CBSE Class 7 Science Advanced Chemistry Acids, Bases and Salts Long Answer Questions Set 01

Long Answer Questions

Teacher's Note:
1. Make the two tables neatly.
2. Include at least one example in each class.

Question 2. Write a short note on different types of salts.
Answer:
1. Normal salts: all the \(\ce{H+}\) of the acid are replaced by metal ions (complete neutralization). Examples: \(\ce{Na2SO4}\), \(\ce{Ca3(PO4)2}\).
2. Acidic salts: some replaceable H still remains (partial neutralization of a dibasic or tribasic acid). They give \(\ce{H+}\) in water. Examples: \(\ce{NaH2PO4}\), \(\ce{Na2HPO4}\).
3. Basic salts: some replaceable OH remains (partial neutralization of a diacidic or triacidic base). They give \(\ce{OH-}\) in water. Example: \(\ce{Ca(OH)Cl}\).
4. Double salts: a combination of two simple salts; they give the tests of all ions of both salts. Examples: Mohr's salt \(\ce{FeSO4.(NH4)2SO4.6H2O}\) and potash alum \(\ce{K2SO4.Al2(SO4)3.24H2O}\).
5. Mixed salts: contain more than one basic or acidic radical. Examples: \(\ce{NaKCO3}\) and bleaching powder \(\ce{CaOCl2}\).
6. Complex salts: contain one or more simple ions and a complex ion, which behaves as one unit and does not break into atoms. Examples: \(\ce{K4[Fe(CN)6]}\) (potassium ferrocyanide), which dissociates as \(\ce{K4[Fe(CN)6] -> 4K+ + [Fe(CN)6]^4-}\), and \(\ce{Na[Ag(CN)2]}\).
In simple words: Salts are grouped by what ions they have and how they were formed.

Teacher's Note:
1. Give one equation or formula for each type.
2. Six types are in the syllabus of this book.

Question 3. Give the chemical properties of acids and bases by giving balanced chemical equations.
Answer:
Chemical properties of acids
1. With active metals, acids give hydrogen: \(\ce{Zn + H2SO4 -> ZnSO4 + H2}\); \(\ce{Mg + 2HCl -> MgCl2 + H2}\).
2. With metal oxides they give salt and water: \(\ce{ZnO + H2SO4 -> ZnSO4 + H2O}\); \(\ce{CaO + 2HNO3 -> Ca(NO3)2 + H2O}\).
3. With metal carbonates they give salt, water and carbon dioxide: \(\ce{CaCO3 + 2HCl -> CaCl2 + H2O + CO2}\); \(\ce{MgCO3 + H2SO4 -> MgSO4 + CO2 + H2O}\).
4. With bases (neutralization): \(\ce{2KOH + H2SO4 -> K2SO4 + 2H2O}\).
Chemical properties of bases
1. Action of heat: NaOH and KOH only melt. Hydroxides such as calcium hydroxide and aluminium hydroxide decompose: \(\ce{Ca(OH)2 -> CaO + H2O}\) (on heating); \(\ce{2Al(OH)3 -> Al2O3 + 3H2O}\) (on heating).
2. Ammonium hydroxide decomposes even on slight heating: \(\ce{NH4OH -> NH3 + H2O}\) (on heating).
3. With acids (neutralization): \(\ce{Ca(OH)2 + 2HNO3 -> Ca(NO3)2 + 2H2O}\).
4. Acids and some alkalies are corrosive to skin.
In simple words: Acids react with metals, oxides, carbonates and bases; bases decompose on heating and neutralize acids.

Teacher's Note:
1. Write each equation fully balanced.
2. Neutralization is common to both.

Question 4. Describe the methods of preparation of acids and bases with the help of examples.
Answer:
Preparation of acids
1. Direct combination of elements: hydrogen and chlorine combine to give hydrogen chloride gas, which dissolves in water to give hydrochloric acid: \(\ce{H2 + Cl2 -> 2HCl}\).
2. From nonmetals: a nonmetal burns in oxygen to give an acidic oxide, which dissolves in water to give an acid: \(\ce{C + O2 -> CO2}\), \(\ce{CO2 + H2O -> H2CO3}\); \(\ce{S + O2 -> SO2}\), \(\ce{SO2 + H2O -> H2SO3}\); \(\ce{2SO2 + O2 -> 2SO3}\), \(\ce{SO3 + H2O -> H2SO4}\). Oxides that are insoluble in water and do not give an acid (such as NO and CO) are neutral oxides.
Preparation of bases
1. From metals: a metal burns in oxygen to give a basic oxide, which dissolves in water to give a base: \(\ce{4Na + O2 -> 2Na2O}\), \(\ce{Na2O + H2O -> 2NaOH}\); \(\ce{2Mg + O2 -> 2MgO}\), \(\ce{MgO + H2O -> Mg(OH)2}\).
2. Slaked lime from quicklime: \(\ce{CaO + H2O -> Ca(OH)2 + heat}\). Slaked lime dissolves very little and settles; the clear water over it is lime water.
In simple words: Burn the element in oxygen and dissolve the oxide in water: nonmetals give acids and metals give bases.

Teacher's Note:
1. Give one equation for each method.
2. Neutral oxides give neither.

Question 5. Give equations for the dissociation of the following compounds in their solutions and write the formulae of the salts formed from them.
(a) \(\ce{H2SO4}\)   (b) \(\ce{Mg(OH)2}\)
Answer:
(a) \(\ce{H2SO4 -> 2H+ + SO4^2-}\). It can form the normal salt \(\ce{Na2SO4}\) (with NaOH) and the acidic salt \(\ce{NaHSO4}\) (partial neutralization).
(b) \(\ce{Mg(OH)2 <=> Mg^2+ + 2OH-}\). With hydrochloric acid it can form the normal salt \(\ce{MgCl2}\) and the basic salt \(\ce{Mg(OH)Cl}\).
Complete: \(\ce{H2SO4 + Mg(OH)2 -> MgSO4 + 2H2O}\), giving magnesium sulphate.
In simple words: Dibasic sulphuric acid and diacidic magnesium hydroxide can each form two kinds of salts.

Teacher's Note:
1. Write the ionization first, then the salts.
2. Dibasic acids give acidic and normal salts.

Teacher's Note:
1. Learn the three suffixes.
2. Use 'hydrogen' for acidic salts.

Teacher's Note:
1. Make a colour table.
2. A neutral solution gives no change in litmus.

Teacher's Note:
1. Write the use and the property side by side.
2. Give at least two uses for each base.