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The Hidden Power of Halogens: Properties of Group 17 Elements

  The Hidden Power of Halogens – Understanding Group 17 Elements When you hear the word halogen , you might not feel any excitement at first. But these elements, hidden in Group 17 of the periodic table, are some of the most fascinating, dangerous, and useful substances on Earth. From the fluoride in your toothpaste to the chlorine that keeps swimming pools clean, halogens are all around us — changing lives, saving lives, and sometimes even threatening them. Let’s take a deeper, human look into these elements and uncover what makes them so unique. What Makes Group 17 So Special? Group 17 contains a lineup of nonmetals known as halogens. The group includes fluorine, chlorine, bromine, iodine, and astatine — and a very rare artificial member, tennessine. What binds them together is not just their position on the periodic table, but their desperate need for one more electron. That’s right. All halogens have seven electrons in their outermost shell, and they want eight. This mis...
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Ch 07 acid base chemistry 9th exercise solutions with mcqs and short question answer for free

 

Acids and Bases: MCQs with Answers and Explanations

(i) Which acid is not used as a food or mixed with food?

(a) Tartaric acid
(b) Ascorbic acid
(c) Citric acid
(d) Formic acid ✔

Explanation: Formic acid is highly corrosive and not safe for consumption, whereas tartaric, ascorbic, and citric acids are commonly found in food and beverages.

(ii) While baking, which gas is responsible for raising the bread and making it soft?

(a) Oxygen
(b) Carbon dioxide ✔
(c) Nitrogen
(d) Carbon monoxide

Explanation: Baking soda or baking powder releases carbon dioxide (CO₂) when heated or mixed with acids, causing dough to rise and become soft.

(iii) Predict the main characteristics of the reactions of metals with acids.

(a) Metals are dissolved
(b) Metals are converted into salts
(c) Hydrogen gas is evolved
(d) All the above-mentioned characteristics are true ✔

Explanation: Metals react with acids to form salt and release hydrogen gas, which is why all the mentioned characteristics are correct.

(iv) How many hydroxide ions will calcium hydroxide release in water?

(a) 1
(b) 2 ✔
(c) 0
(d) 3

Explanation: Calcium hydroxide (Ca(OH)₂) dissociates into two OH⁻ ions per molecule in water, making it a strong base.

(v) In a neutralization reaction between KOH and H₃PO₄, how many molecules of KOH will react with one molecule of H₃PO₄?

(a) 2
(b) 1
(c) 3 ✔
(d) 4

Explanation: H₃PO₄ (phosphoric acid) has three replaceable H⁺ ions, so it requires three molecules of KOH to fully neutralize and form K₃PO₄ (potassium phosphate).

(vi) Which acid is used in the preparation of soap?

(a) Tartaric acid
(b) Citric acid
(c) Stearic acid ✔
(d) Oxalic acid

Explanation: Stearic acid is a long-chain fatty acid used in the production of soap. It reacts with sodium hydroxide (NaOH) to form soap through saponification.

(vii) Which compound is formed when SO₂ is dissolved in water?

(a) SO₃
(b) H₂SO₃ ✔
(c) H₂SO₄
(d) H₂S₂O₇

Explanation: Sulfur dioxide (SO₂) reacts with water to form sulfurous acid (H₂SO₃), which is a weak acid.

(viii) Which of the following contains oxalic acid?

(a) Tomato
(b) Orange
(c) Tamarind
(d) Sour milk ✔

Explanation: Oxalic acid is present in dairy products like sour milk and also in vegetables like spinach. It is responsible for the sour taste and can contribute to kidney stones in excessive amounts.

(ix) Which compound in the following reaction is behaving as a conjugate base?

CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺

(a) CH₃COOH
(b) CH₃COO⁻ ✔
(c) H₂O
(d) H₃O⁺

Explanation: A conjugate base is formed when an acid loses a proton (H⁺). Acetic acid (CH₃COOH) loses a proton to form CH₃COO⁻, making it the conjugate base.

(x) When a chemical reaction is carried out with a substance Z, a gas is produced that turns red litmus paper blue. What is the reaction?

(a) Reaction of an acid with a metal carbonate
(b) Reaction of an acid with ammonium salt
(c) Reaction of an alkali with a metal carbonate ✔
(d) Reaction of an alkali with ammonium salt

Explanation: When an alkali reacts with a metal carbonate, CO₂ gas is released, making the solution basic. The basicity turns red litmus paper blue.


