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Grade 10 Chemistry Notes – Chapter 6 Salt

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Chemistry Notes – Grade 10

National Curriculum Pakistan - NCP

Chapter 6: Salts

Aligned with National Curriculum Pakistan (Federal Board, NBF, PTB)

Salts Questions and Answers

  1. What is salt?
    A salt is a chemical compound formed when the hydrogen ions (H⁺) of an acid are replaced by metal ions or ammonium ions during a neutralization reaction. For example, when hydrochloric acid reacts with sodium hydroxide, the resulting salt is sodium chloride.
  2. What are the three types of substances that can react with acids to form salts?
    The three types of substances are:
    Metals, which react with acids to form a salt and hydrogen gas.
    Insoluble bases (such as metal oxides), which react with acids to form a salt and water.
    Carbonates, which react with acids to form a salt, water, and carbon dioxide gas.
  3. What is formed when a metal reacts with an acid?
    When a metal reacts with an acid, it produces a salt and hydrogen gas. This reaction is often visible through the fizzing or bubbling of hydrogen. For example, magnesium reacting with hydrochloric acid forms magnesium chloride and hydrogen gas.
  4. What is the test for hydrogen?
    To test for hydrogen gas, a lit splint is brought near the mouth of the test tube. If hydrogen is present, it reacts with the flame, making a characteristic ‘pop’ sound due to its flammability.
  5. What is the general word equation for acid + carbonate?
    The general word equation is:
    Acid + Carbonate → Salt + Water + Carbon Dioxide
    This type of reaction is easily recognized by the effervescence (bubbling) caused by the release of carbon dioxide gas.
  6. What is the test for carbon dioxide?
    Carbon dioxide can be tested by bubbling the gas through limewater (calcium hydroxide solution). If carbon dioxide is present, the limewater turns milky or cloudy, indicating the formation of calcium carbonate.
  7. What is meant by the term ‘precipitate’?
    A precipitate is a solid substance that forms during a chemical reaction in solution and does not dissolve in water. It appears when two soluble substances react and produce an insoluble compound that settles or turns the solution cloudy.
  8. What is a precipitation reaction?
    A precipitation reaction is a chemical reaction where two aqueous (soluble) solutions are mixed and an insoluble salt forms as a solid product. This solid is called a precipitate, and it can be separated through filtration.
  9. State the solubility of common sulfates?
    Most sulfates are soluble in water, allowing them to dissolve easily. However, there are exceptions—barium sulfate, lead(II) sulfate, and calcium sulfate are all insoluble, and they form precipitates when mixed with appropriate ions.
  10. Describe how a soluble salt is made from an acid and an insoluble base?
    First, warm the dilute acid gently to increase the rate of reaction.
    Then add the insoluble base (like copper(II) oxide) in small amounts until no more dissolves, indicating the acid is neutralized.
    Filter the mixture to remove the excess base.
    Evaporate the filtrate to crystallize the soluble salt, and let it cool to collect the crystals.
  11. Describe how a soluble salt is made from an acid and a soluble base (alkali)?
    Since both substances are solutions, a titration must be carried out to find the exact volume of acid needed to neutralize the alkali.
    Use an indicator (like phenolphthalein) during the titration to determine the end point.
    Repeat the reaction using the exact volumes but without the indicator to get a pure salt solution.
    Finally, evaporate the water gently and let the solution crystallize.
  12. Describe how an insoluble salt is made?
    Choose two soluble salts whose ions will form an insoluble salt upon mixing.
    Mix the solutions together and a precipitate will form.
    Use filtration to separate the solid precipitate from the liquid.
    Then, wash the precipitate with distilled water to remove any impurities and dry it using absorbent paper.
  13. Why is titration used when preparing salts with soluble bases (alkalis)?
    Titration is used because both the acid and the alkali are clear, colorless solutions, so it's impossible to tell when neutralization is complete without an indicator. It ensures the salt is formed without any excess reactant, making it pure.
  14. Why does barium sulfate form a precipitate but sodium sulfate does not?
    Barium sulfate is insoluble in water, so when it forms in a reaction, it appears as a solid precipitate. In contrast, sodium sulfate is soluble, so it remains dissolved in the solution and does not form a precipitate.
  15. Why are silver and lead halides often used in precipitation reactions?
    Silver and lead halides (like AgCl or PbI₂) are typically insoluble in water, so they form visible precipitates when mixed with halide ions. This makes them ideal for demonstrating or testing precipitation reactions.
  16. What observations help confirm a precipitation reaction has taken place?
    A precipitation reaction is confirmed when a cloudy mixture appears or a solid forms and settles at the bottom of the container. The liquid may also become opaque or require filtering to separate the solid product.
  17. Why does the pH change when making salts?
    The pH changes because the acid (low pH) is gradually neutralized by the base or carbonate, increasing the pH towards neutral as the salt forms.
