Chapter 3: Let us answer these (Page-48)

Q1. What is the nature of metal oxides – acidic or basic?
Ans:- Basic as when they react with with acids, they form a salt and water.

Q2. Two metallic compounds A and B react separately with acid to form salt and water. The compound A also produces an effervescence. What could be A and B ?
Ans:-            A +B = Salt + water
           A is a metal carbonate or metal hydrogen carbonate.
           B is a metal oxide.

Q3. When milk becomes curd lactic acid is produced as one of the products. Give reason             why is it not advisable to keep curd in copper and brass vessels ?
Ans:- Since Lactic acid, C₂H₄(OH)COOH reacts with copper or brass vessels to produce copper lactate which is toxic in human.That is why it is not advisable to keep curd in copper and brass vessels.

Q4. Carbon dioxide reacts with lime water, Ca(OH)₂ to form salt, CaCO₃ and water as
                             Ca(OH)₂ (base) + CO₂ --------> CaCO₃ (salt) + H₂O (water)
Indicate the nature of CO₂. What will be the general nature of non-metallic oxides?
Ans:- The nature of CO₂ is acidic.
           Hence, the general nature of non-metallic oxide is acidic.

Q5. Write the reaction between sodium hydroxide and sulphur dioxide (SO₂).
Ans:-       2NaOH + SO₂ Na₂SO₃ + H₂O.

Q6. What will happen when CO₂ gas is passed into water. Write the chemical equation.
Ans:-  When CO₂ gas is passed through water, carbonic acid is formed.
                                       H₂O + CO₂ H₂CO₃

Q7. Write the reaction that takes place between water and sulphur dioxide.
Ans:- Water reacts with sulphur dioxide to form sulphurous acid.
                        SO₂ + H₂O H₂SO₃

Q8. Give definition of base.
Ans:- The ionic compounds that produce negative hydroxide (OH⁻) ions when dissolved in water are called bases.

Types of Bases

• Strong base – It is a compound that has an ability to remove a proton from a very weak acid. Or they completely dissociate into its ions when in water. Examples are potassium hydroxide (KOH), sodium hydroxide (NaOH).
• Weak base – There is incomplete dissociation when in water. The aqueous solution contains both the weak base as well as its conjugate acid. Examples are ammonia (NH₃), water (H₂O), pyridine (C₅H₅N).
• Superbase – These bases are better at deprotonation when compared to a strong base. These have very weak conjugate acids. They can be obtained by mixing an alkali metal with its conjugate acid. It can’t sustain in aqueous solution as it is a stronger base than hydroxide ion. Examples are sodium hydride (NaH), ortho-diethynylbenzene dianion (C₆H₄(C₂)₂)²⁻
• Neutral base – It forms a bond with a neutral acid share an electron pair.
• Solid base – It is active in solid form. Examples are silicon dioxide and sodium hydroxide mounted on alumina.

Properties of Base

• Aqueous base solution dissociates into ions to conduct electricity.
• It has a pH value greater than 7.
• They form salts on reacting with acids.
• They help in promoting certain chemical reactions.
• They are bitter to taste if placed in alkali solutions.
• Strong or concentrated bases are caustic.
• It changes the indicator colour from red litmus paper to blue litmus paper.
• It has the ability to accept protons from proton donors.
• It contains OH⁻ ions.
• They vigorously react when in contact with acids.
• They are slippery to touch.
• They conduct electricity when dissolved in water.

Q9.Define acids.
Ans:-  Acids are chemical agents that release hydrogen ions when added to water

 Eg. Lewis acid, Hydrochloric acid, Sulfuric acid, Hydrofluoric acid, Acetic acid

Q10. Give the definition of acidic oxides.
Ans:- Those compounds of non-metallic oxides like C0₂ , SO₂ etc. can neutralize the bases to form salt and water. Such non-metallic oxides are called acidic oxides.

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