SHORT ANSWERS TYPE QUESTIONS
Q.1. Identify the acid and base which form sodium hydrogen carbonate. Write chemical equation in support of your answer. State whether this compound is acidic, basic or neutral. Also write its pH value. [Delhi 2019]
Ans. Sodium hydrogen carbonate - NaHCO3
Sodium hydrogen carbonate is formed by the combination of an acid - H2CO3 (Carbonic acid) and base (NaOH)
NaOH + H2CO3 → NaHCO3 + H2O
The compound is neutral pH value = 8.3
Q.2. A White chemical compound becomes hard on mixing proper quantity of water. It is also used to maintain broken founts in fixed position.
Name the chemical compound and write its chemical formula. Write the chemical equation to show what happens when water is added to this compound in proper quantity? [CBSE Sample Paper 2018]
Ans. The compound is Plaster of Paris. Its formula is CaSO4.1/2 H2O
It change into gypsum when proper quantity of water is added to it.
Q.3. What is water of crystallisation? Name and give formula of two salts which contain water of crystallisation. [CBSE 2018 for blind]
Ans. The water molecules present in crystalline salts are called water of crystallisation.
Hydrated copper sulphate (Blue vitroil)
CuSO4 . 5H2O
Hydrated ferrous sulphate (Green vitroil)
FeSO4 . 7H2O
Washing soda Na2CO3. 10H2O
(a) For the preparation of cakes, baking powder is used. If at home your mother uses baking soda instead of baking powder, how will it affect the taste of the cake and why?
(b) How is baking soda be converted into baking powder?
(c) What makes the cake soft and spongy? [CBSE 2018(C)]
(a) Baking powder consists of sodium hydrogen- carbonate (baking soda) and tartaric acid. If only baking soda is used in making cake, then sodium carbonate is formed on heating which will give a bitter taste to cake.
(b) Baking soda can be converted into baking powder by adding starch and tartaric acid.
(c) Sodium hydrogen carbonate gives carbon dioxide (CO2) gas which makes the cakes soft and spongy.
Q.5. The pH of a salt used to make tasty and crispy pakoras is 14. Identify the salt and write a chemical equation for its formation. List its two uses. [CBSE 2018]
(a) The salt is sodium hydrogen carbonate (NaHCO3) or sodium bicarbonate.
(b) Manufacture of baking soda is as follows:
NaCl + H2O + CO2 + NH3 → NH4Cl + NaHCO3
(i) It is used for making baking powder.
(ii) It is used in soda acid fire extinguishers.
(iii) It is used as ingredient of antacid.
Q.6. 2 mL of sodium hydroxide solution is added to a few pieces of granulated zinc metal taken in a test tube. When the contents are warmed, a gas evolves which is bubbled through a soap solution before testing. Write the equation of the chemical reaction involved and the test to detect the gas.
Name the gas which will be evolved when the same metal reacts with dilute solution of a strong acid. [CBSE 2018]
(a) Zinc reacts with NaOH to form sodium zincate and hydrogen gas
(b) When a burning splinter is brought near the gas, the splinter will gives out a ‘pop’ sound which shows the gas evolved is hydrogen.
(c) Hydrogen gas will be evolved when Zn metal reacts with dilute solution of strong acid.
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2↑
Q.7. 1 g of solid sodium chloride is taken in a clean and dry test tube and 2 mL of Conc. sulphuric acid is added to it. If the gas evolved is tested first with dry and then with wet blue litmus paper, in which case will the litmus paper change colour? Give reason for your answer with chemical equation for the reaction. [CBSE 2018C]
Ans. The colour change will observed in wet blue litmus paper because moisture/water is needed for the formation of hydronium ions due to which colour change of litmus paper take place.
NaCl + H2SO4 → Na2SO4 + HCl
Q.8. How are bases different from alkalis? Are all bases alkalis? [CBSE 2017]
Ans. Bases which are soluble in water and produce OH- ions are called alkalis. Some examples are:
Sodium hydroxide NaOH → Na+ + OH-
Potassium hydroxide KOH → K+ + OH-
All alkalis are bases but all bases are not alkalis.
For example, Al(OH)3 is a base but not an alkali because it does not dissolve in water.
Q.9. Answer the following questions: [CBSE 2016]
(i) State the colour of phenolphthalein in soap solution.
(ii) Name the by-product of chlor-alkali process which is used for the manufacture of bleaching powder.
(iii) Name one indicator which specifies the Various levels of H+ ion concentration.
(iii) Universal indicator
Q.10. On passing excess carbon dioxide gas through lime water, it first turns milky and then becomes colourless. Explain why? Write all the chemical equations of the reactions involved. [CBSE 2016]
Ca(OH)2 + CO2 → CaCO3(s) + H2O
CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2
Lime water turns milky due to the formation of calcium carbonate. It becomes colourless when excess of CO2 is passed due to formation of Ca(HCO3)2 which is soluble in water.
