Q.1 Give an example of a metal which
(i) is a liquid at room temperature.
(ii) can be easily cut with a knife.
(iii) is the best conductor of heat.
(iv) is a poor conductor of heat.
Ans: Malleable: Substances that can be beaten into thin sheets on hammering are called malleable. For example – most of the metals like gold are malleable.
Ductile: Substances that can be drawn into thin wires are called ductile. For example -most of the metals like copper and aluminium are ductile.
Q.2 Explain the meanings of malleable and ductile.
Ans: Malleable: Substances that can be beaten into thin sheets on hammering are called malleable. For example – most of the metals like gold are malleable. Ductile: Substances that can be drawn into thin wires are called ductile. For example -most of the metals like copper and aluminium are ductile.
Q.3 Why is sodium kept immersed in kerosene oil?
Ans: Sodium is a very reactive metal. It combines explosively with air as well as water. Hence, it catches fire if kept in open. Therefore, to prevent accidental fires and accidents, sodium is kept immersed in kerosene oil.
Q.4 Write equations for the reactions of:
(i) iron with steam
(ii) calcium and potassium with water
(iii) potassium with water
Ans: (i) 3Fe(s) + 4 H2O (g) → Fe3O4(aq)+ 4 H2( g)
(ii) Ca(s) + 2 H2O(l) → Ca (OH)2 (aq) + H2(g) + Heat
(iii) 2K(s) + 2 H2O(l) → 2 KOH (aq) + H2(g) + Heat
Q.5 Samples of four metals A, B, C and D were taken and added to the following solution one by one. The results obtained have been tabulated as follows
Use the table above to answer the following questions about metals A, B, C and D.
(i) Which is the most reactive metal?
(ii) What would you observe if B is added to a solution of copper (II) sulphate?
(iii) Arrange the metals A, B, C and D in the order of decreasing reactivity.
: Ans: (i) B is the most reactive metal.
(ii) If B is added to a solution of copper (II) sulphate, then it would displace copper.
B + CuSO4 → Displacement
(iii) The arrangement of the metals in the order of decreasing reactivity is:
B > A > C > D
A + FeSO4 → No reaction, A is less reactive than iron
A + CuSO4 → Displacement reaction, A is more reactive than copper
B + FeSO4 → Displacement reaction, B is more reactive than iron
B + ZnSO4 → No reaction, B is less reactive than zinc
C + FeSO4 → No reaction, C is less reactive than iron
C + CuSO4 → No reaction, C is less reactive than copper
C + ZnSO4 → No reaction, , C is less reactive than zinc
C + AgNO3 → Displacement, C is more reactive than silver
D + FeSO4 / CuSO4 / ZnSO4 / AgNO3 → No reaction, D is less reactive than
iron, copper, zinc and silver.
Q.6 What would you observe when zinc is added to a solution of iron (II) sulphate? Write the chemical reaction that takes place.
Ans: Zinc is more reactive than iron. Therefore, if zinc is added to a solution of iron (II) sulphate, then it would displace iron from the solution.
Zn (s) + FeSO4(aq) → ZnSO4(aq) + Fe (s)
Q.7 How do metals and non-metals react?
Ans: In the above activities, you saw the reactions of metals with a number of reagents. Why do metals react in this manner? Let us recall what we learnt about the electronic configuration of elements in Class IX. We learnt that noble gases, which have a completely filled valence shell, show little chemical activity. We, therefore, explain the reactivity of elements as a tendency to attain a completely filled valence shell.
Q.8 Write the electron-dot structures for sodium, oxygen and magnesium
The representation of elements with valence electrons as dots around the elements is referred to as electron-dot structure for elements. This is the electron-dot structures for sodium.This is the electron-dot structures for oxygen. And this is the electron-dot structures for magnesium.
Q.9 Show the formation of Na2O and MgO by the transfer of electrons
Q.10 What are the ions present in these compounds?
Ans: The ions present in Na2O are Na+ and O2– ions and in MgO are Mg2+ and O2– ions.
Q.11 Why do ionic compounds have high melting points?
Ans: Ionic compounds have strong electrostatic forces of attraction between the ions. Therefore, it requires a lot of energy to overcome these forces. That is why ionic compounds have high melting points.
Q.12 Define the following terms.
(i) Mineral (ii) Ore (iii) Gangue
Ans: (i) Mineral: The naturally occurring compounds of elements are known as Mineral.
(ii) Ore: Minerals from which metals can be extracted profitably are known as ores.
