Acid Bases And Salts short type ncert solved question

Acid Bases And Salts 

Question (1): Define the terms: Acid, alkali and salt.

 Answer:  An acid is a compound, which releases hydronium ions (H₃O⁺) as the only positive ions in solution. An alkali is a compound, which releases hydroxyl ions (OH^-) as the only negative ions in solution. A salt is one of the products of neutralization between an acid and a base; water being the only other product. OR 
A salt gives positive ions other than H⁺ ion and negative ions other than OH^- ion in solution.

Question (2): Identify the number of replaceable hydrogen ions (H⁺) in the following acids: HCl, CH₃COOH, H₂SO₄, H₃PO₄.

Answer:  HCl = 1    CH₃COOH = 1       H₂SO₄ = 2     H₃PO₄ = 3.

Question (3): What is a neutralization reaction?

Answer:  Neutralization is essentially a chemical reaction between H₃O⁺ ions of an acid with OH^- ions of the base, to give undissociated molecules of water.

Question (4): What are strong and weak acids? Give one example of each?

Answer:  A strong acid is one, which is almost completely dissociated in solution. Examples: Dilute nitric acid, dilute sulphuric acid and dilute hydrochloric acid. A weak acid is one, which is only partially ionized in solution (degree of dissociation is >30%). Examples: Acetic acid, carbonic acid and sulphurous acid.

Question (5): Why is acetic acid called a weak acid though there are 4 'H' atoms in the molecule? 

Answer:  Acetic acid is called a monobasic acid because only one of the 4 'H' atoms of the acid is released as H⁺ ion in solution.

Question (6): How does a strong acid differ from a concentrated acid?

Answer:  The strength of an acid depends upon its dissociation power whereas concentration depends on water content in the acid.

Question (7): Name a salt of a strong acid HNO₃ and a weak base like NH₄OH. Represent the reaction that takes place.

Answer:  The salt that results due to the above reaction is Ammonium nitrate (NH₄NO₃). The reaction is represented as: HNO₃ + NH₄OH --------------> NH₄NO₃   + H₂O

Question (8): 1) Name a strong base and a weak base. 2) Name a hydrated salt.

Answer: 
1) A strong base is sodium hydroxide (NaOH) and a weak base is ammonium hydroxide (NH₄OH).
2) A hydrated salt is copper sulphate crystals (CuSO₄.5H₂O).

Question (9): Name the following: 1) Two non-hydrated crystalline salts 2) Two neutral salts
3) Two basic salts 4) Two acid salts

Answer:  

1) Two non-hydrated crystalline salts are: sodium chloride (NaCl) and potassium nitrate (KnO₃)
2) Two neutral salts are: sodium chloride (NaCl) and sodium sulphate (Na₂SO₄)
3) Two basic salts are: basic copper carbonate (CuCO₃.Cu(OH)₂) and basic lead carbonate (PbCO₃.Pb(OH)₂)
4) Two acid salts are: sodium bicarbonate (NaHCO₃) and sodium phosphate (NaH₂PO₄)

Question (10): Name the salts of sulphuric acid.

Answer:  The salts of sulphuric acid are bisulphate and sulphate. Examples: NaHSO₄, KHSO₄ and Na₂SO₄.

 Question (11): Define the term "pH"; what does" pH" stand for?

Answer:  The term "pH" is defined as the negative logarithm of H⁺ ion concentration of a given solution; the concentration being expressed as moles per litre. Mathematically pH = - log [H⁺] 'pH' stands for: Power of hydrogen ion concentration, 'p' for power and 'H' for H⁺ ion concentration.

Question (12): What is 'pH' scale? Explain briefly.

Answer:  The strength of an acid or a base is expressed in terms of hydronium ion concentration. This is expressed on a scale known as 'pH' scale. It is a 14 point scale; i.e., it has values ranging from 0 to 14, indicating the value of negative logs of H⁺ ion concentration of the solution. Some important benchmark values in the pH scale are: pH = 7 indicates neutral solutions e.g., aqueous solutions. pH > 7 to 14 indicates alkaline solutions and pH < 7 to 0 indicate acidic solutions

Question (13): What is the 'pH' of pure water and that of rain water? Explain the difference. 

