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

Short Answer Questions

Teacher's Note:
1. Draw a two-column table in the exam.
2. Remember: all organic acids are weak.

Question 2. Give an example of a dibasic acid. How is it obtained from the corresponding nonmetal? Give balanced equations.
Answer:
Sulphuric acid, \(\ce{H2SO4}\), is a dibasic acid. It is obtained from sulphur in three steps:
\(\ce{S + O2 -> SO2}\)
\(\ce{2SO2 + O2 -> 2SO3}\)
\(\ce{SO3 + H2O -> H2SO4}\)
In simple words: Burn sulphur, oxidise the gas, and dissolve the product in water.

Teacher's Note:
1. Show all three equations and balance them.
2. Carbonic acid is another dibasic acid, from carbon.

Question 3. What are active metals? Give examples with equations.
Answer:
Metals that displace hydrogen from strong acids such as HCl and \(\ce{H2SO4}\) are called active metals.
Examples: \(\ce{Zn + H2SO4 -> ZnSO4 + H2}\) and \(\ce{Mg + 2HCl -> MgCl2 + H2}\).
In simple words: Active metals are reactive enough to push hydrogen out of acids.

Teacher's Note:
1. Zinc and magnesium are common examples.
2. Copper is not active enough.

Question 4. Explain the corrosive action of acids and alkalies. Give examples.
Answer:
All acids and some alkalies form painful blisters on skin; this is their corrosive action.
Examples:
1. \(\ce{H2SO4}\) absorbs water from the skin tissues.
2. \(\ce{HNO3}\) reacts with skin proteins and forms a pulp-like mass.
3. NaOH and KOH are called caustic soda and caustic potash because of their burning action on skin.
In simple words: Strong acids and alkalies damage skin on contact.

Teacher's Note:
1. Mention at least one acid and one alkali.
2. Wash the area with plenty of water at once.

Question 5. Distinguish between an acid and base with respect to definitions.
Answer:
An acid is a substance that gives \(\ce{H+}\) ions as the only positive ions when dissolved in water.
A base is a substance that gives \(\ce{OH-}\) ions as the only negative ions when dissolved in water.
Example: \(\ce{HCl -> H+ + Cl-}\) (acid) and \(\ce{NaOH -> Na+ + OH-}\) (base).
In simple words: Acids give H+ in water and bases give OH- in water.

Teacher's Note:
1. Add one equation for each.
2. The splitting into ions is called ionization.

Question 6. Define the following and give an example for each.
(a) efflorescent substances   (b) deliquescent salts   (c) hygroscopic salts
Answer:
(a) Efflorescent substances lose water of crystallization fully or partly on exposure to air. Example: \(\ce{Na2CO3.10H2O}\) (washing soda).
(b) Deliquescent salts absorb moisture from air and dissolve in it to become a solution. Examples: calcium chloride, zinc chloride.
(c) Hygroscopic substances absorb moisture from air without changing their physical state. Examples: calcium oxide, cobalt chloride.
In simple words: Efflorescent loses water, deliquescent turns to liquid, hygroscopic stays solid.

Teacher's Note:
1. Define by what happens to the substance in air.
2. Learn one example for each.

Question 7. What are mineral acids? How can they be further classified on the basis of nature of the constituents?
Answer:
Acids obtained from the minerals of the earth's crust are mineral acids (inorganic acids), e.g. HCl, \(\ce{H2SO4}\), \(\ce{HNO3}\).
On the basis of their constituents they are of two types:
1. Hydracids contain hydrogen and another nonmetal only. Examples: HCl, HBr, \(\ce{H2S}\).
2. Oxyacids contain oxygen as well as hydrogen and a nonmetal. Examples: \(\ce{H2SO4}\), \(\ce{HNO3}\), \(\ce{H3PO4}\).
In simple words: Hydracids have no oxygen and oxyacids have oxygen.

Teacher's Note:
1. Give two examples of each type.
2. Remember: hydracids are named 'hydro...ic acid'.

Teacher's Note:
1. All organic acids are weak.
2. Add the ionization equations.

Teacher's Note:
1. Give at least three examples.
2. Fewer oxygens: 'ous', more oxygens: 'ic'.

Question 10. What is meant by neutralization? Give balanced equations showing neutralization reactions of the following:
(a) slaked lime and sulphuric acid   (b) sodium bicarbonate and hydrochloric acid   (c) magnesium carbonate and sulphuric acid
Answer:
The reaction of an acid with a base to give salt and water is called neutralization.
(a) \(\ce{Ca(OH)2 + H2SO4 -> CaSO4 + 2H2O}\)
(b) \(\ce{NaHCO3 + HCl -> NaCl + CO2 + H2O}\)
(c) \(\ce{MgCO3 + H2SO4 -> MgSO4 + CO2 + H2O}\)
In simple words: An acid and a base cancel each other and form a salt and water.

