Acids, Bases And Salts (LAQs)

Physical Science | 3. Acids, Bases And Salts – LAQs:
Welcome to LAQs in Chapter 3: Acids, Bases And Salts. This page contains the most Important FAQs for Long Answer Questions in this Chapter. Each answer is provided in simple English, with a Telugu explanation, and formatted according to the exam style. This will support your preparation and help you secure top marks in your exams.

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LAQ-1 : Observe the given information and write the answers from the following question.

S.No.	1	2	3	4	5	6	7	8
Solution	HCl	NaOH	NH4OH	HNO3	H2SO64	CH3COOH	Mg(OH)2	KOH
Nature of the solution	acid	base	base	acid	acid	acid	base	base

i.Which of the above solutions turn the blue litmus into bed?

ii.Which of the above solutions turn the red litmus into blue?

iii.Which of the above solutions change the colour of methyl orange indicator into (orange) yellow?

iv.Which of the above solutions change the colour phenolphthalein indicator into pink?

Introduction

We are dealing with acids (HCl, HNO₃, H₂SO₄, and CH₃COOH) and bases (NaOH, NH₄OH, Mg(OH)₂, and KOH). They react differently with indicators and litmus papers, and understanding these reactions is crucial for various chemical experiments and processes.

Turning Blue Litmus into Red

1. Acids: HCl, HNO₃, H₂SO₄, and CH₃COOH
2. Behavior: Acids turn blue litmus paper red.

Turning Red Litmus into Blue

1. Bases: NaOH, NH₄OH, Mg(OH)₂, and KOH
2. Behavior: Bases turn red litmus paper blue.

Changing Methyl Orange Indicator into (Orange) Yellow

1. Bases: NaOH, NH₄OH, Mg(OH)₂, and KOH
2. Behavior: Bases change methyl orange indicator to (orange) yellow.

Changing Phenolphthalein Indicator into Pink

1. Bases: NaOH, NH₄OH, Mg(OH)₂, and KOH
2. Behavior: Bases change phenolphthalein indicator to pink.

Summary

Understanding the behavior of acids and bases with different indicators and litmus papers is fundamental in chemistry. Acids typically turn blue litmus paper red, while bases have the opposite effect, turning red litmus paper blue and causing other specific color changes in various indicators.

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LAQ-2 : Write the required material and experimental procedure for the experiment, “Hydrochloric acid reacts with Zn pieces and liberates H₂”. (OR) Mention the precautions to be taken in the experiment to show that hydrogen gas is evolved when metals react with acids. Mention the experimental procedure.

Introduction

This experiment demonstrates that when acids react with metals, hydrogen gas is liberated. We will use hydrochloric acid and zinc as reactants. Safety and correct procedure are paramount to successfully conduct the experiment and obtain the desired results.

Materials/Chemicals Required

1. Test tube
2. Delivery tube
3. Glass trough
4. Candle
5. Soap water
6. Dilute Hydrochloric acid (HCl)
7. Zinc granules
8. Cork

Procedure

1. Setup the Apparatus: Arrange the apparatus as necessary.
2. Preparation of Acid Solution: Pour approximately 10 ml of dilute HCl into a test tube.
3. Add Zinc Granules: Add a few zinc granules to the dilute HCl in the test tube.
4. Observe the Reaction: Watch for the formation of gas bubbles on the zinc granules’ surface, indicating a reaction.
5. Pass the Gas through Soap Solution: Use a delivery tube to pass the formed gas through a soap solution in a trough.
6. Test the Gas: Bring a burning candle near to the gas-filled bubble in soap water. If the gas burns with a pop sound and extinguishes the candle, hydrogen gas is confirmed.

Chemical Reaction

The general reaction between an acid and a metal:
$$\text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen}$$
Specific reaction in this experiment:
$$2 \text{HCl (aq)} + \text{Zn(s)} \rightarrow \text{ZnCl}_2 \text{(aq)} + \text{H}_2 \text{(g)}$$

Result

Hydrogen gas is liberated when acids, such as hydrochloric acid, react with metals, as evidenced by the pop sound and the extinguishing of the candle flame.

