
• Properties of acids


  • Acidic solutions have a sour taste
  • Acidic solutions can conduct electricity
  • Acids react with alkalis
  • Acids react with many materials including metals
• Uses of acids


  • Used as preservatives
  • Removing stains
  • Making industrial chemicals
• Acids have a sour taste. Acidic solutions can conduct electricity.
• Acids can be used as preservatives to prevent food from going bad. Some household cleaners contain acids to help remove stains. Acids are also commonly used in making industrial chemicals.
• Vinegar contains ethanoic acid, it can be used to help preserve food.
• Alkalis found at home and in laboratory

  • Home: Glass cleaner, Kitchen (glass) cleaner, Toothpaste, Soap, Alkaline batteries, Baking soda
  • Laboratories: Sodium hydroxide (corrosive), Ammonia solution (corrosive), Calcium hydroxide (limewater), Potassium hydroxide
• Properties of alkalis


  • Alkalis usually have a bitter taste
  • Alkalis have a slippery feel
  • Alkaline solutions can conduct electricity
  • Alkalis react with acids
• Uses of alkalis


  • Removing grease
  • Making industrial chemicals
• Alkalis have a bitter taste and a slippery feel. Alkaline solutions can conduct electricity.
• Some household cleaners contain alkalis to help remove grease. Alkalis also have many industrial uses. For example, they can be used in making batteries.
• Handling acids and alkaline safely in the laboratory

  • Read the hazard warning labels on the reagent bottles (Corrosive)
  • Wear safety goggles to protect your eyes// wear protective gloves to avoid direct contact with acids and alkalis
  • Pour the acids and alkalis wastes into appropriate waste bottles
  • Wash your hands thoroughly after the experiments
  • NEVER taste right acids or alkalis used in the laboratory
  • Do NOT mix acids and alkalis with other chemicals without your tracher's permission
• What should we do if accidents involving acids and alkalis happen

  1. Wash the affected area under slow running water for 10 minutes

  2. Carefully remove any contaminated clothing and avoid making any contact with the chemical

  3. If you get the acid or alkali into your eye, wash it immediately with the eyewash bottle for at least 10 minutes

  4. For more serious injuries, call an ambulance. Keep washing the affected area until the ambulance arrives. Take a sample of the acid or alkali to the hospital for reference
• Many acids and alkalis used in the school laboratory can burn our skin and eyes. We must take safety precautions in handling them.
• Acid-alkali indicators


  We use indicators to show the presence of acids and alkalis
• Natural indicators


  • Some plants contain pigments which change color when mixed with acids and alkalis. Extracts from these plants can be used as natural indicators to distinguish acid and alkalis.
• Litmus


  It is a mixture of pigments extracted from lichen, which can be found on tree trunks. It shows different colors in acidic and alkaline conditions and can be used to distinguish between acidic and alkaline situations.
• Blue litmus


  Used for testing acidic solutions, turns red in acidic solutions
• Red litmus


  Used for testing alkaline solutions, turns blue in alkaline solutions
• Universal indicator


  A mixture of several indicators that shows a range of different colors depending on the degrees of acidity and alkalinity of the substance mixed with it.
• pH value


  Expressed as a number from 0 to 14, the lower the pH value the more acidic the substance, the higher the pH value the more alkaline the substance, a neutral substance has a pH value of 7.
• We can use the universal indicator solution or pH paper to find the pH value of a substance.
• The lower the pH value, the more acidic the substance. The higher the pH value, the more alkaline the substance. A neutral substance has a pH value of 7.
• A pH meter can measure the pH value of a substance more accurately than universal indicator.
• Neutralization


  The chemical reaction that occurs when an acid and an alkali are mixed together.
• When an acid and an alkali neutralize each other, a salt and water are produced, and heat is released.
• Neutralisation word equation


  acid + alkaline (reactants) = salt + water (products)
• Chemical names of salts formed by neutralization

  Alkaline name first then acid name
• Mass is conserved in neutralization.
• Applications of neutralization


  • Prevention of tooth decay
  • Relieving stomach upset
  • Treatment of insect stings and bites
  • Treatment of industrial waste containing acids or alkalis
  • Regulation of pH value of soil
• Dilute hydrochloric acid reacts with some metals (eg. magnesium) to produce salt and hydrogen.
• Word equation for reactions between acids and metals

  Acid + metal (reactants) -> salt + hydrogen (products)
• Magnesium and aluminum also react with sulphuric acid to form a salt and hydrogen.
• Hydrogen burns with a 'pop' sound

  Whenever hydrogen is brought into contact with heat, it expands so fast that it breaks the sound barrier
• When the metal reacts with the acid

  Its size decreases slightly
• Corrosion


  The acid corrodes the metal
• We shouldn't store acidic drinks in metal containers, because the metal containers will be corroded by the acid
• Acids reacting with metals


  • Salt and hydrogen are produced during the reaction
• Reaction between acids and metals

  Acid + metal (reactants) -> salt + hydrogen (products)
• Metals reacting with hydrochloric acid

  • Magnesium
  • Aluminum