  18. Why is it important to add excess base or carbonate?
    To ensure that all the acid is fully reacted and no acid remains in the solution, which ensures purity of the salt.
  19. How do you know when enough base has been added to an acid?
    When no more base dissolves and the solution becomes saturated, indicating all the acid has been neutralized.
  20. How can a salt be recovered from a solution?
    By filtering out unreacted solid and then evaporating the water from the solution to crystallize the salt.
  21. Why do we heat the acid when reacting with an insoluble base?
    Heating increases the reaction rate and helps dissolve more of the base to complete the reaction faster.
  22. Explain the process of preparing a soluble salt using titration. Include a detailed description of the steps and reasoning.
    In titration:
    An alkali (soluble base) is placed in a conical flask.
    An acid is added slowly from a burette using an indicator (e.g., methyl orange) to detect neutralization.
    Once the endpoint is reached (colour change), the experiment is repeated without the indicator using exact volumes.
    The neutral solution is then evaporated to crystallize the salt.
  23. Describe the properties of the salts that are soluble at STP and have high melting points?
    Soluble salts dissociate into ions in water at STP and conduct electricity. They are typically ionic compounds, which results in high melting points due to strong electrostatic forces between ions.
  24. Explain the solubility rules for salts (beyond the exceptions for chlorides, bromides, and iodides).
    All nitrates and salts of sodium, potassium, and ammonium are soluble.
    Most sulfates are soluble, except barium, calcium, and lead sulfates.
    Most carbonates and hydroxides are insoluble, except those of sodium, potassium, and ammonium.
  25. Describe what happens when two soluble compounds form ions then influence their solubility.
    When two soluble salts are mixed, their ions may combine to form an insoluble salt (precipitate) if their combination results in a product that is insoluble in water according to the solubility rules.
  26. Compare the preparation, properties, and purification of salts using at least two different methods.
    Method 1 (Acid + Insoluble Base): Heat acid, add excess base, filter, evaporate. Used for insoluble bases.
    Method 2 (Titration with Soluble Alkali): Use indicator to find neutralization point, repeat without indicator, evaporate. Used for soluble bases.
    Both methods yield soluble salts, but the purification steps differ.
  27. Design an experiment to make magnesium sulfate crystals using available ingredients.
    Warm dilute sulfuric acid in a beaker.
    Add magnesium oxide in small portions until excess remains.
    Filter the mixture to remove unreacted solid.
    Evaporate the filtrate to concentrate the solution.
    Allow it to cool and form crystals of magnesium sulfate.
  28. In your opinion is it better to use a metal or base to prepare salts? Why?
    A base is often safer and easier to handle than a reactive metal. Bases allow better control over the reaction and minimize risks like hydrogen gas evolution and splashing.
  29. What would you do if some students add excess copper(II) carbonate?
    Filter the solution to remove the excess solid. Then proceed to evaporate the filtrate to crystallize the copper(II) salt.
  30. What is the reaction of a metal with dilute sulfuric acid?
    Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
  31. Which chlorides, bromides, and iodides are insoluble?
    Silver and lead chlorides, bromides, and iodides.
  32. Are all common sulfates soluble?
    Most are, except for those of barium, calcium, and lead.
  33. Which carbonates are soluble?
    Only those of sodium, potassium, and ammonium.
  34. Which hydroxides are soluble?
    Sodium, potassium, and ammonium hydroxides.
  35. What is used to prepare a soluble salt with a soluble base (alkali)?
    A titration method using an indicator.
  36. What is the indicator used in titration for preparing salts?
    Methyl orange or phenolphthalein.
  37. What is done after neutralization in titration?
    The solution is evaporated to obtain salt crystals.
  38. What does a Think Tank question ask about crystal making?
    Design an experiment to make magnesium sulfate crystals using available ingredients.
  39. What determines the solubility of salts?
    The type of ions present in the salt.
  40. Why do silver and lead halides form precipitates?
    Because they are insoluble in water.
  41. What happens when barium sulfate is formed?
    A white precipitate forms because it's insoluble.
  42. What do all common sodium, potassium, and ammonium salts have in common?
    They are soluble in water.
  43. What gas is evolved when an acid reacts with a carbonate?
    Carbon dioxide (CO₂).
  44. What gas is evolved when an acid reacts with a metal?
    Hydrogen (H₂).
  45. Why are excess solids added in base or carbonate reactions?
    To ensure all acid is reacted and neutralized.
  46. How can you tell when excess base or carbonate has been added?
    When it no longer dissolves or reacts.
  47. What is the purpose of crystallization in salt preparation?
    To obtain pure, solid salt crystals.
  48. What happens if too much heat is used during crystallization?
    The salt may decompose or form powder instead of crystals.
  49. What safety precaution is needed when heating during salt preparation?