Q.11. Name of type of chemical reaction represented by the following equation : [CBSE 2016]
(i) CaO + H2O → Ca(OH)2
(ii) 3BaCl2 + Al2 (SO4)3 → 2AlCl3 + 3BaSO4
(i) Combination reaction.
(ii) Double displacement reaction.
(iii) Decomposition reaction.
(a) Define olfactory indicators. Name two substances which can be used as olfactory indicators.
(b) Choose strong acids from the following :
CH3COOH, H2SO4, H2CO3, HNO3 [CBSE 2016]
(a) Indicators which work on the basis of odour are called olfactory indicators.
Onion and vanilla extracts can be used as olfactory indicators.
(b) H2SO4 and HNO3 are strong acids out of the four acids given.
(a) A student detected the pH of four unknown solutions A, B, C and D as follows: 11, 5, 7 and 2.
Predict the nature of the solution.
(b) Explain how an antacid works. [CBSE 2015]
(a) A is basic, ‘B’ is acidic ‘C’ is neutral and ‘D’ is strongly acidic.
(b) Hyperacidity is Caused by excess of hydrochloric acid in stomach. Antacid is basic in nature. It neutralises excess of acid and releases CO2 gas which gives relief from pain caused by hyperacidity.
Q.14. Explain the action of dilute hydrochloric acid on the following with chemical equations: [CBSE 2015]
(a) Magnesium ribbon
(b) Sodium hydroxide
(c) Crushed egg shells
(a) Hydrogen gas will be formed, e.g.
Mg (s) + 2HCl (dil) → MgCl2 (aq) + H2 (s)
(b) Sodium chloride and water will be formed, e.g.
NaOH + HCl → NaCl + H2O (Neutralization reaction)
(c) Crushed egg shells are made up of CaCO3 which reacts with dil HCl to give brisk effervescence due to CO2, e.g.
CaCO3 (s) + 2HCl → CaCl2 + H2O + CO2
Q.15. What is meant by water of crystallisation? How would you show that copper sulphate crystals contain water of crystallisation? [CBSE 2015]
Ans. Water which is present in the crystals of a compound is called water of crystallisation. It is defined as the fixed number of water molecules present in one formula unit of a salt. Copper sulphate crystals are blue in colour. On heating solid copper sulphate crystals, they lose water of crystallisation and become white. On adding a few drops of water to the white powder, blue colour of copper sulphate is restored. This shows that copper sulphate crystals contain water of crystallisation.
Q.16. Define water of crystallisation with two examples. How will you prove their existence in the examples given by you? [CBSE 2015]
Ans. Number of fixed water molecules associated with one formula unit of a substance is called water of crystallisation.
(i) Gypsum on heating loses half molecule of water and forms plaster of Paris.
On adding water, it again converts to gypsum.
(ii) Copper sulphate on heating loses water molecules and is converted to white anhydrous CuSO4. On adding water to anhydrous copper sulphate, the blue colour of copper sulphate is restored.
(a) Three acidic solutions A, B and C have p H = 0, 3 and 5 respectively.
(i) Which solution has highest concentration of H+ ions?
(ii) Which solution has the lowest concentration of H+ ions?
(b) How concentrated sulphuric acid can be diluted? Describe the process. [CBSE 2015]
(i) Solution A has the highest concentration of H+ ions.
(ii) Solution C has the lowest concentration of H+ ions.
(b) While diluting concentrated sulphuric acid, water is not added to the acid, rather acid is added to water.
Take some water in a beaker. Add cone, sulphuric acid in small lots to water. Stir the mixture with glass rod constantly. Cool the beaker under water if it becomes hot.
Q.18. Give the chemical name and formula of bleaching powder. What happens when it is exposed to air? Mention two uses of bleaching powder. [CBSE 2015]
Ans. Chemical name of bleaching powder is calcium oxychloride.
Formula: CaOCl2 CaOCl2 On exposure to air, it absorbs moisture.
(i) As an oxidising agent in chemical industries.
(ii) For disinfecting drinking water to make it free from germs.
LONG ANSWER TYPE QUESTION
(a) The soil in a field is highly acidic. List any two materials which can be added to this soil to reduce its acidity. Give the reason for your choice. [CBSE 2016]
(b) A gas produced in the laboratory is highly soluble in water. Its colourless solution turns pink when a few drops of phenolphthalein is added to it. What is the nature of this gas?
(i) Lime, (CaO) can be added to neutralize acidity.
(ii) Chalk, (CaCO3) can also be added to neutralise acidity.
It is because CaO and CaCO3 are basic in nature which neutralize acid present in soil.