Precautions

1. Safety Measures: Since hydrogen gas reacts explosively with oxygen, ensure all air inside the apparatus is displaced before collecting the hydrogen gas.
2. Use Protective Gear: Wear fireproof gloves while testing the presence of hydrogen gas with a candle to avoid accidents.

Summary

The experiment successfully demonstrates the liberation of hydrogen gas when an acid (hydrochloric acid in this case) reacts with a metal (zinc). Proper safety precautions, such as wearing fireproof gloves and ensuring no air is in the apparatus, are essential for conducting the experiment safely and effectively.

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LAQ-3 : Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9 respectively, Classify the solutions as given below.

a) Neutral b) strongly alkaline c) strongly acidic d) weakly acidic e) weakly alkaline.

Introduction

The pH scale is used to determine the acidity or basicity of a solution. A pH of 7 is neutral, less than 7 is acidic, and greater than 7 is basic or alkaline. Based on the given pH values of solutions A, B, C, D, and E, we can classify them into categories of neutral, strongly alkaline, strongly acidic, weakly acidic, and weakly alkaline.

Classification of Solutions

1. Solution A (pH 4):
   • Classification: Weakly Acidic
   • Reason: A pH of 4 is less than 7 but not extremely low, indicating a weakly acidic solution.
2. Solution B (pH 1):
   • Classification: Strongly Acidic
   • Reason: A pH of 1 is significantly lower than 7, indicating a strongly acidic solution.
3. Solution C (pH 11):
   • Classification: Strongly Alkaline
   • Reason: A pH of 11 is significantly higher than 7, indicating a strongly alkaline solution.
4. Solution D (pH 7):
   • Classification: Neutral
   • Reason: A pH of 7 is considered neutral, meaning it is neither acidic nor basic.
5. Solution E (pH 9):
   • Classification: Weakly Alkaline
   • Reason: A pH of 9 is above 7 but not extremely high, indicating a weakly alkaline solution.

Summary

In summary, the solutions are classified based on their pH values as follows:

• Solution A: Weakly Acidic (pH 4)
• Solution B: Strongly Acidic (pH 1)
• Solution C: Strongly Alkaline (pH 11)
• Solution D: Neutral (pH 7)
• Solution E: Weakly Alkaline (pH 9)

This classification helps in understanding the nature of different solutions and can be crucial for various chemical processes and reactions.

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LAQ-4 : Answer the following questions by using above information.

i.Which of the above is neutral solution?

ii.Which of the above is used to neutralize the acidicty in stomach?

iii.What is the strong acid among the above solutions?

iv.What is the colour of phenolphthalein indicator in NaOH solution?

Sample solution	Milk	Gastric juice	Distilled water	NaOH solution	Milk of magnesia	Washing soda
pH value	6.8	1.2	7	14	10.5	12.6

Introduction

Let’s answer the questions using the given information about different solutions and their pH values.

1. Neutral Solution:
   • Answer: Distilled Water
   • Explanation: Distilled water is neutral with a pH of 7, which is neither acidic nor basic.
2. Solution to Neutralize Stomach Acidity:
   • Answer: Milk of Magnesia
   • Explanation: Milk of Magnesia is a basic solution used to neutralize excess stomach acid.
3. Strong Acid Among the Solutions:
   • Answer: Gastric Juice
   • Explanation: Gastric juice, with a pH of 1.27, is highly acidic and helps in the digestion of food.
4. Color of Phenolphthalein Indicator in NaOH Solution:
   • Answer: Pink Colour
   • Explanation: Phenolphthalein turns pink in basic solutions like NaOH.

Summary

In summary, distilled water is neutral, milk of magnesia is used to neutralize stomach acidity, gastric juice is a strong acid, and the color of phenolphthalein in NaOH solution is pink. Understanding these properties is fundamental in various applications, including medical and laboratory contexts.

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LAQ-5 : List out the material for the experiment to investigate whether all compounds containing Hydrogen are acids or not and write the experimental procedure.

Introduction

The experiment aims to demonstrate that not all compounds containing hydrogen are acids. Substances like alcohols and glucose contain hydrogen but do not exhibit acidic properties. This will be proven by testing the conductivity of hydrochloric acid, alcohol, and glucose solutions.

