1. 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: To identify the contents of each test tube using only red litmus paper:
- Dip the red litmus paper into the first test tube containing the distilled water. If the litmus paper remains red, it indicates that the solution is neutral (neither acidic nor basic).
- Then, dip the same red litmus paper into the second test tube. If the litmus paper turns blue, it indicates that the solution is basic.
- Finally, dip the red litmus paper into the third test tube. If the litmus paper remains red, it indicates that the solution is acidic.
2. Why should curd and sour substances not be kept in brass and copper vessels?
Curd and sour substances should not be kept in brass and copper vessels because both brass and copper react with acids. The acidic nature of curd and sour substances can accelerate this reaction, leading to the formation of toxic compounds such as copper acetate or copper sulfate, which can be harmful if ingested.
3. 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 usually liberated. For example, when hydrochloric acid reacts with zinc metal, it produces zinc chloride and hydrogen gas according to the following equation:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
To test for the presence of hydrogen gas, one can hold a lighted splint near the mouth of the test tube where the reaction is occurring. If hydrogen gas is present, it will ignite with a “pop” sound.
4. 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.
The reaction described suggests that the metal compound A is likely a metal carbonate, as carbonates typically react with acids to produce carbon dioxide gas, which extinguishes a burning candle. Therefore, considering calcium chloride as one of the products, the balanced chemical equation for the reaction would be:
Metal carbonate (A) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
If A represents calcium carbonate, the reaction can be represented as:
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
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?
Acids like HCl and HNO3 become acidic when mixed with water because they break down into tiny particles called ions, like H+ ions. These ions make the solution acidic. But things like alcohol and glucose don’t do this, so they don’t make the solution acidic.
6. Why does an aqueous solution of an acid conduct electricity? Why does dry HCl gas not change the colour of the dry litmus paper?
Acids in water have ions, like H+ ions, that can carry electricity. So, when you put them in water, they make the water able to conduct electricity. But when they’re dry, they don’t have these ions, so they can’t change the color of litmus paper, which needs those ions to work.
7. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
When we dilute acid, it’s safer to pour the acid into water. Adding water to acid can make it really hot and splashy because it’s like pouring water onto a fire. Pouring acid into water is safer because it’s easier to control, and it doesn’t get too hot.
8. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
When you dilute an acid by adding water to it, you’re basically spreading out the acid molecules in a larger volume of water. This means there are fewer acid molecules in each unit of volume. Since the amount of acid remains the same but the volume increases, the concentration of hydronium ions (H3O+) decreases. In simpler terms, diluting the acid makes the solution less acidic because there are fewer H3O+ ions per unit of volume.
9. How is the concentration of hydroxide ions (OH–) affected when excess base is dissolved in a solution of sodium hydroxide?
When excess base is added to a solution of sodium hydroxide (NaOH), it reacts with the sodium hydroxide present in the solution. This reaction forms more hydroxide ions (OH⁻) because the base contributes these ions to the solution. As a result, the concentration of hydroxide ions increases in the solution. In simpler terms, adding more base increases the amount of OH⁻ ions, making the solution more basic.
10. 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?
Solution B has a higher pH (8), it indicates a lower concentration of hydrogen ions compared to Solution A (pH 6). Therefore, Solution A has a higher hydrogen ion concentration and is more acidic, while Solution B has a lower hydrogen ion concentration and is more basic.
11. What effect does the concentration of H+ solution?
The concentration of H+ ions in a solution determines its acidity.
Higher concentrations of H+ ions result in lower pH values, indicating stronger acidity.
Lower concentrations of H+ ions result in higher pH values, indicating weaker acidity or basicity.
12. Do basic solutions also have H+ (aq) ions have on the nature of the (aq) ions? If yes, then why are these basic?
Basic solutions do not have a high concentration of H+ ions compared to acidic solutions.
However, they still have a very low concentration of H+ ions due to the presence of water molecules that can undergo auto-ionization to form H+ and OH- ions.
Basic solutions have a higher concentration of OH- ions than H+ ions, making them basic.
Despite the presence of a small amount of H+ ions, the dominance of OH- ions makes the solution basic.
13. 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)?
- Farmers treat soil with quicklime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate) under acidic soil conditions.
- These calcium compounds are alkaline and can neutralize the acidity of the soil by reacting with the excess hydrogen ions present.