Acids and Bases – Questions and Answers

Short Answer Questions

  1. Choose Arrhenius acids among the following compounds: HF, NH₃, H₂SO₄, SO₂, H₂S, H₂O.
    Answer: HF, H₂SO₄, and H₂S are Arrhenius acids because they increase H⁺ ion concentration in aqueous solution.

  2. How does calcium metal react with dilute H₂SO₄?
    Answer: Calcium reacts with dilute sulfuric acid to form calcium sulfate (CaSO₄) and hydrogen gas (H₂). The reaction is:
    Ca + H₂SO₄ → CaSO₄ + H₂↑

  3. Which salt is formed when HCl reacts with BaCO₃?
    Answer: Barium chloride (BaCl₂) and carbon dioxide (CO₂) are formed. The reaction is:
    BaCO₃ + 2HCl → BaCl₂ + CO₂ + H₂O

  4. How will you justify that HSO₄⁻ is a Bronsted-Lowry acid?
    Answer: HSO₄⁻ is a Bronsted-Lowry acid because it donates an H⁺ ion in a reaction:
    HSO₄⁻ → SO₄²⁻ + H⁺

  5. What chemical name will you give to soap as a compound?
    Answer: Soap is chemically known as sodium or potassium salt of a fatty acid, produced through saponification.

  6. Why is HCl not edible although it is present in the stomach?
    Answer: HCl is highly corrosive and can cause burns, ulcers, and tissue damage if ingested directly, even though dilute HCl in the stomach aids digestion.

  7. Is water an acid, a base, or both?
    Answer: Water is amphoteric, meaning it can act as both an acid and a base. In the reaction H₂O + HCl → H₃O⁺ + Cl⁻, water acts as a base by accepting H⁺ ions.

  8. Why does Na₂CO₃ behave like a base in water?
    Answer: Sodium carbonate (Na₂CO₃) reacts with water to release hydroxide (OH⁻) ions, making the solution basic.

  9. Is NaHCO₃ a base or an acid?
    Answer: Sodium bicarbonate (NaHCO₃) is amphoteric, meaning it can act as both an acid and a base, but it is generally considered a weak base.

  10. What is the difference between a strong acid and a concentrated acid?
    Answer: A strong acid completely ionizes in water (e.g., HCl), while a concentrated acid has a high amount of acid per unit volume, regardless of strength.

Constructed Response Questions

  1. Explain Arrhenius' concept of acids and bases.
    Answer: Arrhenius acids increase H⁺ ions in water, while Arrhenius bases increase OH⁻ ions. This concept applies only to aqueous solutions.

  2. Compare Arrhenius and Bronsted-Lowry concepts of acids and bases.
    Answer: Arrhenius acids and bases are limited to aqueous solutions, whereas Bronsted-Lowry acids donate H⁺ and bases accept H⁺, making the latter more general.

  3. How does sulfuric acid react with the following compounds?

    • With NH₄Cl: H₂SO₄ + NH₄Cl → NH₄HSO₄ + HCl
    • With NH₄NO₃: H₂SO₄ + NH₄NO₃ → NH₄HSO₄ + HNO₃
    • With MgO: H₂SO₄ + MgO → MgSO₄ + H₂O
    • With MgCO₃: H₂SO₄ + MgCO₃ → MgSO₄ + CO₂ + H₂O

  4. What happens when a base reacts with a non-metallic oxide?
    Answer: A base neutralizes a non-metallic oxide to form a salt and water. Example:
    Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
    This shows that non-metallic oxides are acidic.

  5. Why does dry HCl gas not show acidic properties, but its aqueous solution does?
    Answer: Dry HCl gas does not ionize, while in water, it ionizes to release H₃O⁺ ions, which give it acidic properties.
    HCl + H₂O → H₃O⁺ + Cl⁻

Investigative Questions

  1. Acids play significant roles within the human body. Comment.
    Answer: Acids are essential for digestion (HCl in the stomach), metabolism (carbonic acid in blood), and genetic material (nucleic acids like DNA and RNA).

  2. What is observed when CO₂ is passed through lime water?

    • For a short duration: Lime water turns milky due to calcium carbonate formation.
      Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
    • For a long duration: The milkiness disappears as calcium bicarbonate forms.
      CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂

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