(iii) Gangue: The impurities present in the ore such as sand, rocks etc are non as gangue
Q.13 Name two metals which are found in nature in the free state.
Ans: The metals at the bottom of the reactivity series are mostly found in free state. For example: gold, silver, and platinum.
Q.14 What chemical process is used for obtaining a metal from its oxide?
Ans: A metal can be extracted from its oxide by the process of reduction.
Q.15 Which metals do not corrode easily?
Ans: Metals which have low reactivity such as silver, gold does not corrode easily.
Q.16 What are alloys?
Ans: An alloy is the homogeneous mixture of two or more metals or metals and non-metals. For example brass is an alloy of copper and zinc.
Q.17 You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?
Ans: Step 1: Let us mark the three test tubes as A, B, and C.
Step 2: A drop of the solution in test tube A is put on the red litmus paper. Same is repeated with solutions B and C. If either of them changes colour to blue, then it is basic. Therefore, out of three, one is eliminated.
Step 3: Out of the remaining two, any one can be acidic or neutral. Now a drop of basic solution is mixed with a drop of each of the remaining two solutions separately and then a drop of each solution is put on the red litmus paper.
If the colour of red litmus turns blue, then that solution is neutral and if there is no change in colour, then that solution is acidic. This is because acidic and basic solutions neutralise each other. Hence, we can distinguish between the three types of solutions.
Q.18 Why should curd and sour substances not be kept in brass and copper vessels?
Ans: Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, metals react with the acids to form harmful toxic products, thereby spoiling the food and damaging our health.
Q.19 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?
Ans: Hydrogen gas is usually liberated when an acid reacts with a metal.
Procedure:
Step 1: Take few pieces of zinc granules and add 5 ml of dilute H2SO4.
Step 2: Shake it and pass the gas produced into a soap solution. Bubbles are formed in the soap solution. These soap bubbles contain hydrogen gas.
Zn + dil. H2SO4 → ZnSO4 + H2
Test for hydrogen gas: The evolved hydrogen gas can be tested by bringing a burning candle near the soap bubbles. Hydrogen gas burns with a pop sound.
Q.20 Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.
Ans: Metal compound A should be a compound of Ca as a calcium salt is formed in the product. We know that CO2 extinguishes a burning candle. Also, since carbon dioxide is liberated, therefore, it must be a carbonate. Hence, it is calcium carbonate.
CaCO3 (S) + 2HCl (aq) → CaCl2 (S) + CO2(g) + H2O (l)
Q.21 Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
HCl and HNO3 show acidic character because they dissociate in the presence of water to form hydrogen or hydronium ions.Although aqueous solutions of glucose and alcohol contain hydrogen, these do not dissociate in water to form hydrogen or hydronium ions. Hence, they do not show acidic character.
Q.22 Why does an aqueous solution of acid conduct electricity?
Ans: Acids dissociate in aqueous solutions to form charged particles called ‘ions’. These ions are responsible for conduction of electricity.
Q.23 Why does dry HCl gas not change the colour of the dry litmus paper?
Ans: Dry HCl gas does not change the colour of the dry litmus paper because it does not contain hydrogen or hydronium ions.
Q.24 While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Ans:It is recommended that the acid should be added to water and not water to the acid because the process of dissolving an acid in water is exothermic. If water is added to acid, since large amount of acid is present, a large amount of heat is generated at once. Hence, the mixture can splash out and cause burns. But, if acid is added to water, then heat is evolved gradually and easily absorbed by the large amount of water.
Q.25 How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Ans: When an acid is diluted, the concentration of hydronium ions (H3O+) per unit volume decreases. This means that the strength of the acid decreases.
Q.26 How is the concentration of hydroxide ions (OH-) affected when excess base is dissolved in a solution of sodium hydroxide?
Ans: The concentration of hydroxide ions (OH-) would increase when excess base is dissolved in a solution of sodium hydroxide.
Q.27 You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?
Ans: A pH value of less than 7 indicates an acidic solution, while greater than 7 indicates a basic solution. The pH of a solution is inversely proportional to its hydrogen ion concentration. Hence, the solution having lower pH will have more hydrogen ion concentration. Therefore, the solution with pH = 6 is acidic and has more hydrogen ion concentration than the solution of pH = 8 which is basic.
Q.28 What effect does the concentration of H+(aq) ions have on the nature of the solution?
Ans: If the concentration of H+ ions is increased, the solution becomes more acidic. If the concentration of H+ ions is decreased, the solution becomes less acidic or more basic.