Teacher's Note:
1. Check that each equation is balanced.
2. Carbonates and bicarbonates also give \(\ce{CO2}\).

Question 11. What is the action of heat on the following?
(a) ammonium hydroxide   (b) aluminium hydroxide   (c) sodium hydroxide
Answer:
(a) Ammonium hydroxide decomposes quickly on slight heating: \(\ce{NH4OH -> NH3 + H2O}\) (on heating).
(b) Aluminium hydroxide decomposes on strong heating: \(\ce{2Al(OH)3 -> Al2O3 + 3H2O}\) (on heating).
(c) Sodium hydroxide only melts on strong heating and does not decompose.
In simple words: Heat breaks some hydroxides into oxides and water, but \(\ce{NaOH}\) only melts.

Teacher's Note:
1. \(\ce{NaOH}\) and \(\ce{KOH}\) do not decompose.
2. Ammonium hydroxide gives ammonia gas.

Teacher's Note:
1. Learn the colour table.
2. Neutral solutions give no change in litmus.

Teacher's Note:
1. Examples: \(\ce{NaH2PO4}\) (acidic), \(\ce{Ca(OH)Cl}\) (basic).
2. Write all four points.

Question 14. Mention four important properties of salts.
Answer:
1. Salts are solids at room temperature.
2. Salts have high melting points.
3. Most salts dissolve in water; carbonates, sulphides and some sulphates of metals are exceptions.
4. Salts of sodium and potassium are colourless, while salts of iron, copper and chromium are coloured.
5. Aqueous solutions of salts conduct electricity, so salts are electrolytes.
In simple words: Salts are hard, high-melting solids that give conducting solutions.

Teacher's Note:
1. Four points are enough; the fifth is a bonus.
2. Give a coloured salt example, such as blue copper sulphate.

Question 15. Mention two uses of the salts given below:
(a) sodium chloride   (b) sodium bicarbonate   (c) calcium sulphate hemihydrate
Answer:
(a) Sodium chloride: food additive and a preservative for pickles; used to manufacture chlorine and NaOH.
(b) Sodium bicarbonate (baking soda): raising agent in cakes; used in fire extinguishers and in antacid tablets.
(c) Calcium sulphate hemihydrate (plaster of Paris): making statues and setting broken bones.
In simple words: Common salt flavours and preserves food, baking soda makes cakes rise, and plaster of Paris makes casts.

Teacher's Note:
1. Give two uses for each salt.
2. Learn the uses table.

Teacher's Note:
1. Write the use and the property side by side.
2. \(\ce{HCl}\) is also used in dyeing; nitric acid in explosives.

Question 17. Though phosphorus acid contains three hydrogen molecules, it is a dibasic acid. Give reasons.
Answer:
The formula of phosphorous acid is \(\ce{H3PO3}\).
In water it gives only two \(\ce{H+}\) ions. Only two of the three hydrogen atoms are replaceable; the third is non-replaceable.
Basicity is the number of replaceable H, so it is a dibasic acid.
In simple words: Only two of its three hydrogens can come off as H+.

Teacher's Note:
1. Basicity is not decided by the number of H atoms in the formula.
2. Phosphoric acid \(\ce{H3PO4}\) is different: it is tribasic.

Question 18. While showing some common compounds used in our daily life in the laboratory, the teacher showed sodium chloride compound and told that it is also called common salt. Next day, one of the students brought common salt from home to the laboratory and requested the teacher to show sodium chloride as well. He kept both the common salt and sodium chloride in two different watch glasses and left. The next day, during lunch break the student went to the laboratory and saw that the common salt brought from home became sticky, whereas the compound sodium chloride remained as it is. What would be the justification given by the teacher for the above observation?
Answer:
Common salt from home contains small amounts of impurities such as \(\ce{MgCl2}\) and \(\ce{CaCl2}\).
These impurities are deliquescent: they absorb moisture from the air and turn into a solution.
This makes the common salt sticky. Pure sodium chloride is not deliquescent, so it stayed dry.
In simple words: Impurities in household salt soak up moisture, and pure sodium chloride does not.

Teacher's Note:
1. Table salt is not 100 percent \(\ce{NaCl}\).
2. Deliquescent impurities cause the stickiness in the rainy season.