ACIDS, BASES AND SALTS 1. What is an indicator? Name three common indicators. An indicator is a dye that changes colour when it is put into an acid or a base. It tells us whether the substance we are testing is an acid or a base. The three common indicators are litmus, methyl orange and phenolphthalein. 2. What is an olfactory indicator? Name two such substances. Those substances whose smell changes in acidic or basic solutions are called olfactory indicators. Onion or vanilla extract are olfactory indicators. 3. 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? a) Put the red litmus paper turn by turn in each of the three test tubes. The solution which turns the red litmus paper to blue will be a basic solution. The blue litmus paper formed can now be used to test the acidic solution. b) Put this blue litmus paper in the remaining two test tubes one by one. The solution which turns blue litmus paper to red will be acidic solution. c) The solution which has no effect on any litmus paper will be neutral and hence it will be distilled water. 4. Why should curd and sour substances not be kept in brass and copper vessels? Curd and sour substances are acidic in nature. When these substances are kept in brass or copper vessels, the metal reacts with the acid liberating hydrogen and other harmful substances (metal salts) which will spoil the food.
Transcript
ACIDS, BASES AND SALTS
1. What is an indicator? Name three common indicators.An indicator is a dye that changes colour when it is put into an acid or a base. It tells us whether the substance we are testing is an acid or a base. The three common indicators are litmus, methyl orange and phenolphthalein.
2. What is an olfactory indicator? Name two such substances.Those substances whose smell changes in acidic or basic solutions are called olfactory indicators. Onion or vanilla extract are olfactory indicators.
3. 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?a) Put the red litmus paper turn by turn in each of the three test tubes. The solution which
turns the red litmus paper to blue will be a basic solution. The blue litmus paper formed can now be used to test the acidic solution.
b) Put this blue litmus paper in the remaining two test tubes one by one. The solution which turns blue litmus paper to red will be acidic solution.
c) The solution which has no effect on any litmus paper will be neutral and hence it will be distilled water.
4. Why should curd and sour substances not be kept in brass and copper vessels?Curd and sour substances are acidic in nature. When these substances are kept in brass or copper vessels, the metal reacts with the acid liberating hydrogen and other harmful substances (metal salts) which will spoil the food.
5. 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 ionize in aqueous solution to produce H+ ions. Hence they show acidic character. Compounds like C2H5OH and glucose do not ionize in the aqueous solution to give H+ ions,i.e., hydrogen present in them is non-ionizable.Hence they do not show acidic character.
6. Why does aqueous solution of an acid conduct electricity?An acid in the aqueous solution ionizes to produce H+ ions and the corresponding negative ions.Eg. HNO3 gives H+ and NO3
- ions. Due to the presence of ions in the solution it conducts electricity.
7. Why does dry HCl gas not change the colour of dry litmus paper?Dry HCl does not ionize to produce H+ ions as ionization takes place in aqueous solution. Hence it does not show acidic properties and therefore does not change the colour of dry litmus paper.
8. What is known as dilution of an acid or a base?
Mixing of an acid or a base with water results in the decrease in the concentration of H3O+ ions for acids and OH- ions for bases per unit volume. Such a process is called dilution and the acid or base is said to be diluted.
9. While diluting an acid why is it recommended that the acid should be added to water and not water to the acid?Dilution of concentrated acid with water is a highly exothermic process. If water is added into acid, the heat produced is so large that the solution may splash out causing burns and the beaker in which it is carried out may break due to excessive heating. Hence dilution is done by adding acid into a larger volume of water which can absorb the heat.
10. Do basic solutions also have H+ ions? If yes, then why are these basic?Basic solutions also have H+ ions in addition to OH- ions. They are basic because in these solutions OH- concentration is greater than H+ ion concentration.
11. Why does distilled water not conduct electricity whereas rain water does?Distilled water does not conduct electricity because it does not contain any ionic compounds like acids bases or salts dissolved in it. Rain water while falling on earth through the atmosphere dissolves acidic gases like CO2, SO2 etc. from the air to form acids like carbonic acid (H2CO3) and Sulphurous acid(H2SO3)These acids provide H=Ions and they can conduct electricity.
12. Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equations for the reaction if one of the compounds formed is calcium chloride. Since the gas extinguishes fire, it must be CO2 gas (CO2 is a non-supporter of combustion). Since A gives carbon dioxide with dilute HCl, A can be a carbonate. Since the salt formed is CaCl2 , A must be calcium carbonate. CaCO3 (s) + 2HCl(aq) CaCl2 (aq) + CO2 (g) + H2O (l)
13. Equal lengths of magnesium ribbons are taken in test tubes A and B. hydrochloric acid 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?Hydrochloric acid is a strong acid and acetic acid is a weak acid. The fizzing is due to the evolution of H2 gas is formed by the action of acid on the magnesium ribbon. Being a strong acid, HCl solution contains a much greater amount of hydrogen ions in it. Due to this the fizzing will occur more vigorously in test tube A.Mg + 2HCl fast MgCl2 + H2
Mg +2CH3COOH slow (CH3 COO)2 Mg + H2
Acetic acid Magnesium acetate
14. Why do acids not show acidic behavior in the absence of water?
ACIDS, BASES AND SALTSAcidic nature of a substance is due to the formation of H+ (aq) ions in solution. Hydrogen ions are produced in the presence of water. The separation of H+ ions from acid like HCl molecule cannot occur in the absence of water. So acids do not show acidic behavior in the absence of water.