Q.29 Do basic solutions also have H+(aq) ions? If yes, then why are these basic?
Ans: Yes, a basic solution also has H+(aq) ions. These come from the ionization of water in which the base is dissolved. However, their concentration is less as compared to the concentration of OH-(aq) ions that makes the solution basic.
Q.30 Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?
Ans: If the soil is too acidic and improper for cultivation, then to neutralise the acidity of the soil, the farmer would treat the soil with bases like quick lime or slaked lime or chalk.
Q.31 What is the common name of the compound CaOCl2?
Ans: The common name of the compound CaOCl2 is bleaching powder.
Q.32 Name the substance which on treatment with chlorine yields bleaching powder?
Ans: Calcium hydroxide or slaked lime [Ca(OH)2], on treatment with chlorine, yields bleaching powder.
Q.33 Name the sodium compound which is used for softening hard water.
Washing soda (Na2CO3.10 H2O) is used for softening hard water.
Q.34 What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Ans: When a solution of sodium hydrogencarbonate is heated, sodium carbonate and water are formed with the evolution of carbon dioxide gas.
Q.35 Write an equation to show the reaction between Plaster of Paris and water.
Ans: The chemical equation for the reaction of Plaster of Paris and water can be represented as:
Q.36 A solution turns red litmus blue, its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10
Ans: Concept insight: Bases turn red litmus blue and acids turn blue litmus red. Basic solution has a pH value more than 7. Since the solution turns red litmus blue, its pH is likely to be 10.
Q.37 A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
(a) NaCl
(b) HCl
(c) LiCl
(d) KCl
Ans: HCl
Concept insight: Crushed egg shells contain calcium carbonate. Calcium carbonate reacts with HCl to liberate CO2 gas which turns lime water milky.
Q.38 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount of HCl solution (the same solution as before) required to neutralise it will be
(a) 4 mL
(b) 8mL
(c) 12 mL
(d) 16 mL
Ans: (d) 16 mL
Concept insight: 16 mL of HCl solution will be required since the volume of NaOH is doubled.
Q.39 Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Ans: (c) Antacid
Concept insight: Antacid is used for treating indigestion because antacids are mild bases that are able to neutralise excess of acid produced during indigestion.
Q.40 Write word equations and then balanced equations for the reaction taking place when –
(a) Dilute sulphuric acid reacts with zinc granules.
(b) Dilute hydrochloric acid reacts with magnesium ribbon.
(c) Dilute sulphuric acid reacts with aluminium powder.
(d) Dilute hydrochloric acid reacts with iron filings.
Ans: (a) Sulphuric acid + Zinc ⇨ Zinc sulphate + Hydrogen
H2S04(aq) + Zn(s) ⇨ ZnSO4 (aq) + H2 (g)
(b) Hydrochloric acid + Magnesium ⇨ Magnesium chloride + Hydrogen
2HCl (aq) + Mg (s) ⇨ MgCl2 (aq) + H2 (g)
(c) Sulphuric acid + Aluminium ⇨ Aluminium sulphate + Hydrogen
3 H2SO4(aq) + 2 Al(s) ⇨ Al2(SO4)3 (aq) + 3 H2(g)
(d) Hydrochloric acid + Iron ⇨ Ferric chloride + Hydrogen
6 HCl (aq) + 2 Fe (s) ⇨ 2 FeCl3 (aq) + 3H2 (g)
Concept insight: First convert the word equation into skeletal equation and then balance the different atoms on both the sides of the equation. Also, do not forget to write the symbols for different states.
Q.41 Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.
Ans:
Activity Procedure:
Step 1: Two nails are fitted on a cork and are kept it in a 100 mL beaker.
Step 2: The nails are then connected to the two terminals of a 6-volt battery through a bulb and a switch.
Step 3: Some dilute HCl is poured in the beaker and the current is switched on.
Step 4: The same experiment is then performed with glucose solution and alcohol solution.
Observations: It will be observed that the bulb glows in the HCl solution and does not glow in glucose and alcohol solution.
HCl dissociates into H+ (aq) and Cl- (aq) ions. These ions conduct electricity in the solution which results in the glowing of the bulb. On the other hand, glucose and alcohol solution do not dissociate into ions. Therefore, they do not conduct electricity.
Conclusion:
From this activity, it can be concluded that all acids contain hydrogen but not all compounds containing hydrogen are acids.
That is why, though alcohols and glucose contain hydrogen, they are not categorised as acids.
Concept insight: You should always mention the complete acitivity with complete procedure with diagram, observations, result in a sequence.
