CBSE Class 7 Science Acid Bases and Salts Worksheet Set A

Acid Bases and Salts 

Question. What do you understand by the terms, acid and base?
Answer. 
(a) Acids are defined as compounds which contain one or more hydrogen atoms, and when dissolved in water, they produce hydronium ions (H3O+), the only positively charged ions.
(b) Hydronium ion
(c) H₃O⁺

Question. Explain the formation of hydronium ion. Write the ionization of sulphuric acid showing the formation of hydronium ion.
Answer. Hydronium ions: They are formed by the reaction of H+ (from acid) and water. It reacts with water to form H₃O⁺ (Hydronium ion).
H⁺ + H₂O → H₃O⁺
Ionization of sulphuric acid showing the formation of hydronium ion:

Question. Water is never added to acid in order to dilute it why?
Answer. If water is added to a concentrated acid, the heat generated causes the mixture to splash out and cause severe burns. Thus, water is never added to acid in order to dilute it.

Question. Define the term ‘basicity’ of an acid. Give the basicity of: nitric acid, sulphuric acid and phosphoric acid.
Answer.  Basicity: The basicity of an acid is defined as the number of hydronium ions (H₃O⁺) that can be produced by the ionization of one molecule of that acid in aqueous solution.
The basicity of following compounds are:
Nitric acid: Basicity = 1
Sulphuric acid: Basicity =2
Phosphoric acid: Basicity =3

Question. Give two examples of each of the following:
(a) oxy-acid
(b) hydracids
(c) monobasic acid
(d) dibasic acid
(e) tribasic acid
Answer. 
(a) Oxyacids: - HNO₃, H₂SO₄
(b) Hydracid:- HCl, HBr
(c) Monobasic acid:- HCl, HBr
(d) Dibasic acid: - H₂SO₄ , H₂CO₃
(e) Tribasic acid:- H₃PO₄, H₃PO₃

Question. Name the:
(a) acidic anhydride of the following acids:
(i) sulphurous acid
(ii) nitric acid
(iii) phosphoric acid
(iv) carbonic acid
(b) acids present in vinegar, grapes and lemon
(c) (i) ion that turns blue litmus red,
(ii) ion that turns red litmus blue.
Answer. 
(a) The anhydride of following acids are:
(i) Sulphurous acid: SO₂
(ii) Nitric acid: N₂O₅
(iii) Phosphoric acid: P₂O₅
(iv) Carbonic acid : CO₂
(b) Acids present in following are:
Vinegar: Acetic acid
Grapes: Tartaric acid and Malic acid
Lemon: Citric acid
(c) (i) H+ ion turns blue litmus red.
(ii) OH- ion turns red litmus blue.

Question. What do you understand by the statement ‘acetic acid is a monobasic acid?
Answer.  Acetic acid is a monobasic acid which on ionization in water produce one hydronium ion per molecule of the acid.

Question.
(a) Give a balanced equation for reaction of nitrogen dioxide with water.
(b) How many types of salts does dibasic acid produce when it reacts with caustic soda solution? Give equation(s)
Answer. 
(a) 2NO₂ + H₂O ⟶ HNO₂ + HNO₃
(b) Two types of salts are produced when dibasic acid reacts with caustic soda. One is acidic salt and other normal salt.
Acid salts:
H₂SO₄ + NaOH ⟶ NaHSO₄ + H₂O
Normal salts:
H₂SO₄ + 2NaOH ⟶ Na₂SO₄ + 2H₂O

Question. Carbonic acid gives an acid salt but hydrochloric acid does not. Explain.
Answer. 
The strength of an acid is the extent to which the acid ionizes or dissociates in water.
The strength of an acid depends on the degree of ionization and concentration of hydronium ions [H₃O⁺] produced by that acid in aqueous solution.

Question. What do you understand by the strength of an acid? On which factor does the strength of an acid depend?
Answer. 
(a) Carbonic acid is a dibasic acid with two replaceable hydrogen ions; therefore it forms one acid salt or one normal salt.
Hydrochloric acid is a monobasic acid with one replaceable hydrogen ion and so forms only one normal salt.
(b) Strength of an acid is the measure of concentration of hydronium ions it produces in its aqueous solution. Dil. HCl produces high concentration of hydronium ion compared to that of concentrated acetic acid. Thus, dil. HCl is stronger acid than highly concentrated acetic acid.
(c) H₃PO₃ is not a tribasic acid because in oxyacids of phosphorus, hydrogen atoms which are attached to oxygen atoms are replaceable. Hydrogen atoms directly bonded to phosphorus atoms are not replaceable.
(d) The salt produced is insoluble in the solution so the reaction does not proceed. Hence, we do not expect lead carbonate to react with hydrochloric acid.
(e) NO₂ is called double acid anhydride because two acids – nitrous acid and nitric acid – are formed when it reacts with water.
2NO₂ + H₂O → HNO₂ + HNO₃