Materials/Chemicals Required

1. Two different colored electrical wires
2. Three 100ml beakers
3. Solutions of hydrochloric acid, alcohol, and glucose
4. Six graphite rods
5. A 100-watt bulb with a holder
6. An AC plug

Procedure

1. Setup: Connect two differently colored electrical wires to graphite rods placed separately in a 100 ml beaker. Ensure the setup is as shown in the corresponding figure.
2. Circuit Completion: Attach the free ends of the wires to a 230 volts AC plug, and complete the circuit by connecting a bulb to one of the wires.
3. Adding Solutions: Pour some dilute HCl into one of the beakers and switch on the current.
4. Conducting the Experiment: Repeat the experiment with the other two beakers containing glucose and an alcohol solution.
5. Observation: Note that the bulb glows in the HCl solution, indicating conductivity, but does not glow in glucose and alcohol solutions, indicating non-conductivity.
6. Analysis: The ions present in HCl (H+ and Cl-) conduct electricity in the solution, leading to the glowing of the bulb. In contrast, glucose and alcohol solutions do not dissociate ions, hence do not conduct electricity.

Result

From this activity, it is clear that while all acids contain hydrogen, not all compounds with hydrogen are acids. Substances like alcohols and glucose, even though they contain hydrogen, do not exhibit acidic characteristics, as proven by their inability to conduct electricity in this experiment.

Summary

This experiment successfully demonstrates the difference between hydrogen-containing compounds and acids, showing that not all hydrogen-containing compounds exhibit acidic properties. Understanding this distinction is crucial for various scientific applications and investigations.

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LAQ-6 : List out the materials for the experiment “When hydrochloric acid reacts with NaHCO₃ and evolves CO₂”. Write the experiment procedure.

Aim

To demonstrate that when acids, like hydrochloric acid, react with carbonates or hydrogen carbonates, a salt is produced along with the liberation of carbon dioxide (CO₂).

Materials/Chemicals Required

1. Stand with clamp
2. Cork
3. Thistle funnel
4. Delivery tube
5. Two test tubes
6. Dilute hydrochloric acid (HCl)
7. Sodium hydrogen carbonate (NaHCO₃)
8. Lime water (Ca(OH)₂ solution)

Procedure

1. Preparation:
   • Label two test tubes as ‘A’ and ‘B’.
   • Add approximately 0.5 gm of sodium hydrogen carbonate (NaHCO₃) to test tube ‘A’.
2. Adding Acid:
   • Carefully add about 2 ml of dilute hydrochloric acid (HCl) to test tube ‘A’.
3. Collection of Gas:
   • Close the mouth of the test tube with a cork having a thistle funnel and a delivery tube.
   • Direct the other end of the delivery tube into another test tube containing lime water (Ca(OH)₂ solution).
4. Observation:
   • Watch for the gas produced from the reaction in test tube ‘A’ passing through the delivery tube.
   • Note the change in the lime water, which will turn milky, confirming the presence of carbon dioxide gas.
5. Recording Reaction:
   • Record the chemical reaction as follows:
     $$\text{NaHCO}_3 (s) + \text{HCl} (aq) \rightarrow \text{NaCl} (aq) + \text{H}_2 \text{O}(l) + \text{CO}_2 (g)$$

Result

From the above activity, it is evident that the reaction of sodium hydrogen carbonate with hydrochloric acid results in the formation of a salt (sodium chloride), water, and the liberation of carbon dioxide gas. This experiment successfully proves the general reaction of metal carbonates or hydrogen carbonates with acids to produce a corresponding salt, carbon dioxide, and water.

Summary

Understanding the reactions of acids with carbonates and hydrogen carbonates is essential for various chemical applications. This experiment clearly and effectively demonstrates this reaction, providing practical insight into the theoretical knowledge.

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LAQ-7 : How does baking powder make the cake soft and spongy?