Q.42 Why does distilled water not conduct electricity, whereas rain water does?
Ans:
Distilled water is a pure form of water and is devoid of any ionic species. Therefore, it does not conduct electricity. When rain water falls to the earth, it dissolves an acidic gas ‘carbon dioxide’ from the air and forms carbonic acid. This acid provides some hydrogen ions and carbonate ions to rain water. Hence, due to presence of these ionic species, rain water conducts electricity.
Concept insight: For answering this question, you should always remember that ions are charge carriers. If there are no free ions avalaible, electricity will not be conducted.
Q.43 Why do acids not show acidic behaviour in the absence of water?
Ans: Acids do not show acidic behaviour in the absence of water because there are no hydrogen ions available for conduction of electricity. Acids dissociate in the presence of water to give free hydrogen ions. It is the hydrogen ions that are responsible for the acidic behaviour.
Concept insight: For answering this question, you should always remember that acidic nature is because of the presence of H+ or H3O+ ions.
Q.44 Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is
(a) neutral?
(b) strongly alkaline?
(c) strongly acidic?
(d) weakly acidic?
(e) weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration.
Ans: (a) Neutral- Solution D with pH 7
(b) Strongly alkaline- Solution C with pH 11
(c) Strongly acidic- Solution B with pH 1
(d) Weakly acidic- Solution A with pH 4
(e) Weakly alkaline- Solution E with pH 9
pH is inversely proportional to hydrogen ion concentration. Hence, the pH can be arranged in the increasing order of the concentration of hydrogen ions as: 11 < 9 < 7 < 4 < 1
Concept insight: While answering this question, you should remember that when pH < 7, the solution is acidic, when pH > 7, it is basic and when pH = 7, it is a neutral solution. The lower the pH more will be the acidic nature. The higher the pH more will be the basic nature.
Q.45 Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?
Ans: The fizzing will occur strongly in test tube A, in which hydrochloric acid (HCl) is added. This is because HCl is a stronger acid than CH3COOH and contains a much greater amount of hydrogen ions; therefore it produces hydrogen gas at a faster speed due to which fizzing occurs.
Concept insight: The key to this answer lies in the fact that HCl is a strong acd while CH3COOH is a weak acid.
Q.46 Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
The pH of milk is 6. As it changes to curd, the pH will reduce because curd is acidic in nature due to formation of lactic acid.
Concept insight: Always remember that acids have pH < 7 and bases have pH >7.
Q.47 A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Ans: (a) The milkman shifts the pH of the fresh milk from 6 to slightly alkaline because in alkaline condition, milk does not get sour easily.
(b) Since this milk is slightly basic than usual milk, lactic acid produced to set the curd is neutralised by the base. Therefore, it takes a longer time for the curd to set.
Concept insight: Always remember that acids have pH < 7 and bases have pH >7.
Q.48 Plaster of Paris should be stored in a moisture-proof container. Explain why?
Ans: Plaster of Paris should be stored in a moisture-proof container because Plaster of Paris, a powdery mass, absorbs water (moisture) to form a hard solid known as gypsum.
Concept insight: While answering this question, recall the formula of Plaster of Paris and what will happen when it will react with water.
Q.49 What is a neutralisation reaction? Give two examples.
Ans: A reaction in which an acid and base react with each other to give a salt and water is termed as neutralisation reaction. In this reaction, energy is evolved in the form of heat.
For example:
(i) NaOH (base) + HCl (acid) ⇨ NaCl (salt) + H2O (water)
(ii) During indigestion (caused due to the production of excess of hydrochloric acid in the stomach), we administer an antacid (generally milk of magnesia, Mg(OH)2which is basic in nature). The antacid neutralises the excess of acid produced and thus gives relief from indigestion.
Mg(OH)2 + 2 HCl ⇨ MgCl2 + 2H2O
Concept insight: Always remember that the reaction of an acid and base to give salt and water is a neutralisation reaction.
Q.50 Give two important uses of washing soda and baking soda.
Ans: Two important uses of washing soda and baking soda are as follows:
(1) Washing soda:
(a) It is used in glass, soap, and paper industries.
(b) It is used to remove permanent hardness of water.
(2) Baking soda:
(a) It is used as baking powder. Baking powder is a mixture of baking soda and a mild acid known as tartaric acid. When it is heated or mixed in water, it releases CO2 gas that makes bread or cake fluffy.
(b) It is used in soda-acid fire extinguishers.
Concept insight: For answering this question, recall any two important uses of baking soda and washing soda.