Acids and Alkalis Notes

9 Common acids and alkalis

9.1 Common acids and alkalis

  • Acids:

    • Taste sour.

    • Found in various kinds of vinegar (ethanoic acid), lemons, kiwi fruit, oranges, soft drinks, and yoghurt.

    • Also present in household products like hair conditioner, lotion, and toilet cleaner.

  • Alkalis:

    • Give a slippery feel (like soap).

    • Some have a bitter taste.

    • Found in foods like Chinese noodles and lye water rice dumplings.

    • Also present in household products like kitchen cleaner, alkaline batteries, baking soda, bleach, soap, and toothpaste.

  • Many acids and alkalis (especially in household cleaners) are irritant or corrosive.

    • Handle with care.

    • Never taste them except in food.

  • Acids commonly used in the laboratory:

    • Hydrochloric acid HCl

    • Sulphuric acid H2SO4

    • Nitric acid HNO3

  • Alkalis commonly used in the laboratory:

    • Sodium hydroxide NaOH

    • Ammonia solution NH3

    • Calcium hydroxide Ca(OH)2 (lime water).

  • Most acids and alkalis used in the laboratory are irritant or even corrosive.

    • Safety precautions must be taken when handling them.

Safety precautions when handling acids and alkalis in the laboratory:

  • Read hazard warning symbols on chemical bottles carefully.

  • Wear a laboratory coat and disposable or protective gloves.

  • Wear safety spectacles.

  • Do NOT mix acids or alkalis with other chemicals without teacher approval.

  • Pour waste solutions into suitable waste bottles, not the sink.

  • Wash hands thoroughly after handling acids or alkalis.

Emergency treatment for accidents involving acids and alkalis:

  • If acids or alkalis splash onto:

    • Skin: Immediately wash the affected area under slow-running water for at least 10 minutes.

    • Eyes: Immediately wash the affected eye(s) using an eye wash bottle for at least 10 minutes.

    • Clothes: Carefully remove the affected clothes and avoid contact with any acids or alkalis on the clothes.

  • If the accident is serious, take the injured person to hospital as soon as possible.

Uses of acids:

  • For food preservation:

    • Acids inhibit the growth of microorganisms that cause food spoilage.

    • Vinegar (ethanoic acid) is commonly used to preserve vegetables.

  • For stain removal:

    • Hydrochloric acid is present in many toilet cleaners.

    • It reacts with dirt and stains, which are then easily removed when the toilet is flushed.

Uses of alkalis:

  • For making noodles:

    • Lye water (alkaline solution) is added to Chinese noodles to make them chewy.

  • For grease removal:

    • Sodium hydroxide is present in many kitchen cleaners.

    • It reacts with grease on kitchen stoves, allowing it to be easily wiped or washed away.

9.2 Distinguishing acids and alkalis

  • Natural Indicators:

    • Some substances in nature change color when they come into contact with acids or alkalis.

    • These substances can be used as acid-alkali indicators.

    • Examples:

      • Red cabbage extract

      • Butterfly pea flower extract

      • Red onion skin extract

  • Robert Boyle discovered the use of plant extracts as natural indicators in the 17th century

  • Litmus:

    • Commonly used acid-alkali indicator in the laboratory.

    • Extracted from several kinds of lichens.

    • Two kinds: blue litmus and red litmus.

    • Available in solution and paper form (litmus paper).

    • Behavior in different substances:

      • Acidic: Blue litmus turns red, red litmus remains red.

      • Alkaline: Blue litmus remains blue, red litmus turns blue.

      • Neutral: Both blue and red litmus show no color change.

  • pH Scale:

    • Used to describe the relative acidity or alkalinity of substances.

    • Ranges from 0 to 14.

      • Acidic: pH < 7

        • The lower the pH, the more acidic the substance.

      • Neutral: pH = 7

      • Alkaline: pH > 7

        • The higher the pH, the more alkaline the substance.

  • Universal Indicator:

    • A solution made up of several indicators that shows a range of colors depending on the acidity or alkalinity of the substance being tested.

    • Also available in paper form (pH paper).

  • pH Meter:

    • Electronic instrument used to measure the pH values of solutions.

    • Must be calibrated before use.

    • Provides a more accurate pH value than universal indicator.

9.3 Neutralization

  • Definition: The reaction between an acid and an alkali.

  • Change in pH value:

    • Acids have pH values lower than 7.

    • Alkalis have pH values higher than 7.