    Use a heatproof mat and wear protective equipment.
  50. What is the difference between a soluble and insoluble salt?
    A soluble salt dissolves in water to form an aqueous solution, while an insoluble salt does not dissolve and forms a precipitate when mixed in water.
  51. What does the term precipitation mean in chemistry?
    Precipitation is the formation of a solid (precipitate) when two aqueous solutions react and produce an insoluble salt.
  52. How can you identify a precipitate in a reaction?
    A precipitate appears as a solid that forms and settles or makes the solution cloudy when two solutions are mixed.
  53. Why does barium sulfate form a precipitate but sodium sulfate does not?
    Barium sulfate is insoluble in water, so it forms a precipitate, whereas sodium sulfate is soluble, so it remains dissolved in solution.
  54. What are the general rules of solubility?
    All sodium, potassium, and ammonium salts are soluble.
    All nitrates are soluble.
    Most chlorides, bromides, and iodides are soluble except those of silver and lead.
    Most sulfates are soluble except barium, calcium, and lead.
    Most carbonates and hydroxides are insoluble, except those of sodium, potassium, and ammonium.
  55. Which ions are always present in soluble salts?
    Sodium (Na⁺), potassium (K⁺), and ammonium (NH₄⁺) ions are always found in soluble salts.
  56. Why are silver and lead halides often used in precipitation reactions?
    Because silver and lead halides (like AgCl and PbI₂) are insoluble in water, they form precipitates easily, making them useful in precipitation experiments.
  57. What observations help confirm a precipitation reaction has taken place?
    The formation of a cloudy solution or solid particles that settle at the bottom indicates a precipitate has formed.
  58. Why is it important to learn solubility rules in salt preparation?
    Solubility rules help determine whether a salt will be soluble or insoluble, guiding the choice of method (e.g., precipitation or crystallization) for salt preparation.
  59. Why is it important to add an insoluble base in excess during salt preparation?
    Adding the base in excess ensures that all the acid reacts completely. This prevents any unreacted acid from remaining in the final solution, which could contaminate the salt.
  60. How do you confirm that a reaction between an acid and a carbonate has finished?
    The reaction is complete when no more bubbles of carbon dioxide are observed and the solid carbonate stops dissolving, indicating all acid has been neutralized.
  61. What is the purpose of using filtration in salt preparation?
    Filtration separates the unreacted excess base or carbonate from the salt solution, leaving a clear solution that can be crystallized to obtain pure salt.
  62. What observation indicates the formation of a soluble salt in a reaction with an acid and a metal?
    Effervescence or bubbling occurs due to the release of hydrogen gas, and the metal gradually dissolves as the salt forms in solution.
  63. Why are nitrates generally soluble in water?
    Nitrate ions do not form strong ionic lattices with metal ions, allowing them to dissolve easily in water regardless of the cation.
  64. Why is titration used when preparing salts with alkalis?
    Because both the acid and alkali are solutions and no visible reaction (like a gas or precipitate) occurs, an indicator is necessary to detect the exact point of neutralization.
  65. What does the formation of a precipitate indicate during salt reactions?
    It suggests the salt formed is insoluble in water and thus separates from the solution as a solid, indicating a double displacement reaction has occurred.
  66. Why is evaporation used after filtration in salt preparation?
    To remove water from the salt solution, leaving behind solid crystals of the desired salt, which can then be collected and dried.
  67. How does the choice of acid affect the type of salt formed?
    The anion from the acid combines with the cation from the base or metal. For example, HCl forms chlorides, H₂SO₄ forms sulfates, and HNO₃ forms nitrates.
  68. What happens if the acid is not fully neutralized during salt preparation?
    The resulting salt will be contaminated with excess acid, making it impure and potentially corrosive or unsafe for further use.
  69. Why is crystallization preferred over simple evaporation for salt preparation?
    Crystallization gives well-formed, pure crystals, whereas rapid evaporation may result in an impure or powdered form of the salt.
  70. What is the difference between a soluble and insoluble salt in terms of practical applications?
    Soluble salts are used in solutions (like in medicine or food), while insoluble salts are often used in applications like pigments, materials, or water treatment.
  71. Why is lead(II) sulfate considered dangerous despite being a salt?
    Lead compounds are toxic and can cause poisoning; even though it’s a salt, its insolubility does not reduce its harmful potential if ingested or inhaled.
  72. What is the general pattern in the solubility of sulfates?
    Most sulfates are soluble except for those of barium, lead, and calcium, which form insoluble compounds and precipitate out in solution.
  73. Why should acid be heated before reacting it with an insoluble base?
    Heating increases the rate of reaction and helps dissolve the base more effectively, ensuring a complete reaction in a shorter time.
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