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Acids , Bases and Salts

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NCERT Solutions for Class 10 Science Chapter 2

Absolutely, here’s a summary of the Acids Bases and Salts chapter from your 10th standard science textbook:

Acids:

  • Sour taste
  • Turn blue litmus paper red (indicator)
  • Release hydrogen (H+) ions in water solution
  • Examples: Sulfuric acid (H2SO4), Nitric acid (HNO3), Acetic acid (CH3COOH)

Bases:

  • Bitter taste
  • Slippery touch (soapy)
  • Turn red litmus paper blue (indicator)
  • Release hydroxide (OH-) ions in water solution

Salts:

  • Can be acidic, basic or neutral depending on the strength of the acid and base used
  • Examples: Sodium chloride (NaCl), Calcium carbonate (CaCO3), Ammonium sulfate ((NH4)2SO4)

Key Reactions:

  • Acid + Metal -> Salt + Hydrogen gas (e.g., Zinc + Sulfuric acid)
  • Acid + Metal carbonate -> Salt + Water + Carbon dioxide (e.g., Sulfuric acid + Sodium carbonate)
  • Acid + Base -> Salt + Water (Neutralization)

Important Points:

  • Concentrated acids are corrosive and can cause burns. Always add acid to water slowly with stirring, never the other way around.
  • Weak acids and weak bases partially ionize in water, while strong acids and strong bases completely ionize.
  • Indicators like litmus paper help identify acidic or basic solutions by color change.

NCERT Solutions for Class 10 Science Chapter 2 : Acids Bases and Salts

Questions (page 18)

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 : 

  1. Dip the red litmus paper into each test tube solution separately.
  2. Observe the color change of the litmus paper:
    • If the red litmus paper stays red in a solution, it indicates either distilled water or an acidic solution (since red is its natural color for acidic or neutral solutions).
    • If the red litmus paper turns blue in a solution, it confirms the presence of a basic solution.
  3. Identify the basic solution: The test tube where the red litmus paper turned blue clearly contains a basic solution.
  4. Distinguish between distilled water and acidic solution:
    • Take a small piece of the blue litmus paper from the basic solution test tube (since it’s no longer needed there).
    • Divide this blue litmus paper into two halves.
    • Dip each half of the blue litmus paper into the remaining two test tube solutions (one that didn’t change the red litmus paper color).
      • If the blue litmus paper stays blue in one solution, it indicates distilled water (as neutral solutions don’t change the color of blue litmus paper).
      • If the blue litmus paper turns red in a solution, it confirms the presence of an acidic solution (since acids turn blue litmus paper red).

NCERT Solutions for Class 10 Science Chapter 2 : Acids Bases and Salts

Questions (page 22)

1. Why should curd and sour substances not be kept in brass and copper vessels ?

Ans : Curd and sour substances shouldn’t be stored in brass or copper vessels for two reasons:

  1. Reaction:  Sour substances like curd contain acids (lactic acid in this case). These acids react with the brass or copper, causing the vessels to corrode.
  2. Harmful compounds:  This reaction can also create toxic salts that are not safe for consumption and can lead to stomach upset or food poisoning.

2. 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 : The gas usually liberated when an acid reacts with a metal is hydrogen gas (H2).

Example: When zinc metal (Zn) reacts with dilute sulfuric acid (H2SO4), zinc sulfate (ZnSO4) salt is formed along with hydrogen gas:

Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)

Testing for Hydrogen Gas:

There are two ways to test for the presence of hydrogen gas:

  1. Pop Test: Carefully hold a burning splint near the mouth of the test tube where the reaction is taking place.In the presence of hydrogen gas, a spark or flame can ignite it, producing a characteristic pop sound.
  2. Caution:  Never point the test tube opening directly at yourself or others while performing this test.
  3. Inflation Test: If a flexible balloon is attached to the test tube where the reaction is occurring, hydrogen gas will inflate the balloon as the reaction progresses.