- This process helps to raise the pH of the soil, making it more neutral or slightly basic, which is favorable for many crops to grow effectively.
14. What is the common name of the compound CaOCl2
The common name of the compound CaOCl2 is bleaching powder.
15. Name the substance which on treatment with chlorine yields bleaching powder.
The substance that, on treatment with chlorine, yields bleaching powder is calcium hydroxide [Ca(OH)2].
16. Name the sodium compound which is used for softening hard water.
The sodium compound used for softening hard water is sodium carbonate (Na2CO3), also known as washing soda.
17. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
When a solution of sodium hydrocarbonate (NaHCO3) is heated, it decomposes to form sodium carbonate (Na2CO3), water (H2O), and carbon dioxide gas (CO2).
The equation for the reaction is:
2NaHCO3→Na2CO3+H2O+CO2
18. Write an equation to show the reaction between Plaster of Paris and water.
Plaster of Paris, which is chemically calcium sulfate hemihydrate (CaSO4·1/2H2O), reacts with water to form gypsum (calcium sulfate dihydrate, CaSO4·2H2O).
The equation for the reaction is: CaSO4⋅½H2O(s) + 1½H2O(l) → CaSO4⋅2H2O(s)
Exercise Question
1. A solution turns red litmus blue, its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10
Answer: (d) 10
Explanation: A solution turning red litmus blue indicates that it is basic. Therefore, the pH of the solution is likely to be around 10, making option (d) correct.
2. 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
Answer: (b) HCl
The gas produced from the reaction with crushed egg-shells turning lime-water milky indicates the presence of carbon dioxide gas. This gas is produced when acids react with carbonates or bicarbonates. Among the options, only HCl (hydrochloric acid) is an acid, so option (b) is correct.
3. 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 HCl solution (the same solution as before) required to neutralise it will be
(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL
Answer: (d) 16 mL
Explanation: Based on equivalent proportions in neutralization reactions.
If 10 mL of NaOH needs 8 mL of HCl, then 20 mL of NaOH will need double the amount.
So, for 20 mL of NaOH, initially 16 mL of HCl seems required. But considering 8 mL already used, only an additional 8 mL is needed.
Total HCl required for 20 mL NaOH is 8 mL (previous) + 8 mL (additional) = 16 mL. Hence, option (d) is correct.
4. Which one of the following types of medicines is used for treating indigestion? (a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic
Ans: (c) Antacid
Antacids are medicines used for treating indigestion by neutralizing excess stomach acid. Therefore, option (c) is correct.
5. Write word equations and then balanced equations for the reaction taking place when
- (a) Dilute sulphuric acid reacts with zinc granules:
- Word equation: Sulphuric acid + Zinc → Zinc sulphate + Hydrogen gas
- Balanced equation: H2SO4+Zn→ZnSO4+H2
- (b) Dilute hydrochloric acid reacts with magnesium ribbon:
- Word equation: Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen gas
- Balanced equation: 2HCl+Mg→MgCl2+H2
- (c) Dilute sulphuric acid reacts with aluminium powder:
- Word equation: Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen gas
- Balanced equation: H2SO4+2Al→Al2(SO4)3+3H2
- (d) Dilute hydrochloric acid reacts with iron filings:
- Word equation: Hydrochloric acid + Iron → Iron chloride + Hydrogen gas
- Balanced equation: 2HCl+Fe→FeCl2+H2
6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Ans: Take three test tubes. Label them A, B, and C.
Put some water in each tube.
Add a few drops of universal indicator solution to each tube. Tube A serves as a control, and tubes B and C are for testing.
In tube B, add a small amount of dilute hydrochloric acid, and in tube C, add a small amount of alcohol or glucose solution.
Observe the color changes in the tubes. Tube B should turn acidic (red), indicating the presence of hydrogen ions from the acid. However, tube C remains unchanged or may turn slightly alkaline (green or blue), indicating that alcohols and glucose do not release hydrogen ions like acids.
7. Why does distilled water not conduct electricity, whereas rain water does?
Distilled water does not conduct electricity because it lacks ions. It has been purified through distillation, removing most impurities, including ions.
Rainwater, on the other hand, can conduct electricity to some extent because it contains dissolved gases and ions from the atmosphere and the surroundings. These dissolved ions, such as chloride, sulfate, and nitrate ions, contribute to its conductivity, although it is still relatively low compared to solutions with higher ion concentrations.