15. What is a neutralization reaction? Give two examples.When an acid reacts with a base it forms salt and water, it is called neutralization reaction. NaOH + HCl NaCl + H2O Sodium Hydroxide Sodium Chloride KOH + HNO3 KNO3 + H2OPotassium hydroxide Potassium nitrate
16. 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?When an acid reacts with a metal , hydrogen gas is liberated.E.g. Zinc reacts with dilute sulphuric acid.Zn (s) + H2SO4 (aq) ZnSO4 (aq) + H2 (g) Methoda) Take 5 ml of dilute H2SO4 in a test tube. And add few pieces of Zn granules to it.
We will observe the formation of gas bubbles on the surface of Zinc granules.b) Pass the gas being evolved through the soap solution.
Observation-Gas filled bubbles are formed in the soap solution which rise into air.c) Bring a burning candle near a gas filled soap bubble.
Observation-The gas present in soap bubble burns with a pop sound. d) Conclusion-This proves that the gas is hydrogen.
Draw fig. 2.1 in page 19
17. How will you show that all the compounds containing H are not acids? ORCompounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.Methoda) Fix two iron nails on a rubber cork and place the cork in a beaker.b) Connect the nails to the terminals of the 6V battery through a bulb and a switch.c) Pour some alcohol or glucose in a beaker and switch on the current.
Observation- The bulb does not glow showing that alcohol solution does not conduct electricity.Conclusion- The Hydrogen containing compound such as alcohol are not categorized as acids because they do not dissociate in water to produce H+(aq) ions.Draw fig2.3 in page 22
18. A salt X when dissolved in distilled water gives a clear solution which turns red litmus blue. Explain this phenomenon.Basic solutions turn red litmus blue. The salt of a weak acid and a strong base gives a basic solution. Na2CO3 + 2H2O 2NaOH + H2CO3 Being a strong base, NaOH is fully ionized and gives a large amount of OH- ions.H2CO3 being a weak acid produces only a small amount of H+ ions. The solution has a large number of OH- ions and hence basic in nature.
19. What is meant by pH Scale?A scale for measuring hydrogen ion concentration in a solution is called pH scale. Higher the hydrogen ion concentration lower is the pH value.
20. Under what soil conditions do you think a farmer would treat the soil of his field with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?Most of the plants grow best when the pH of the soil is close to 7. If the soil is too acidic, farmers add quicklime, slaked lime or chalk to neutralize it as these substances are basic in nature.
21. Differentiate between strong and weak acids?
Strong acids Weak acidsAn acid which is completely ionized in water and thus produces a large amount of hydrogen ions
An acid which is partially ionized in water and thus produces a small amount of hydrogen ions
They react very rapidly with other substances They react quite slowly with other substancesThey have high electrical conductivity They have low electrical conductivityE.g. H2SO4, HNO3, HCl All organic acids, Carbonic acid, Sulphurous
acid
22. Plaster of Paris should be stored in a moisture-proof container. Explain why?Plaster of Paris is a white powder and on mixing with moisture or water, it changes to gypsum giving a hard solid mass. CaSO4.1/2H2O + 1 1 H2O CaSO4.2H2O
Plaster of Paris 2 Gypsum
23. Which acid is produced in our stomach? What happens if there is an excess of acid in the stomach? How can its effect be cured?Hydrochloric acid is produced in our stomach. The excess acid in the stomach causes indigestion which produces pain and irritation. This effect can be cured by using antacids. Being basic in nature, antacids react with excess acid in the stomach and neutralize it. The two common antacids used for curing acidity are Milk of Magnesia and Baking soda.
ACIDS, BASES AND SALTS
24. What happens when a bee stings? What is its remedy?When a bee stings a person, it injects an acidic liquid into the skin which causes immense pain and irritation. By rubbing a mild base, baking soda neutralizes the acidic liquid injected by bee sting and cancels its effect.
25. With the help of a chemical equation, explain how a fire extinguisher helps in putting out a fire.Soda acid fire extinguisher contains sodium bicarbonate and sulphuric acid in separate containers. When the knob of the fire extinguisher is pressed, then sulphuric acid mixes with sodium bicarbonate solution and produces a lot of CO2 gas, which forms a blanket over the fire and cuts it off from the supply of the air to the burning substance and the fire stops.2NaHCO3 + H2SO4 Na2SO4 + 2H2O + 2CO2
Sodium bicarbonate Sodium sulphate
26. What is meant by water of crystallization? Explain that crystalline salts contain water of crystallization.Water of crystallization is a fixed number of water molecules present in one formula unit of a salt. One formula unit of copper sulphate contains five water molecules. These water molecules form part of the crystal structure. When hydrated salts are heated strongly, they lose their water of crystallization.On strong heating blue copper sulphate crystals turn white due to the loss of water of crystallization.CuSO4.5H2O CuSO4 + 5 H2OHydrated salt Anhydrous (Blue) (White)Anhydrous copper sulphate turns blue on adding water. CuSO4 + 5H2O CuSO4 .5H2O Anhydrous hydrated
(White) (Blue)
27. How would you distinguish between baking powder and washing soda by heating?The chemical formula of baking powder is sodium hydrogen carbonate (NaHCO3), whereas that of washing soda is sodium carbonate (Na2CO3.10H2O)Sodium hydrogen carbonate on heating gives CO2gas which will turn limewater milky whereas no such gas is obtained from sodium carbonate. 2NaHCO3 Heat Na2CO3 + H2O + CO2