3. 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 : Based on the information provided, the metal compound A is likely calcium carbonate (CaCO3). Here’s the balanced chemical equation :

CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)

Explanation:

  • CaCO3(s): This represents calcium carbonate, the metal compound A that reacts.
  • 2HCl(aq): This represents dilute hydrochloric acid. The “2” indicates two molecules of hydrochloric acid are needed for the reaction.
  • CaCl2(aq): This represents calcium chloride, one of the products formed.
  • CO2(g): This represents carbon dioxide gas, the effervescent gas that extinguishes the candle. Extinction occurs because CO2 displaces oxygen (needed for combustion) around the flame.
  • H2O(l): This represents water, another product formed in the reaction.

NCERT Solutions for Class 10 Science Chapter 2 : Acids Bases and Salts

Questions (Page 25)

1. Why do HCl, HNO3, etc show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character ?

Ans : The acidic nature of HCl, HNO3, etc. in water boils down to how they behave when dissolved:

  • Acids like HCl and HNO3 ionize in water. This means they break apart into separate ions: hydrogen ions (H+) and a negatively charged ion (like Cl- for hydrochloric acid).
  • H+ ions are what give acids their acidic character. They’re very reactive and can donate a proton (H+) to other molecules, making the solution acidic.

2. Why does an aqueous solution of an acid conduct electricity ?

Ans : Acids in water split into charged ions (H+ and others). These ions move freely and carry electricity, making the solution conductive.

3. Why does dry HCl gas not change the colour of the dry litmus paper ?

Ans : Dry HCl gas won’t change the color of dry litmus paper because two key conditions for the reaction are missing:

  1. Aqueous Environment: Litmus paper works by detecting H+ ions in solution. Dry HCl gas is just molecules of HCl, not separated ions. Acids only dissociate (split into H+ and other ions) in water.
  2. Moisture: Litmus paper itself needs to be slightly moist to function. Dry litmus paper can’t conduct the reaction or absorb the H+ ions even if they were present.

4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid ?

Ans : Here’s why you should always add acid to water when diluting:

  • Heat:  Adding acid to water releases heat due to the reaction. Water has a larger heat capacity, meaning it absorbs the heat more readily, preventing the solution from getting too hot and potentially boiling or splashing.
  • Safety:  Adding water to concentrated acid can cause a violent reaction. The heat released can vaporize some of the water instantly, causing concentrated acid to splatter. This poses a risk of burns.

5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted ?

Ans : When you dilute an acid solution, the concentration of hydronium ions (H3O+) decreases.

6. 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⁻) in a sodium hydroxide solution increases when excess base is dissolved.

NCERT Solutions for Class 10 Science Chapter 2 : Acids Bases and Salts

Questions (Page 28)

1. 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 : The concentration of hydrogen ions (H+) is higher in solution A.  A solution’s pH is a measure of the concentration of hydrogen ions (H+) in that solution. Lower pH indicates a higher concentration of H+ ions, and vice versa. Since solution A has a pH of 6 and solution B has a pH of 8, solution A must have a higher concentration of H+ ions.

2. What effect does the concentration of H+ (aq) ions have on the nature of the solution ?

Ans : The concentration of H+ (aq) ions directly affects the nature of a solution. 

3. Do basic solutions also have H+ (aq) ions ? If yes, then why are these basic ?

Ans : Yes, basic solutions also have H+ (aq) ions, but the key lies in their concentration compared to hydroxide ions (OH-).

4. 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 : Farmers would treat their fields with quick lime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate) when the soil condition is acidic.

NCERT Solutions for Class 10 Science Chapter 2 : Acids Bases and Salts

Questions (Page 33)

1. What is the common name of the compound CaOCl2 ?

Ans : The common name for the compound CaOCl2 is bleaching powder.

2. Name the substance which on treatment with chlorine yields bleaching powder.

Ans : The substance that reacts with chlorine to yield bleaching powder is calcium hydroxide (Ca(OH)2).  Also known as slaked lime, calcium hydroxide is treated with chlorine gas (Cl2) to produce bleaching powder (CaOCl2).

3. Name the sodium compound which is used for softening hard water.

Ans : Sodium carbonate (Na2CO3), also known as washing soda, is a chemical compound effective in softening hard water by removing calcium and magnesium ions. It’s commonly known as washing soda.

