Q 1. Why is it advised not to use copper or brass vessels to store pickles or curd?
Solution Acids present in pickles or curd will react with metals like copper or zinc and produces poisonous salts which are harmful to health.
Q 2. What are Concentrated and dilute acids? How should we dilute an acid?
Solution Concentrated acid contains a very small amount of water or no water whereas dilute acids have far more amount of water than its own mass. The dilution of a concentrated acid should always be done by adding concentrated acid to water gradually with stirring and not by adding water to concentrated acid.
Q 3. Name the gas evolved when sodium hydrogen carbonate is made to react with dilute hydrochloric acid. How will you test the gas?
Solution Carbon dioxide is evolved when sodium hydrogen carbonate is made to react with dil. Hydrochloric acid. Test for the gas: Pass the gas through lime water. If it turns lime water milky, then the gas is carbon dioxide.
(i) What change will you observe in the colour of red litmus paper when it is dipped into a solution of sodium sulphate? Give a reason to explain your observation.
(ii) A bottle filled with concentrated sulphuric acid up to brim is left open in the atmosphere by mistake. Will there be any change in the level of liquid? Explain your answer with reason.
(i) It will not undergo any colour change because the solution of Na2SO4 (sodium sulphate) in water is almost neutral.
(ii) Concentrated sulphuric acid is highly hygroscopic. It absorbs moisture from the air and gets diluted. Since the volume increases, the acid starts flowing out of the bottle.
Q 5. What can you say about the pH of salts?
Salts of strong acids and bases are neutral with a pH value of 7. Salts of a strong acid and weak base are acidic with pH value less than 7 and those of strong base and weak acid are basic in nature, with pH value more than 7.
Q 6. The growth of plants depends to a large extent on the availability of proper pH conditions. Elaborate on the effect of soil pH on plant growth and ways to treat acidic and basic soil.
Most of the plants grow best when the pH of the soil is close to 7. If the soil is too acidic or too basic, the plants grow badly or do not grow at all. If the soil is too acidic, then it is treated with substances like quicklime, slaked lime or chalk. These substances are basic in nature and hence react with the excess acid present in soil and reduce its acidity. If the soil is too basic, then its basicity is reduced by adding decaying organic matter (manure or compost) which contains acidic materials.
Q 7. Classify the following into acids and bases:(a) Gastric juice(b) Milk of magnesia(c) Lemon juice(d) Ammonium hydroxide
(a) Gastric juices – Acid (b) Milk of magnesia – Base (c) Lemon juice – Acid (d) Ammonium hydroxide – Base
Q 8. Classify the following into acids, bases or neutral solution on the basis of their pH:
(a) A, pH = 3.2
(b) B ,pH = 4.4
(c) C, pH = 7.2
(d) D, pH = 12
(a) A, pH = 3.2 – Acidic
(b) B, pH = 4.4 – Acidic
(c) C, pH = 7.2 – Neutral
(d) D, pH = 12 – Basic
Q 9. What is the role of acids and bases in our digestive system?
Hydrochloric acid produced in our stomach helps in digestion of food but if it becomes excessive, the pH falls, and pain and irritation occurs. To get rid of this, antacids like milk of magnesia (Mg(OH)2) is generally used to adjust the pH in the stomach.
Q 10. 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 is an exothermic process. If water is added to concentrated acid, the heat generated may cause the mixture to splash out and cause burns. When the acid is added to water slowly with constant stirring, the mixture will not splash out.
(i) Why are some salts called hydrated salts?
(ii) Give two examples of white coloured hydrated salts with their chemical formula.
(i) Salts which contain water of crystallization are called as hydrated salts. (ii) Examples of white coloured hydrated salts are:
(a) CaSO4.2H2O – Gypsum
(b) Na2CO3.10H2O – Washing soda
Q 12. Glucose has hydrogen in it, therefore can we classify it as an acid?
A common feature of all acids is that they dissolve in water to produce hydrogen ions. Since glucose has hydrogen but on dissolving in water, it does not produce hydrogen ions, therefore it is not an acid.
Q 13. Classify acids on the basis of their origin. Give an example of each kind.