Question. Dil. HCl acid is stronger than highly concentrated acetic acid. Explain.
Answer.  Acid rain is a by-product of a variety of human activities which release oxides of sulphur and nitrogen in the atmosphere. Burning of fossil fuels, coal, oil, petrol and diesel produces sulphur dioxide and nitrogen oxide which pollute the air. Polluted air also contains many oxidising agents which produce oxygen because of excessive heat. This oxygen combines with the oxides of sulphur and nitrogen and rain water to form acids.
2SO₂ + O₂ + 2H₂O → 2H₂SO₄
4NO₂ + O₂ + 2H₂O → 4HNO₃

Question. How is an acid prepared from a
(a) Non-metal
(b) Salt?
Give an equation for each.
Answer. 
Acids are prepared from non-metals by their oxidation. For example :
Sulphur or phosphorus is oxidized by conc. Nitric acid to form sulphuric acid or phosphoric acid.
S + 6HNO₃ H₂SO₄ + 2H₂O + 6NO₂
P + 5HNO₃ H₃PO₄ + H₂O + 5NO₂
Acids are prepared from salt by displacement reaction. For example :
Nitric acid is prepared by using H₂SO₄ and sodium chloride.
NaCl + H₂SO₄ NaHSO₄ + HNO₃

Question. Name an acid used:
(a) to flavor and preserve food,
(b) in a drink,
(c) to remove ink spots,
(d) as an eyewash
Answer. 
(a) SO₂ +H₂O H₂SO₃
(b) P₂O₅ +3H₂O 2H₃PO₄
(c) CO₂ + H₂O H₂CO₃
(d) SO₃ + H₂O H₂SO₄

Question. Give equations to show how the following are made from their corresponding anhydrides.
(a) sulphurous acid
(b) phosphoric acid,
(c) carbonic acid
(d) sulphuric acid
Answer. 
(a) Citric acid
(b) Carbonic acid
(c) Oxalic acid
(d) Boric acid

Question. What do you understand by an alkali? Give two examples of:
(a) strong alkalis
(b) weak alkalis
Answer. 
An alkali is a basic hydroxide which when dissolved in water produces hydroxyl ions (OH-) as the only negatively charged ions.
(a) Strong alkalis: Sodium hydroxide , Potassium hydroxide
(b) Weak alkalis: Calcium hydroxide , Ammonium hydroxide

Question. What is the difference between:
(a) an alkali and a base,
(b) an alkali and a metal hydroxide?
Answer. 
(a) An alkali and a base:
1. Alkalis are soluble in water whereas bases may be or may not be soluble in water.
2. All alkalis are bases but all bases are not alkalis.
(b) An alkali and metal hydroxide :
1. Alkalis are soluble in water whereas metal hydroxides may be or may not be soluble in water

Question. Define in terms of ionization:
(a) an acid
(b) an alkali
Answer. 
(a) An acid: Acids are defined as compounds which when dissolved in water produce hydronium ions.
(b) An alkali: Alkalis are compounds which when dissolved in water produces hydroxyl ions.

Question. Name the ions furnished by:
(a) bases in solution,
(b) a weak alkali
(c) an acid
Answer. 
(a) Bases in solution give hydroxide ion.
(b) Weak alkali gives hydroxide ions.
(c) An acid gives a hydronium ion.

Question. Give one example in each case:
(a) A basic oxide which is soluble in water,
(b) A hydroxide which is highly soluble in water,
(c) A basic oxide which is insoluble in water,
(d) a hydroxide which is insoluble in water,
(e) A weak mineral acid,
(f) a base which is not an alkali
(g) An oxide which is a base,
(h) A hydrogen containing compound which is not an acid,
(i) A base which does not contain a metal ion.
Answer. 
(a) Barium oxide
(b) Sodium hydroxide
(c) Manganese oxide
(d) Cupper hydroxide
(e) Carbonic acid
(f) Ferric hydroxide
(g) Copper oxide
(h) Ammonia
(i) Ammonium hydroxide

CBSE Class 7 Science Chapter 5 Acid Bases and Salts Very Short Answer

Question. What are neutral solutions? 
Answer. The solutions which do not change the colour of either red or blue litmus are called neutral solutions.

Question. Where we find the oxalic acid? 
Answer. Oxalic acid is found in spinach

Question. Define phenolphthalein? 
Answer. An indicator we have not used so far is called phenolphthalein.

Question.Where we find potassium hydroxide? 
Answer. Potassium hydroxide found in the soap.