4. What will happen if a solution of sodium hydrogen carbonate is heated. Give the equation of the reaction involved ?

Ans : When a solution of sodium hydrogen carbonate (NaHCO3), also known as baking soda, is heated, it decomposes to form three products:

  • Sodium carbonate (Na2CO3): This is commonly known as washing soda.
  • Water (H2O)
  • Carbon dioxide gas (CO2)

The chemical equation for this reaction is:

2NaHCO3(s) Δ -> Na2CO3(s) + H2O(l) + CO2(g)

5. Write an equation to show the reaction between plaster of Paris and water.

Ans : The reaction between plaster of Paris (CaSO4 • ½H2O) and water (H2O) can be represented by the following equation:

CaSO4 • ½H2O (s) + ¾H2O (l) → CaSO4 • 2H2O (s)

NCERT Solutions for Class 10 Science Chapter 2 : Acids Bases and Salts

Exercises

1. A solution turns red litmus blue, its pH is likely to be

(a) 1

(b) 4

(c) 5

(d) 10

Ans : (d) 10

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

Ans : (b) HCl

3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HC1. If we take 20 mL of the same solution of NaOH, the amount of HC1 solution (the same solution as before) required to neutralise it will be

(a) 4 mL

(b) 8 mL

(c) 12 mL

(d) 16 mL

Ans : (d) 16 mL

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

5. .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 filing

Ans : 

(a) Dilute Sulphuric Acid + Zinc Granules:

Word Equation:

Sulphuric acid (dilute) + Zinc  → Zinc sulphate + Hydrogen gas

Balanced Equation:

H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)

(b) Dilute Hydrochloric Acid + Magnesium Ribbon:

Word Equation:

Hydrochloric acid (dilute) + Magnesium  → Magnesium chloride + Hydrogen gas

Balanced Equation:

2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)

(c) Dilute Sulphuric Acid + Aluminium Powder:

Word Equation:

Sulphuric acid (dilute) + Aluminium  → Aluminium sulphate + Hydrogen gas

Balanced Equation:

3H₂SO₄(aq) + 2Al(s) → Al₂(SO₄)₃(aq) + 3H₂(g)

(d) Dilute Hydrochloric Acid + Iron Filings:

Word Equation:

Hydrochloric acid (dilute) + Iron  → Iron chloride + Hydrogen gas

Balanced Equation:

Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

6. Compounds such as alcohol and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.

Ans : Alcohols and glucose do contain hydrogen, but they aren’t classified as acids because they don’t readily release hydrogen ions (H+) in water solution.

Procedure:

  1. Label the test tubes: “Acid,” “Alcohol,” and “Glucose.”
  2. Carefully add a few drops of dilute hydrochloric acid to the “Acid” test tube.
  3. Add a few drops of alcohol solution to the “Alcohol” test tube.
  4. Add a few drops of glucose solution to the “Glucose” test tube.
  5. Using a universal indicator: Add a drop of universal indicator to each test tube. Observe the color change in each solution.
    • OR (if using separate litmus papers)
    • Dip a clean piece of red litmus paper into each test tube. Observe the color change.

7. Why does distilled water not conduct electricity, whereas rainwater does ?

Ans : Distilled water doesn’t conduct electricity well because it lacks ions,  the charged particles that carry electricity in a solution. Rainwater, on the other hand, contains dissolved impurities that become ions when it interacts with air and dust particles. 

8. Why do acids not show acidic behaviour in the absence of water ?

Ans : Acids need water for two reasons:

  1. Dissolving: Water pulls apart the acid molecule, releasing H+ ions (the acidic part).
  2. Stability: Water stabilizes these H+ ions, allowing them to act acidic.

No water, no acidic behavior for the acid.

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 : 

  • (c) Strongly acidic: Solution B with a pH of 1 has the lowest pH, indicating the strongest concentration of hydrogen ions (H+) and thus the most acidic character.
  • (d) Weakly acidic: Solution A with a pH of 4 is mildly acidic, with a higher pH than strongly acidic solutions but lower than 7 (neutral).
  • (e) Weakly alkaline: Solution E with a pH of 9 is mildly basic, with a pH higher than 7 (neutral) but lower than strongly alkaline solutions.
  • (a) Neutral: Solution D with a pH of 7 has equal concentrations of H+ and hydroxide (OH-) ions, making it neutral.
  • (b) Strongly alkaline: Solution C with a pH of 11 has the highest pH, indicating the strongest concentration of hydroxide ions (OH-) and thus the most basic character.