On the basis of their origin, acids can be classified as: 1.Organic acids: Acids which are obtained from plants are called organic acids. They contain carbon atom also along with hydrogen atom. Example: Tartaric acid is present in tamarind. 2.Inorganic acids: Acids which are usually obtained from minerals of the earth are known as inorganic acids. They do not contain carbon except carbonic acid(H2CO3). Example: Sulphuric acid(H2SO4)
Q 14. What is the pH of human blood?
The pH of human blood is 7.4.
Q 15. What are the indicators? Give an example.
Indicators are weak organic compounds [acids or bases] which change colour in accordance with the pH of the solution. For example, Blue litmus paper turns red in acidic solutions and re litmus paper turns blue in basic solutions.
Q 16. With the help of equations show the amphoteric nature of zinc oxide.
Zinc oxide is amphoteric because it reacts with both acids and bases to form salts.
ZnO + 2HCl → ZnCl2 + H2O
2NaOH + ZnO → Na2ZnO2 + H2O
Q 17. A milkman added a small amount of baking soda to fresh milk.
(a) Why does he shift the pH of fresh milk to slightly alkaline?
(b) Why does this milk take a longer time to set as a curd?
(i) Fresh milk is acidic and gets spoiled easily. In presence of baking soda, milk becomes alkaline and can be stored for a longer time.
(ii) When milk sets to curd, pH decreases. The alkali does not allow it to become more acidic easily. So it takes more time.
Q 18. The pH of the mouth of a person is lower than 5.5. What changes will occur in his mouth? How these changes can be controlled? Write any two measures.
When the pH of the mouth of a person is lower than 5.5 tooth decay starts. Tooth enamel made up of calcium phosphate is corroded. Measures:
(i) Clean the mouth after eating food.
(ii) Use of toothpaste (generally basic in nature).
Q 19. “Dry HCl gas does not change the colour of dry blue litmus paper”. Give reasons.
In dry state, HCl gas does not give any H+ ions. It does not behave as an acid. That is why the colour of litmus paper does not change.
Q 20. HCl and HNO3 show acidic characters in aqueous solution while alcohol and glucose solutions do not. Give reasons.
HCl and HNO3 produce H+ ions in aqueous solution. Hence, they show an acidic character. Alcohol and Glucose do not produce H+ ions in aqueous solution, hence, they do not show acidic characters.
Q 21. What are acids? Give physical properties of acids.
Acids are substances that dissolve in water to produce hydronium ions (H3O+). Physical properties of acids: a) Acids have a sour taste. b) They are highly corrosive in nature and cause painful burns on the skin. c) They change the colour of the indicators. d) Some acids are solids and some are liquids at room temperature. e)They are electrolytes, i.e., they conduct electricity in the aqueous state.
(a) State, in brief, the method of preparation of bleaching powder. Give a balanced chemical equation for the reaction involved.
(b) Bleaching powder is used for disinfecting drinking water. Give reason.
(a) When Chlorine gas is passed through dry slaked lime, it forms bleaching powder.
Ca(OH)2 + Cl2 → CaOCl2 + H2O
(b) Bleaching powder is used for disinfecting drinking water due to the Chlorine liberated which kills the germs.
Q 23. Why is it advised to add concentrated acid to water and not water to the acid?
If water is added to concentrated acid, the heat generated may cause the mixture to splash out and cause burns. When the acid is added to water slowly with constant stirring, the mixture will not splash out.
Q 24. What is the chemical formula of Gypsum? How many molecules of water of crystallisation are present in its one molecule?
The chemical formula of Gypsum is CaSO4.2H2O. In its one molecule, two molecules of water of crystallization are present.
Q 25. Explain how pH change in the lake water can endanger the lives of aquatic animals (like fish). What can be done to lessen the danger to the lives of aquatic animals in the lake?
Sometimes, the pH of lake water becomes lower because of too much acid rain. The high acidity of lake water can kill aquatic animals like fish since they can survive within a narrow range of pH change. Calcium carbonate is added to acidic lake water to neutralise the acid that comes from acid rain and this prevents the fish in the lake from being killed.
Q 26. What are strong acids and weak acids? Give two example for each.
Acids that give a high concentration of H+ ions on ionisation in aqueous solution are said to be strong acids and acids which give less H+ ions on ionisation in aqueous solution are said to be weak acids. Examples of Strong acids: Hydrochloric acid and nitric acid Examples of Weak acids: Acetic acid and formic acid
Q 27. The products obtained on electrolysis of concentrated aqueous solution of a substance ‘X’ are NaOH, Cl2 and H2.