8. Why do acids not show acidic behaviour in the absence of water?
Acids show acidic behavior when dissolved in water because they release hydrogen ions (H+) in the presence of water. In the absence of water, acids cannot release these hydrogen ions, so they do not exhibit acidic behavior.
9. 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) The solution with pH 7 is neutral.
(b) The solution with pH 11 is strongly alkaline.
(c) The solution with pH 1 is strongly acidic.
(d) The solution with pH 4 is weakly acidic.
(e) The solution with pH 9 is weakly alkaline.
Increasing Order of Hydrogen-Ion Concentration (Increasing Acidity):
pH 11 (Solution C) > pH 9 (Solution E) > pH 7 (Solution A) > pH 4 (Solution D) > pH 1 (Solution B)
10. 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: Fizzing occurs more vigorously in test tube A (containing hydrochloric acid) compared to test tube B (containing acetic acid).
Hydrochloric acid (HCl) is a stronger acid than acetic acid (CH3COOH). When acids react with metals like magnesium, they release hydrogen gas (H2).
Since hydrochloric acid is stronger, it reacts more vigorously with magnesium, producing more hydrogen gas and resulting in more vigorous fizzing compared to acetic acid.
11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Fresh milk has a pH of around 6, making it slightly acidic due to the presence of lactic acid.
During the process of curd formation, bacteria (Lactobacillus) ferment the lactose (milk sugar) present in milk into lactic acid.
As the fermentation progresses, the concentration of lactic acid increases, leading to a decrease in pH.
Therefore, as milk turns into curd, the pH decreases from its initial slightly acidic value of 6 due to the accumulation of lactic acid, making the curd slightly more acidic.
12. 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?
Fresh milk has a pH of around 6, making it slightly acidic due to the presence of lactic acid.
During the process of curd formation, bacteria (Lactobacillus) ferment the lactose (milk sugar) present in milk into lactic acid.
As the fermentation progresses, the concentration of lactic acid increases, leading to a decrease in pH.
Therefore, as milk turns into curd, the pH decreases from its initial slightly acidic value of 6 due to the accumulation of lactic acid, making the curd slightly more acidic.
13. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Plaster of Paris needs a moisture-proof container because it reacts with water (moisture) in the air. This reaction causes it to harden and become useless for its intended purpose.
Here’s the breakdown:
Plaster of Paris: A powdery substance called calcium sulfate hemihydrate (CaSO4. ½ H2O).
Moisture: Water vapor present in the air.
Reaction: When Plaster of Paris comes in contact with moisture, it absorbs the water and transforms into gypsum (CaSO4. 2H2O).
The problem: Gypsum is a solid material with no molding or casting abilities. So, if Plaster of Paris absorbs moisture before use, it hardens and becomes unusable for its intended purpose, which is typically creating molds or casts.
Moisture-proof container: This container prevents moisture from reaching the Plaster of Paris, keeping it in its dry, usable form (CaSO4. ½ H2O) until you need it.
14. What is a neutralisation reaction? Give two examples.
Ans: A neutralization reaction is a chemical reaction between an acid and a base, resulting in the formation of water and salt.
The hydrogen ions (H+) from the acid react with the hydroxide ions (OH-) from the base to form water (H2O), while the remaining ions combine to form a salt.
Examples:
- Reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH):
Reaction Involved: HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)
- Reaction between sulfuric acid (H2SO4) and potassium hydroxide (KOH):
Reaction Involved: H2SO4(aq) + 2KOH(aq) -> K2SO4(aq) + 2H2O(l)
15. Give two important uses of washing soda and baking soda.
Washing Soda (Sodium Carbonate, Na2CO3·10H2O):
- Used as a water softener in laundry detergents to remove hard water ions, allowing the detergent to work more effectively.
- Employed in cleaning applications due to its alkalinity, acting as a degreaser and stain remover for surfaces and fabrics.
Baking Soda (Sodium Bicarbonate, NaHCO3):
- Used as a leavening agent in baking, reacting with acidic ingredients like vinegar or lemon juice to produce carbon dioxide gas, which causes dough to rise.
- Employed as an antacid to relieve heartburn and indigestion by neutralizing excess stomach acid.