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. In which test tube will the fizzing occur more vigorously and why ?

Ans : The fizzing will occur more vigorously in test tube A, where hydrochloric acid (HCl) is added. 

  • Strength of Acids: Hydrochloric acid (HCl) is a strong acid, meaning it readily dissociates in water to release a high concentration of hydrogen ions (H+).
  • Acetic Acid:  In contrast, acetic acid (CH3COOH) is a weak acid. It dissociates less completely in water, resulting in a lower concentration of H+ ions compared to HCl.
  • Reaction with Magnesium: Both acids react with magnesium metal to produce hydrogen gas (H2) which causes the fizzing:
    Mg (s) + 2H+ (aq) → Mg²⁺ (aq) + H₂ (g)
  • Rate of Reaction:  Since hydrochloric acid releases more H+ ions, the reaction with magnesium in test tube A proceeds faster, generating hydrogen gas at a higher rate. This rapid evolution of gas leads to more vigorous fizzing.

11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd ? Explain your answer.

Ans : The pH of milk will decrease as it turns into curd. 

  • Fresh Milk: Fresh milk has a slightly acidic pH, typically around 6.4 to 6.8. This acidity is due to the presence of various factors like casein proteins, phosphates, and carbonic acid.
  • Curd Formation: During curd production, bacteria like Lactobacillus are introduced. These bacteria ferment the lactose sugar present in milk, converting it into lactic acid.
  • Lactic Acid: Lactic acid is a weak acid. As the bacteria multiply and produce more lactic acid, the overall concentration of H+ ions in the milk increases.
  • pH Decrease: With a higher concentration of H+ ions, the pH of the milk solution goes down. This decrease in pH is what causes the milk to curdle. The curdling process itself is driven by the changing interactions between milk proteins due to the acidic environment.

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 ?

Ans : 

(a) Shift in pH:

  • Fresh Milk pH: Milk naturally has a slightly acidic pH, typically around 6.4-6.8.
  • Baking Soda (Sodium Bicarbonate): Baking soda is a basic compound. When added to milk, it reacts with the acidic components, like lactic acid and casein proteins. This reaction neutralizes some of the H+ ions present, causing a slight increase in pH towards a more alkaline state.

(b) Delay in Curdling:

  • Curd Formation: Curd formation happens when milk sours due to the action of bacteria. These bacteria convert lactose sugar in milk to lactic acid, making the milk more acidic (lower pH). This acidic environment causes milk proteins to coagulate, forming curds.
  • Impact of Baking Soda: By adding a small amount of baking soda, the milkman creates a slightly less acidic environment (higher pH) initially. This reduces the effectiveness of the bacteria in lowering the pH quickly through lactic acid production. As a result, it takes longer for the milk to reach the optimal acidity level needed for curdling, delaying the process.

13. Plaster of Paris should be stored in a moisture proof container. Explain why?

Ans : Plaster of Paris (CaSO4 • ½H2O) needs to be stored in a moisture-proof container because it reacts with water (H2O) to form gypsum (CaSO4 • 2H2O). Here’s why this matters:

  • Plaster of Paris: This powdery substance is a calcium sulfate hemihydrate, meaning it has one water molecule bonded to each calcium sulfate unit.
  • Moisture Reaction: When exposed to moisture in the air, the plaster of Paris reacts with the water molecules. It essentially absorbs additional water to complete its structure, forming gypsum.
  • Loss of Usable Property: Gypsum is a solid hydrate, and once this conversion happens, the plaster of Paris loses its valuable setting property. It hardens prematurely, becoming unusable for casting or molding purposes.

14. What is a neutralisation reaction ? Give two examples.

Ans : A neutralization reaction is a chemical reaction between an acid and a base that produces salt and water.  During this process, the acidic and basic properties of the reactants are neutralized.

