Acid- bases and salts

Acid bases and Salts Class 10 CBSE (Q.1) You have given three test tubes, one of them contain distilled water and the other two contain an acid solution and a basic solution respectively. If you are given only red litmus paper, how will you identify the contents of each test tube? (3 Marks) (Ans)  We put the red litmus paper in all the test tubes, turn by turn. The solution which turns red litmus blue will be a basic solution. The blue litmus paper obtained here, is dipped in the remaining two test tubes, one by one.The solution which turns the blue litmus paper to red will be the acidic solution and the solution which has no effect on any litmus paper will be neutral and hence is distilled water. (Q.2) A metal compound ‘X’ reacts with dilute hydrochloric acid to produce gas (Y). The gas (Y) evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride. Write names of x & y.    (3 Marks) (Ans)  Metal compound ‘X’ is calcium carbonate because it reacts with dilute hydrochloric acid to give calcium chloride, water and carbon dioxide gas which extinguishes a burning candle. Hence, y is carbon dioxide. CaCO3 + 2HCl CaCl2 + CO2 + H2O Calcium carbonate Hydrochloric acid Calcium chloride + Carbon dioxide (Q.3) What is neutralization reaction? (2 Marks) (Ans)  The reaction of an acid with a base to form a salt and water is known as neutralization reaction. Acid + Base Salt + Water HCl + KOH KCl + H2O (Hydro (Potassium (Potassium (Water)chloric hydroxide) Chloride) acid) (Q.4) Why does distilled water not conduct electricity, whereas rainwater does?    (2 Marks) (Ans)  Distilled water is a bad conductor of electricity, so it does not conduct electricity whereas rainwater is a good conductor of electricity due to the presence of some acid. Hence, rainwater conducts electricity. (Q.5) Fresh milk has a pH 6. How do you think the pH will change as it becomes sour? (2 Marks) (Ans)  pH will decrease as it becomes sour. Sour milk contains lactic acid.So,H+ concentration will increase. (Q.6) A compound which is prepared from gypsum has a property of hardening when mixed with water. Identify the compound. Write the chemical equation of its preparation. Mention two uses of the compound.(5 Marks) (Ans)  The compound is plaster of Paris The chemical name of it, is calcium sulphate hemihydrate. Uses of plaster of Paris: It is used for plastering the fractured bones in hospitals. It is used for making toys and statues. (Q.7) How does acetic acid (vinegar) help in the preservation of food? (1 Mark) (Ans)  The growth of micro-organisms responsible for food spoilage is prevented or they are rendered inactive by pickling in vinegar or acetic acid. (Q.) Name the acid found in (a) Soft drinks (b) Vinegar. (1 Mark) (Ans)  (a)Soft drinks:-Carbonic acid (b)Vinegar :- Acetic acid. (Q.8) How does an acid taste? (1 Mark) (Ans)  An acid has a sour taste. (Q.9) Which gas is usually liberated when an acid reacts with a metal?    Illustrate with an example. How will you test for the presence of this gas? (3 Marks) (Ans)  Hydrogen is usually liberated when an acid reacts with a metal. For example, when zinc granules react with dilute sulphuric acid,? zinc sulphate? and hydrogen are formed.Metal + Acid Salt + Hydrogen gasZn + H2SO4 ZnSO4 + H2Zinc Sulphuric acid Zinc sulphate Hydrogen gas When a burning candle is brought near the gas, it burns with a pop sound. It shows presence of hydrogen gas. (Q.10) What are acid & base indicators? (2 Marks) (Ans)  Acid & base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases. For example, phenolphthalein, methyl orange. (Q.11) Why a substance is acidic or basic in nature? (2 Marks) (Ans)  Acidic nature of a substance is due to the formation of H3+O (aq) ions in aqueous solution and basic nature of a substance is due to the furnishing of OH- (aq) ions in solution. (Q.12) What are alkalis? (2 Marks) (Ans)  The bases which are soluble in water are known as alkalis. For example, NaOH, KOH. (Q.13) Why does an aqueous solution of an acid conduct electricity?(2 Marks) (Ans) When acid is dissolved in water, hydrogen ions (H+) become free to move and hence they can conduct electricity. (Q.14) What is baking soda? How is it produced? Write two uses of it.   (5 Marks) (Ans)  Baking soda is sodium hydrogen carbonate ( NaHCO3). It is produced as follows- Na2CO3 +? H2O + CO2 2 NaHCO3(s) Sodium Water Carbon dioxide Bakingcarbonate Uses of baking soda – For making baking powder which is a mixture of baking soda and tartaric acid. It is used as a mild antiseptic (Q.15) Write an equation to show the reaction between Plaster of Paris and water. (1 Mark) (Ans)  CaSO4. 1/2 H2O?? +? 3/2 H2O?? → CaSO4.2H2O Plaster of Gypsum Paris (Q.16) Name the sodium compound which is used for softening hard water. (1 Mark) (Ans)  Washing soda [Na2CO3.10H2O] (Q.17) What is a salt? (1 Mark) (Ans)  The substance formed by the neutralization reaction of an acid with a base is called salt. NaOH + HCl -->? NaCl + H2O (Q.18) Name three mineral acids. (1 Mark) (Ans)  (i) Sulphuric acid (H2SO4) (ii) Nitric acid (HNO3) (iii) Hydrochloric acid (HCl) (Q.19) Why should curd and sour substances  not be kept in brass and copper vessels? (2 Marks) (Ans)  Curd and sour substances contain acids which can react with the metal of the vessel to form poisonous metal compounds. These compounds can cause food poisoning and damage our health. (Q.20) How is concentration of hydroxide ions (OH-) affected when excess of base is dissolved in a solution of sodium hydroxide? (2 Marks) (Ans)  Concentration of hydroxide ions (OH-) becomes high when excess base is dissolved in a solution of sodium hydroxide. (Q21.) Write a short note on pH scale.(5 Marks) (Ans)  pH - The negative logarithm (base 10) of the hydrogen ion concentration in moles per litre is called pH. pH = - log10 [H+] A scale for measuring hydrogen ion concentration in a solution is known as pH scale. On the pH scale we can measure pH from?0 (very acidic) to 14 (very alkaline). pH indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value. The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the PH value increases from 7 to 14, it represents an increase in OH- concentration that is, increase in the strength of alkali. (Q.22) What is bleaching powder? How is it prepared? Write chemical equation involved in the preparation of bleaching powder. Write two uses of bleaching powder. (5 Marks) (Ans)  Bleaching powder is Calcium oxy chloride ( CaOCl2). It is prepared by passing chlorine gas over dry slaked lime. Ca (OH)2 + Cl2 CaOCl2 + H2O Calcium Hydroxide chlorine Calcium oxy chloride Water (Slaked lime) (Bleaching powder) Uses – 1. Bleaching powder is used for bleaching cotton and linen in the textile industry and for bleaching wood pulp in paper industry. 2. It is used for disinfecting drinking water to make it free of germs. (Q.23) What do you mean by universal indicator? (1 Mark) (Ans)  It is a mixture of several indicators having different pH ranges. It shows many colour changes over a wide range of pH. Each colour corresponds to a certain pH value Q.24) Define pH. (1 Mark) (Ans)  It is defined as the negative logarithm (base10) of the hydrogen ion concentration in moles per litre. pH = - log10 [H+] (Q.25) What is an indicator? (1 Mark) (Ans)  An indicator is a ‘dye’ that changes colour when it is put in an acid or a base (alkali)