(b) The nature of gas is basic because it turns phenolphthalein pink. The following reaction takes place in aqueous solution,
Example: NH3 + H2O → NH4+ + OH-
(a) The pH of soil A is 7.5 while that of soil B is 4.5. Which of the two soils A or B should be treated with powdered chalk to adjust its pH and why? [CBSE 2016]
(b) Name the chemical which is injected into the skin of a person:
(i) During an ant’s sting
(ii) During the nettle leaf sting.
How can the effect of these stings be neutralised?
(c) Explain how the pH change in the river water can endanger the lives of aquatic animals like fish?
(a) Soil ‘B’ is acidic. It should be treated with powdered chalk which is basic so as to adjust its pH.
(i) Formic acid, (HCOOH).
(ii) Formic acid, (HCOOH).
The effect of these stings neutralized by rubbing the skin with mild base like baking soda (NaHCO3).
(c) If pH of river water changes, amount of oxygen dissolved in water may decrease. Acidic and basic water is harmful for skin of aquatic animals like fish.
Q.3. State reasons for the following statements:
(i) Stain of curry on a white cloth becomes reddish brown when soap is scrubbed on it and turns yellow again when the cloth in washed with plenty of water.
(ii) Curd should not be kept in copper or brass vessels. What is done to protect it? [CBSE 2016]
(i) Turmeric reacts with sodium hydroxide present in soap to form red coloured compound. It turns yellow again because sodium hydroxide becomes very dilute on adding lot of water and reaction stops.
(ii) Curd contains lactic acid which reacts with copper or brass vessels and taste changes. Curd should be kept in glass, steel or ceramic container which does not react with lactic acid present in it.
(a) What is pH value of salt formed by a
(i) Weak acid and strong base?
(ii) Strong acid and strong base?
(b) 15 mL of water and 10 ml of sulphuric acid are to be mixed in a beaker
(i) State the method that should be followed with reason.
(ii) What is this process called? [CBSE 2015]
(c) What is observed when sulphur dioxide is passed through
(i) water (ii) lime water?
Also write chemical equations for the reactions that take place.
(i) The pH value will be greater than 7.
(ii) The pH value will be less that than 7.
(i) The acid is to be added slowly in water to prevent the mixture to be from being splashed. The reaction is highly exothermic, therefore, constant cooling should be done.
(ii) The process is called dilution.
(i) Sulphurous acid is formed.
SO2 + H2O → H2SO3 (Sulphurous acid)
Lime water turned turns milky due to formation of calcium sulphite.
(a) State the chemical properties on which the following uses of baking soda are based:
(i) as an antacid
(ii) as soda-acid fire extinguisher
(iii) to make bread and cake soft and spongy.
(b) How washing soda is obtained from baking soda? Write balanced chemical equation. [CBSE 2015]
(i) It is weakly basic in nature and neutralises hyperacidity.
(ii) It liberates CO2 with H2SO4 reacts with it, which extinguisher fire.
(iii) It liberates CO2 on heating which makes bread and cake soft and sponge.
Baking soda on heating gives sodium carbonate which on crystallisation from aqueous solution gives washing soda, e.g.
Na2CO3 + 10H2O → Na2CO3.10H2O
Q.6. State reason for the following statements: [CBSE 2015]
(a) Tap water conducts electricity whereas distilled water does not.
(b) Dry hydrogen chloride gas does not turn blue litmus red whereas dilute hydrochloric acid does.
(c) During summer season, a milk man usually adds a very small amount of baking soda to fresh milk.
(d) For a dilution of acid, acid is added into water and not water into acid.
(e) Ammonia is a base but does not contain hydroxyl group.
(a) Tap water contains ions which conduct electricity, distilled water does not contain ions.
(b) Dry HCl does not form ions but HCl gives H+ and Cl-.
(c) Baking soda does not allow milk to change to lactic acid which makes milk sour.
(d) Adding water to acid is highly exothermic and and acid may splash. Therefore, acid is added to water very slowly with continuous stirring.
(e) Ammonia dissolves in water forms OH-. Therefore, it is basic.
NH3 + H2O → NH4+ + OH-
(a) Define universal indicator. Mention its one use.
(b) Solution A gives pink colour when a drop of phenolphthalein indicator is added to it. Solution B gives red colour when a drop of methyl orange is added to it. What type of solutions are A and B and which one of the solutions A and B will have a higher pH value?
(c) Name one salt whose solution has pH more than 7 and one salt whose solution has pH less than 7. [CBSE 2015]
(a) A universal indicator is a mixture of several indicators. It shows different colours at different concentrations of H+ ion concentrations.
(b) Solution A is alkaline in nature.
Solution B is acidic in nature.
Solution A has a higher pH value.
(c) Salts of weak acid and strong base give a solution which have pH more than 7.
i.e. basic salt
Sodium acetate is an example of such salts.
Salts of strong acid and weak base give a solution which has pH less than 7.
i.e. Acidic salt. Ammonium chloride is an example of such salts.