Introduction

Baking powder plays a crucial role in baking recipes, ensuring that cakes and breads rise properly and have a light and airy texture. It contains both an acid (usually a weak acid like tartaric acid) and a base (sodium bicarbonate, commonly known as baking soda). The combination of these substances allows for a chemical reaction that produces carbon dioxide gas (CO₂​) when the baking powder is moistened and heated.

Chemical Reaction

1. Initial Preparation:
   • Baking soda (sodium bicarbonate, NaHCO₃​) is initially produced through the reaction: $$\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl} + \text{NaHCO}_3$$
2. Upon Activation (Heating or Mixing with Water):
   • Sodium hydrogen carbonate (NaHCO₃​) reacts with the acidic component in the baking powder.
   • This reaction can be represented as:
     $$\text{NaHCO}_3 + \text{H}^+ \rightarrow \text{CO}_2 + \text{H}_2 \text{O} + \text{Sodium salt of acid}$$

Result

The CO₂​ gas produced in the reaction gets trapped in the batter’s wet matrix, which when heated, solidifies and gives the cake or bread its risen, soft, and spongy texture. As the batter cooks, it solidifies, holding these air pockets in place and giving the finished product a soft, airy texture. This process is crucial for many baked goods, ensuring they are light and fluffy rather than dense and flat.

Summary

In essence, baking powder leverages chemical reactions to introduce air into baked goods, improving texture, appearance, and taste, and ensuring consistent and reliable results in baking. The creation of carbon dioxide gas and its entrapment in the batter is the key mechanism by which baking powder makes cakes and breads soft and spongy.

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LAQ-8 : Plaster of paris should be stored in moisture – proof container. Explain why?

Introduction

Plaster of Paris $$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$$ is used for various applications, including making molds, casts, and sculptures. It’s crucial to store it in a moisture-proof container to prevent its hydration before use.

Explanation

1. Hydration Reaction:
   • Plaster of Paris $$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$$ is highly sensitive to moisture.
   • When it comes into contact with water, it undergoes a hydration reaction:
     $$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \frac{3}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$$
   • In this reaction, Plaster of Paris absorbs water and converts into gypsum $$\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$$, a hard solid mass.
2. Loss of Utility:
   • Upon hydration to gypsum, Plaster of Paris loses its original properties.
   • It hardens and cannot be used for molding and casting purposes.

Summary

Because of these reasons, it is essential to store Plaster of Paris in a moisture-proof container to preserve its utility and prevent the inadvertent hydration reaction that renders it unusable for its intended applications. Proper storage ensures that Plaster of Paris remains in its useful hemihydrate form until it is intentionally mixed with water for use.

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LAQ-9 : How to test the strength of an acid or base?

Aim

To test and compare the strength of different acids and bases by observing their conductance.

Materials Required:

1. Dilute hydrochloric acid (HCl) solution
2. Dilute acetic acid (CH3COOH) solution
3. Dilute sodium hydroxide (NaOH) solution
4. Dilute ammonium chloride (NH4Cl) solution
5. Beakers (4)
6. Graphite rods (4)
7. Conductivity apparatus (with bulb)
8. Wires

Procedure:

1. Preparation:
   • Label four beakers as A, B, C, and D.
   • Fill beaker A with dilute HCl, B with dilute CH₃COOH, C with dilute NaOH, and D with dilute NH₄Cl.
2. Setup:
   • Insert graphite rods into each beaker ensuring they do not touch each other.
   • Connect the graphite rods to the conductivity apparatus using wires.
3. Observation:
   • Switch on the conductivity apparatus.
   • Observe the brightness of the bulb in each case.
   • Note the intensity of the glow of the bulb.
4. Analysis:
   • The brighter the glow of the bulb, the stronger the acid or base, indicating a higher concentration of ions.
   • For example, the bulb will glow more brightly in the HCl solution (strong acid) compared to the CH₃COOH solution (weak acid).
   • Similarly, the bulb will glow more brightly in the NaOH solution (strong base) compared to the NH₄Cl solution (weak base).

Summary

This experiment demonstrates the comparative strength of acids and bases based on their ionic conductance. The intensity of the bulb’s glow indicates the ability of the acid or base to conduct electricity, which is directly related to the number of ions present in the solution.