(a) Name the substance ‘X’.
(b) What is the special name of this process and why? (c) Which gas is liberated at anode? (d) List one commercial use of NaOH.
(a) Substance ‘X’ = Sodium chloride
(b) Name of the process = Chlor-alkali This name is due to the formation of NaOH (alkali) and Cl2 (chlorine). (c) Gas liberated at anode = Cl2 (d) It is used in making soaps and detergents.
Q 28. Why a large amount of heat is evolved when a concentrated acid is mixed with water?
The process of mixing concentrated acid with water is highly exothermic i.e. heat producing reaction hence a large amount of heat is evolved when a concentrated acid is mixed with water.
Q 29. You might have seen lemon or tamarind juice being used to clean the tarnished surface of copper vessels. Explain why these sour substances are effective in cleaning the vessels?
Lemon juice contains citric acid and tamarind contains tartaric acid. These acids react with a basic layer of copper carbonate on the surface to form soluble salts which are easily removed and surface shines.
Q 30. What is meant by the term pH of a solution? The pH of rain water collected from two cities A and B was found to be 6 and 5 respectively. The water in which city is more acidic?
pH of a solution is a measure of the H+ ion concentration in a solution. Lesser the pH, more acidic is the solution. Thus rainwater of city B is more acidic.
Q 31. You are provided with three test tubes A, B, C which contain distilled water, acidic and basic solutions. If you are given blue litmus paper only, how will you identify the nature of the solutions in three test tubes?
(i) Solution A: Test the solution with blue litmus paper. There will be no change in the colour of blue litmus paper.
(ii) Solution B: Test the solution with blue litmus paper. The colour of blue litmus paper will change to red.
(iii) Solution C: Test the solution with a piece of red litmus paper (formed in step (ii)). The colour of red litmus paper will change back to blue.Again test solution A with a piece of red litmus paper, there will be no change in the colour.Hence, solution A (distilled water) is neutral since it does not bring out any change in the colour of litmus paper. Solution B is acidic since it turns blue litmus paper red.Solution C is basic since it turns red litmus blue.
Q 32. What happens to the orange colour of Methyl orange when treated with a base and an acid?
Methyl orange turns yellow in a basic solution and Reddish pink in an acidic solution.
Q 33. Why does a curry stain on washing turns red?
Turmeric is a natural indicator that turns red when treated with a base. Since soaps are basic in nature, they convert the yellow colour of turmeric to red.
Q 34. A student prepared solutions of
(i) an acid and
(ii) a base in two separate beakers. She forgot to label the solutions and litmus paper is not available in the laboratory. Since both the solutions are colourless, how will she distinguish between the two?
In the absence of litmus paper, reagent phenolphthalein can be used. The solution which develops pink colour with phenolphthalein is a base and the other is acid.
Q 35. What is the name given to the aqueous sodium chloride?
The name given to aqueous solution of sodium chloride is Brine.
Q 36. What happens during a bee sting? What is its remedy?
When a honey-bee stings a person, it injects an acidic liquid into the skin which causes immense pain and irritation. We can get relief by rubbing a mild base like baking soda solution on the stung area of the skin.
Q 37. Name the acids found in: (a) Vinegar (b) Tamarind (c) Lemon (d) Nettle sting
(a) Vinegar contains Acetic acid.
(b) Tamarind has Tartaric acid.
(c) Lemon contains Citric acid.
(d) Nettle sting contains Methanoic acid.
Q 38. A knife, which is used to cut fruit, was immediately dipped into water containing drops of blue litmus solution. If the colour of the solution is changed to red, what inference can be drawn about the nature of the fruit and why?
The fruit may be a citrus fruit or its juice may contain acid and the traces of acid on the knife turned the blue litmus red.
Q 39. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain
The pH of milk decreases from ‘6’ as it turns into curd. The curd is more acidic than milk.
Q 40. Why are acids never stored in metal containers? What types of containers are used to store acids?
Acids are never stored in metal containers because they gradually corrode and eat up the metal container. Containers made of glass and ceramics are used to store acids.