Answer:  The pH of pure water is seven. Rain water is slightly acidic because as rain drop fall, the carbon dioxide in the air dissolves with drops to form very weak carbonic acid. Accordingly, rain water has a pH that is slightly below 7.

Question (14): What is the pH of solution 'A' which liberates CO₂ gas with a carbonate salt? Give the reason?

Answer :  The pH of solution 'A' is lesser than 7. Carbonates salts react with acids (A) to liberate CO₂ gas.

Question (15): What is the pH of solution 'B' which liberates NH₃ gas with an ammonium salt? Give reason?

Answer:  The pH of solution 'B' is lesser than 7 because 'B' is an alkali as it liberates NH₃ gas. Question (16): How do you increase or decrease the pH of pure water? Answer:  By adding a few drops of alkali to pure water, it's pH increases; and by adding a few drops of an acid decreases the pH of pure water.

Question (16):Why alkalies like sodium hydroxide and potassium hydroxide should not be left exposed to air?

Answer : Because they are hygroscopic in nature and absorb moisture from atmosphere in which they ultimately dissolve.

Question (17):State whether an aqueous solution of washing soda is acidic or alkaline.
Answer : An aqueous solution of washing soda is alkaline (turns red litmus blue).On dissolving in water it forms strong base (NaOH) and weak acid(H2CO3)

Na2CO3 + H2O → 2NaOH + H2CO3

Question (18): An old man complained of acute pain in the stomach .Doctor gave him a small antacid tablet and he got immediate relief. What actually happened?
Answer : The old person was suffering from acute acidity. An antacid tablet contains sodium hydrogen carbonate(NaHCO3). It reacts with acid (HCl) formed because of acidity and neutralize its effect.

Question (19): An acidic solution always contain some OH- ions in it .Comment. Ans The statement is correct. Actually , acids are dissolved in water to form the
Answer : Now water will also ionize to small extent as it is a weak electrolyte. The OH- ion released by water will remain in the solution. Therefore, an acidic solution always contain some OH- ions in it.

Question (20): A few drops of phenolphthalein indicator was added to an unknown solution ?

Answer : It acquired pink color. Now another solution B was added to it dropwise and the solution ultimately became colorless. Predict the nature of the solution A and B.

Question (21): Does tartaric acid help in making cake or bread fluffy? Explain.

Answer : No ,tartaric acid does not evolve any carbon dioxide during baking. Its role is to react with sodium carbonate (Na2CO3) formed when sodium hydrogen carbonate (NaHCO3) Decomposes. If it is not done , Na2CO3 will impart bitter taste to the cake.

Question (22): The oxide of a metal M was water soluble . When a blue litmus strip was dipped in this solution, it did not undergo any change in colour. Predict the natureof the oxide.

Answer : The metal oxide is basic in nature. It mixes with water to form metal hydroxide as follows

MO + H2O → M(OH)2

A blue litmus does not change the colour in the basic medium.

Question (23): Explain why?

(a) Common salt becomes sticky during the rainy season.

(b) Blue vitriol changes to white upon heating.
Answer : (a) common salt contains impurity of magnesium chloride which is of deliquescent nature. When exposed to the atmosphere , it becomes moist.

Therefore it becomes sticky during rainy season.

(b) Blue vitriol (CuSO4.5H2O) upon heating changes to anhydrous copper sulphate (CuSO4) which is white in colour

Question (24): If bottle full of concentrated sulphuric acid is left open in the air by accident, the acid starts flowing out of the bottle of its own.

Answer : Concentrated sulphuric acid is highly hygroscopic. It absorbs moisture from air and gets diluted. Since the volume increases, the acid starts flowing out of bottle.

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