Science Class 18 - Odour Observations
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Questions and Answers

What is the fundamental reason for the exothermic nature of dissolving acids and bases in water?

  • The process involves the formation of strong ionic bonds.
  • Acids and bases release gas when mixed with water.
  • Water acts as a solvent with no thermal energy changes.
  • The dissolution leads to a significant release of energy due to ion interaction. (correct)

    • What must be done when mixing concentrated acid with water to ensure safety?

  • Mix equal volumes of acid and water simultaneously.
  • Add concentrated acid slowly to water while stirring continuously. (correct)
  • Use ice water to dilute the acid more effectively.
  • Always add water to acid with constant stirring.

    • When an acid is diluted with water, what happens to the concentration of H3O+ ions?

  • It increases due to increased volume.
  • It becomes equal to the concentration of OH- ions.
  • It decreases due to dilution. (correct)
  • It remains constant in a neutral solution.

    • Why should a user be cautious when handling concentrated nitric or sulfuric acids?

    <p>They may splatter and cause burns due to heat generation.</p> Signup and view all the answers

    What distinguishes substances like HCl and HNO3 from compounds like alcohol and glucose in aqueous solutions?

    <p>HCl and HNO3 ionize in solution to release H+ ions, while alcohol and glucose do not.</p> Signup and view all the answers

    What is formed when a metal displaces hydrogen from an acid?

    <p>A salt</p> Signup and view all the answers

    Which of the following properties distinguishes olfactory indicators from regular indicators?

    <p>Their ability to change odour</p> Signup and view all the answers

    In the experiment with zinc granules, what gas is evolved when zinc reacts with an acid?

    <p>Hydrogen gas</p> Signup and view all the answers

    What should be observed on the surface of zinc granules when they react with dilute sulfuric acid?

    <p>Bubbling and effervescence</p> Signup and view all the answers

    What would you observe when a burning candle is brought near a gas-filled bubble from the reaction of zinc and acid?

    <p>The candle ignites with a popping sound</p> Signup and view all the answers

    Study Notes

    Reactions of Acids and Bases

    • Rinsing cloth strips with water can reveal changes in odour, aiding in identifying olfactory indicators.
    • Common odours tested include dilute vanilla essence and clove oil in reactions with dilute HCl and NaOH.
    • Observations on odour changes provide insight into which substances can act as olfactory indicators.

    Acid-Metal Reactions

    • Metals displace hydrogen from acids, resulting in the formation of salt and hydrogen gas.
    • General reaction formula:
      • Acid + Metal → Salt + Hydrogen gas
    • Notable observation: Gas evolved can be tested with a burning candle, resulting in a pop sound indicating hydrogen presence.
    • Observations differ with various acids (HCl, HNO3, CH3COOH).

    Acid-Base Neutralization Reactions

    • Neutralization of an acid and base results in salt and water.
    • General reaction formula:
      • Acid + Base → Salt + Water
    • Example reaction:
      • H+(aq) + OH−(aq) → H2O(l)
    • Increased safety is required when mixing acids and bases with water due to exothermic reactions.

    Mixing Acids and Bases with Water

    • Mixing concentrated acid (e.g., H2SO4) with water is highly exothermic.
    • Always add acid to water slowly while stirring to avoid splashes and burns.
    • Dilution decreases the concentration of hydrogen and hydroxide ions in the resulting solution.

    Reactions with Carbonates

    • Metal carbonates react with acids to produce salt, carbon dioxide, and water.
    • General reaction formula for carbonates:
      • Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water
    • Notable reactions involve sodium carbonate and sodium bicarbonate with HCl, leading to the formation of NaCl, CO2, and H2O.

    Indicator Reactions

    • Phenolphthalein changes color in response to acids and bases, demonstrating pH differences.
    • A base neutralizes an acid, resulting in a colour change of the phenolphthalein solution.

    Reaction of Metallic Oxides with Acids

    • Copper oxide reacts with hydrochloric acid, resulting in a color change indicating that the metal oxide is dissolved to form salt.

    Production of Common Compounds

    • Process of electrolysis of brine produces sodium hydroxide, chlorine, and hydrogen, relevant for chemical manufacturing:
      • Reaction: 2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
    • Bleaching powder is formed from chlorine reacting with dry slaked lime (Ca(OH)2).
    • Baking soda (sodium hydrogencarbonate, NaHCO3) is used in culinary applications.

    Safety Precautions

    • Caution is necessary when handling concentrated acids and bases, as they can cause severe burns and damage.
    • Always heed warning signs and instructions for safe usage of chemical substances.

    Summary of Learning Outcomes

    • Acid-base indicators help in identifying acidic and basic solutions.
    • Acids produce H+ ions, while bases produce OH− ions in aqueous solutions.
    • Reactions between acids and metals yield hydrogen gas and salts.
    • Electric conductivity in solutions indicates the presence of ions, affecting how acids and bases interact with water.
    • pH scale measures acidity and basicity, with neutral solutions having a pH of 7.

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    Related Documents

    Acids, Bases and Salts PDF

    Description

    In this quiz, we explore the fascinating world of odours through a series of observations and experiments. Students will check the odour of various substances, including vanilla essence and clove oil, and document their findings. This practical approach helps in understanding the chemical properties related to smell.

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