    • When an acid and an alkali are mixed, the pH value of the resulting solution changes, approaching 7 (neutral).

  • Products of Neutralization:

    • Salt and water are formed.

    • Thermal energy (heat) is released during neutralization.

  • Word Equation:

    • acid + alkali → salt + water

    • The type of salt formed depends on the types of acid and alkali that react.

  • Skill Builder 9.1 Writing word equations for neutralization

    The general format of the word equation for neutralization is as follows:
    acid + alkali \rightarrow salt + water

  • Conservation of Mass:

    • The total mass of the reactants (acid and alkali) is the same as that of the products (salt and water).

  • Applications of Neutralization:

    • Treating stomach ache with antacid:

      • Antacids (alkaline) neutralize excess stomach acid (hydrochloric acid).

    • Brushing teeth with toothpaste:

      • Toothpaste (slightly alkaline) neutralizes acids produced by bacteria in the mouth, preventing tooth decay.

    • Treating chemical waste from factories:

      • Waste is neutralized with suitable alkalis or acids before discharge to minimize pollution.

    • Treating insect stings and bites:

      • Bee stings, mosquito bites, and ant bites are acidic and can be neutralized with soap solution (alkaline).

      • Wasp stings are alkaline and can be neutralized with vinegar (acid).

    • Regulating soil acidity and alkalinity:

      • Lime (alkaline) is added to acidic soil to raise the pH.

      • Sulphur is added to alkaline soil, which bacteria convert into sulphuric acid to lower the pH.

9.4 Corrosive nature of acids

  • Effects of acids on metals:

    • Dilute acids corrode some metals (e.g., zinc, magnesium, iron).

    • Hydrogen gas is produced when dilute acids react with metals.

    • Word equation: metal + acid → salt + hydrogen

  • Effects of acids on building materials:

    • Dilute acids corrode building materials containing calcium carbonate (e.g., marble and limestone).

    • Carbon dioxide is produced when dilute acids react with calcium carbonate.

    • Word equation: carbonate + acid → salt + water + carbon dioxide

    • Acidic cleaners should be avoided for marble

9.5 Acid rain

  • Definition: Rainwater with a pH value lower than 5.6.

  • Causes:

    • Burning of fossil fuels (e.g. coal, oil) releases acidic gases into the air (sulphur dioxide, nitrogen oxides).

    • These acidic gases react with oxygen and water in the air to form sulphuric acid and nitric acid.

    • The increased acidity leads to acid rain.
      Animation: Acid rain formation process

  • Effects:

    • Damages plant leaves directly, hindering photosynthesis.

    • Releases toxic minerals into the soil, harming plant growth.

    • Harms or kills living things in water.

    • Corrodes buildings and statues made of calcium carbonate.

    • Corrodes metal objects (e.g., car bodies, metal bridges).

  • Controlling Acid Rain（這part不太重要）:

    • US:

      • Providing financial benefits to the industry for reducing emissions of sulphur dioxide and nitrogen oxides

      • Rewarding petrol refiners if they produce petrol cleaner than required

    • China:

      • Closing inefficient factories which release large amounts of air pollutants

      • Enhancing public awareness on reducing pollution from vehicles

      • Restricting the mining of coal with high sulphur content

    • Germany:

      • Largely adopting renewable energy sources to replace coal as the main source of energy

      • Over 40% of electricity is powered by renewable energy sources

    • Sweden:

      • Restricting the burning of crude oil with high sulphur content

      • Liming in rivers and lakes to improve water quality and decrease the death rate of fish

9.6 Potential hazards related to the use of acids and alkalis

  • Concentrated vs. Dilute Acids and Alkalis:

    • Concentrated acids and alkalis are more corrosive than dilute ones.

  • Mixing Cleansing Products:

    • Mixing different cleansing products can be dangerous.

    • Reactions may occur, releasing toxic substances or thermal energy (potentially leading to explosions).

    • NEVER mix any cleansing products.

  • General Safety Rules When Using Cleansing Products:

    • Keep the room well-ventilated.

    • Wear gloves, a mask, and protective clothing.

    • Wash hands thoroughly after using the products.

    • Prepare 1:99 diluted bleach containing 5.25% of active ingredient.

  • Emergency treatment for accidents:
    *If the cleansing product has splashed onto the body, wash the affected area with a large
    amount of slow-running water.
    *If a harmful gas is inhaled, open the windows and move the injured person to an area with lots of fresh air.
    *If the injury is serious, take the injured person to hospital as soon as possible.