Question. Define neutralisation? 
Answer. The reaction between an acid and

CBSE Class 7 Science Chapter 5 Acid Bases and Salts Short Answer

Question. What is the effect of the indicator on acidic basic and neutral solution 
Answer. When it is used as an indicator, its colour changes to green in basic solution and dark pink colour in acidic solutions. In neutral solution it colour does not change.
Phenolphthalein solution is colourless. When it comes in contact with basic solution, its colour changes to pink.

Question. What do you understand by the term indicators? 
Answer. Indicators are substances that change colour when they are added to acidic or alkaline solutions. Litmus, phenolphthalein, and methyl orange are all indicators that are commonly used in the laboratory.

Question. How does litmus detect acidic and basic solution? 
Answer. The main use of litmus is to test whether a solution is acidic or basic. Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic or alkaline conditions, with the colour change occurring over the pH range. Neutral litmus paper is purple.

Question. How acids are different form salts? 
Answer. 1. An acid is a substance which dissolved in water ionizes and releases hydrogen ions whereas a salt is a compound formed by the partial or complete replacement of the ionisable hydrogen atoms of an acid by a metallic ion.
2. Acid turns blue litmus paper to red whereas in salts there is no effect on litmus paper.

Question. What is china rose and why it is used as indicators 
Answer. China rose converts acidic solution to dark pink, basic solutions to green colour and has no colour change for neutral solutions. China rose is a natural indicator and is used to identify the acidic or basic nature of a solution.

CBSE Class 7 Science Chapter 5 Acid Bases and Salts Long Answer

Question. Explain neutralisation in everyday life? 
Answer. Neutralisation in everyday life are:
1. Indigestion: Our stomach contains hydrochloric acid, but too much of acid in the stomach causes indigestion. Sometimes indigestion is painful. To relieve indigestion, we take an antacid such as milk of magnesia, which contains magnesium hydroxide.
2. Factory wastes: Factory waste is neutralised before depositing it into the water bodies. Factory wastes are neutralized before disposing into water bodies because, factory wastes often contain acid. If it reaches a river, the acid will kill aquatic life. This is prevented by adding a base called slaked lime.
3. Soil treatment: Excessive uses of chemical fertilisers make the soil acidic. Plants do not grow well when the soil is either too acidic or too basic. When the soil is too acidic. It is treated with the bases like quick lime or slaked lime. If the soil is basic is organic matter is added to it.
4. Ant bite: when an ant bites it injects the acidic liquid into the skin. The effect of the acid can be neutralized by rubbing most baking soda.

Question. How litmus paper is different from pH paper? 
Answer. 1. pH paper is an indicator that can be used to determine the pH value of a solution whereas a litmus paper is an indicator that is used to determine the acidity or the basicity of a solution.
2. pH papers can give the exact pH value of a solution whereas litmus papers cannot give a value for the pH of a solution.
3. pH papers give more precise results whereas litmus papers give a less precise indication.
4. pH papers can give a range of colours in the presence of solutions with different pH values whereas litmus papers can give only two colours.

Question. State the difference between acid and base? 
Answer. 1. Acids are sour in taste whereas bases are bitter to taste.
2. Acids do not change the colour of turmeric indicator whereas bases turn the colour of turmeric indicator to red.
3. Acids do not change the colour of red litmus whereas bases turn red litmus blue.
4. Acids have pH value less than 7 whereas bases have pH value greater than 7.
5. Acids do not give soapy touch whereas bases give soapy touch.

Question. Explain about the natural indicators around us? 
Answer. Natural Indicator is a type of indicator that can be found naturally and can determine whether the substance is an acidic substance or a basic substance. Some examples of natural indicators are red cabbage, turmeric, grape juice, turnip skin, curry powder,
cherries, beetroots, onion, tomato, etc.
1. Litmus paper: When you want to determine the acidity or alkalinity of a liquid or water-soluble gas, the easiest way is to perform a litmus test. The test requires litmus papers, which are narrow strips of paper treated with chemical solutions that are sensitive to pH changes.
2. Turmeric as indicator: Turmeric, a common spice in curry, as an indicator for acidity and basicity. Turmeric is yellow in acid and neutral substances, but turns bright red with bases. Learners can use their indicator to test household chemicals and determine which are basic.

Question. Distinguish between acidic salt and basic salts?
Answer. 1. Acidic salts are ionic compounds that can form acidic solutions upon dissolution in water whereas basic salts are ionic compounds that can form basic solutions upon dissolution in water.
2. Acidic salts are less than pH 7.0 whereas basic salts are higher than pH 7.0.
3. Acidic salts reactivity forms protons whereas basic salts reactivity has force water molecules to remove hydroxide ions.
4. Acidic salts examples are ammonium salts, sodium bi sulphate and calcium oxalate whereas basic salts examples are: sodium bi carbonate, sodium sulphide etc.

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