Here are two examples of neutralization reactions:

Example 1: Hydrochloric Acid and Sodium Hydroxide

  • Reactants:
    • Hydrochloric Acid (HCl) – a strong acid
    • Sodium Hydroxide (NaOH) – a strong base
  • Balanced Equation:
    HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
  • Explanation:
    • In this reaction, the H+ ion from hydrochloric acid reacts with the OH- ion from sodium hydroxide to form water (H2O).
    • The remaining Na+ and Cl- ions combine to form sodium chloride (NaCl), a salt.

Example 2: Sulfuric Acid and Magnesium Hydroxide

  • Reactants:
    • Sulfuric Acid (H2SO4) – a strong acid
    • Magnesium Hydroxide (Mg(OH)2) – a weak base
  • Balanced Equation:
    H2SO4(aq) + Mg(OH)2(s) → MgSO4(aq) + 2H2O(l)
  • Explanation:
    • Similar to the first example, the H+ ions from sulfuric acid react with the OH- ions from magnesium hydroxide to form water.
    • Magnesium sulfate (MgSO4) is formed as a salt by the remaining Mg2+ and SO4²⁻ ions.

15. Give two important uses of washing soda and baking soda.

Ans : 

Washing Soda:

  1. Water Softening:  Washing soda is a common water softener. It reacts with the calcium and magnesium ions that cause water hardness, forming insoluble precipitates that are removed from the water. This makes the water softer, improving its cleaning ability.
  2. Cleaning:  Washing soda is a versatile cleaning agent due to its mildly alkaline nature. It can help remove stubborn grease and grime from surfaces like ovens, pots, and floors. When used in laundry, it can boost the cleaning power of detergents, especially in hard water conditions.

Baking Soda:

  1. Leavening Agent:  Baking soda is a popular leavening agent in baking. When heated in the presence of an acidic ingredient (like buttermilk or vinegar), it releases carbon dioxide gas. This gas creates bubbles in the batter, causing it to rise and become fluffy.
  2. Deodorizer:  Baking soda has mild deodorizing properties. It can absorb and neutralize unpleasant odors in various settings. For example, it can be used in refrigerators, freezers, and pet litter boxes to help control odors.

NCERT Solutions for Class 10 Science Chapter 2 : Acids Bases and Salts

FAQs

What is covered in NCERT Solutions for Class 10 Science Chapter 2?

The NCERT Solutions for Class 10 Science Chapter 2 cover key concepts related to Acids, Bases, and Salts, including their definitions, properties, uses, and the reactions between them. Detailed explanations and examples help in understanding the concepts thoroughly.

How do acids and bases react with each other according to Class 10 Science Chapter 2?

According to Class 10 Science Chapter 2, acids and bases react with each other in a neutralization reaction, producing a salt and water. This reaction can be represented by the general equation: Acid + Base → Salt + Water.

What are the key properties of acids discussed in Class 10 Science Chapter 2?

In Class 10 Science Chapter 2, the key properties of acids discussed include their sour taste, ability to turn blue litmus paper red, and their reaction with metals to produce hydrogen gas. Acids also react with bases to form salts and water.

What are the properties of bases as per NCERT Solutions for Class 10 Science Chapter 2?

The NCERT Solutions for Class 10 Science Chapter 2 outline that bases have a bitter taste and a slippery feel. They turn red litmus paper blue and react with acids in a neutralization reaction to form salts and water.

How are salts formed according to the NCERT Solutions for Class 10 Science Chapter 2?

According to the NCERT Solutions for Class 10 Science Chapter 2, salts are formed from the neutralization reaction between an acid and a base. The products of this reaction are a salt and water.

What is the significance of pH value in Class 10 Science Chapter 2?

The significance of pH value in Class 10 Science Chapter 2 lies in its ability to measure the acidity or basicity of a solution. A pH value less than 7 indicates an acidic solution, a pH of 7 indicates a neutral solution, and a pH greater than 7 indicates a basic solution.

How are indicators used in Chapter 2 of Class 10 Science?

In Class 10 Science Chapter 2 , indicators are used to determine whether a substance is acidic or basic. Common indicators include litmus paper, phenolphthalein, and methyl orange. These indicators change color in the presence of an acid or a base.

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