Online solved CBSE Guess questions - Class VII, Science - Acids, Bases and Salts


Class VII CBSE Science - Acids, Bases and Salts

CBSE Guess - CBSE Guide - Solved CBSE Questions

Question: How would you prepare 
(i) crystals of copper sulphate from metallic copper ?
(ii) Calcium chloride from Marble (calcium carbonate) ?
Answer: (i) Take a solution of sulphuric acid and put some copper turnings to it. The solution becomes blue due to the formation of copper sulphate, by the crystallisation of this solution and crystals of copper sulphate can be obtained:
Cu + H2SO4 -------> CuSO4 + H2
(ii) Calcium chloride can be obtained by treating marble (Calcium Carbonate) with hydrochloric acid.
CaCO3 + 2 HCL -------> CaCl2 + H2 + CO2

CBSE Guide NCERT Solution - image

Question: What is an oxide?
Answer: An oxide is the compound of an element with oxygen. For example - water is an oxide of hydrogen, Rust is an oxide of iron.

Question: Name there types of oxides with their properties.
Answer: Three types of oxides are -
(a) Acidic Oxides:
The oxides which on treatment with water form am acids are called acidic oxides. For example,
CO2 + H2 ------> H2CO3 (carbonic acid)
Non-metals burn in oxygen and form acidic oxides. Such oxides turn blue litmus red like - Carbon dioxide (CO2), Sulphur dioxide (SO2).   
(b) Basic or Metallic Oxides:
The oxides which on treatment with water form alkalines are known as basic oxides. Metallic oxides are mostly basic oxides which turn red litmus blue. For example - Na2O, MgO etc.
Na2O + H2O ----> 2NaOH
(c) Amphoteric Oxides:
The oxides which show the properties of both acidic and basic oxides are known as amphoteric oxides for example, Al2O3, SO2 etc.

Question: What is acid ? (Write definition of acids with their examples) How acids are produced?
Answer: Acids are the chemicals which contain hydrogen atom and are sour in taste.
Acids are produced when oxides of non-metals react with water. Examples of some common acids are - HCl (Hydrochloric Acid), H2SO4 (Sulphuric Acid), HNO3 (Nitric Acid).

Question: Why are mineral acids dangerous?
Answer: Mineral Acids are very strong acids and are mostly used in industries other than laboratories.  Being strong acids they are corrosive in nature. Such acids need to be handled very carefully as they can easily corrode skin, human body, metals, clothes and all coming in contact. Hence, mineral acids are dangerous. Some examples of mineral acids are - hydrochloric acid, sulphuric acid, nitric acid, etc.

Question: What are organic acids ? Name some organic acids, which are used as food ingredients.  
Answer: Acids which contain -COOH functional groups are termed as organic acids. Organic acids are weak acids and are present in animal and plant materials.
Examples of organic acids used as food ingredients - acetic acid, citric acid (C6H8O7), tartaric acid.

Question: What are the differences between Dilute and Concentrated acids ?
Answer: Dilute acid - A dilute acid solution is one which contains only small amount of solute or its solution diluted to a definite strength is called dilute acid.
Concentrated Acid - A concentrated acid is one which contains a high proportion of solute.

Question:  Answer to these questions -
(a) Name the acid present in vinegar used in kitchen
(b) Name the acid present in lemon
(c) Which is called "the king of chemicals"
(d) Name the property due to which salts absorb water from atmosphere
Answer: (a) acetic acid. (b) citric acid. (c) sulphuric acid. (d) deliquescence.  

Question: What are bases ? Give the characteristics of bases.
Answer: Bases are the hydroxide of metals, which give up hydroxide ion after dissociation in aqueous solution.
Characteristics of Bases -
  • Bitter in taste.
  • Turn red litmus to blue.
  • React with acid to form salt and water.
  • Bases are soluble in water and known as alkalies.
Question: What are Acids ? Give some characteristics of acids.
Answer: For definition of acids please refer to our post
Characteristics of Acids
  • Sour in taste.
  • Turn blue litmus to red.
  • Dilution of acid in water is an exothermic reaction (heat evolves).
  • React with metals to evolve hydrogen gas.
2HCl + 2Na -------> 2NaCl + H2
H2SO4 + Zn -------> ZnSO4 + H2
  • React with bases to form salt and water. The process called, neutralisation.
HCl + NaOH ------> NaCl + H2O
H2SO4 + Ca(OH)2 ------> CaSO4 + 2H2O
  • React with basic oxides to form salt and water.
CaO + 2HCl -----> CaCl2 + H2O
Na2O + H2SO4 -----> Na2SO4 + H2O
  • React with carbonates to form salt, water and carbon dioxide.
CaCO3 + 2 HCl -----> CaCl2 + H2O + CO2
MgCO3 + H2SO4 -----> MgSO4 + H2O + CO2

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