Acids, bases and pH value MCQs for Grade 9 Exam

Detailed Solution



Lemon juice, Vinegar, and coffee are of acidic character since the ph value is less than 7. Washing soda and Milk of Magnesia are of basic character since they have ph value of more than 7.

Washing the hand immediately with plenty of water and applying a paste of sodium hydrogen carbonate is the correct response when a few drops of a concentrated acid accidentally spill over the hand of a student.

Why is this the correct response?

1. Washing the hand immediately: The first step is to wash the hand immediately with plenty of water to remove the acid from the skin. This helps to dilute and remove the acid, reducing the risk of further damage.

2. Applying a paste of sodium hydrogen carbonate: After washing with water, applying a paste of sodium hydrogen carbonate (also known as baking soda) on the affected area helps to neutralize the acid. Sodium hydrogen carbonate is a weak base that can react with the acid and neutralize its effects. This reduces the chances of further damage to the skin.

3. Why not use saline solution or sodium hydroxide?
- Saline solution: Saline solution is a solution of salt in water, commonly used for rinsing wounds. However, in the case of an acid spill, using only saline solution may not be sufficient to neutralize the acid. It is important to use a substance that can effectively neutralize the acid, such as sodium hydrogen carbonate.
- Sodium hydroxide: Sodium hydroxide is a strong alkali, also known as caustic soda. While it can neutralize acids, it is not recommended to apply it directly on the skin as it can cause further damage. It has the potential to react with the acid and generate heat, which can lead to burns. Therefore, it is safer to use a mild base like sodium hydrogen carbonate.

Summary:

In summary, when a few drops of a concentrated acid accidentally spill over the hand of a student, it is important to wash the hand immediately with plenty of water to remove the acid. After washing, applying a paste of sodium hydrogen carbonate on the affected area helps to neutralize the acid and reduce the risk of further damage. This response is more effective than using saline solution or sodium hydroxide, as it ensures proper neutralization without causing additional harm to the skin.

The complex tetraammine copper (II) ions are formed when a salt is treated with an excess of ammonium hydroxide. The salt in question contains cupric ions. Let's break down the process and explain why the correct answer is option 'C'.

1. Formation of a Complex:
- When a salt is treated with excess ammonium hydroxide, the ammonia molecules act as ligands and form a complex with the metal ions present in the salt.
- In this case, the metal ion is copper (II) ion (Cu2+).
- The ammonia ligands coordinate with the copper (II) ion to form a complex called tetraammine copper (II) ions.
- The formula of this complex is [Cu(NH3)4]2+.

2. Identification of the Metal Ion:
- To determine the metal ion present in the starting salt, we need to consider the anion of the salt.
- The question does not provide information about the anion, so we cannot determine the exact salt.
- However, since the complex formed contains copper (II) ions, the starting salt must contain cupric ions (Cu2+).
- Therefore, the correct answer is option 'C' - cupric ions.

3. Explanation of Other Options:
a) Cuprous ions: Cuprous ions have a +1 charge (Cu+). However, the complex formed in this reaction contains copper (II) ions with a +2 charge (Cu2+), so cuprous ions are not present in the salt.
b) Chloride ions: Chloride ions (Cl-) are not involved in the formation of the complex. The complex is formed by the coordination of ammonia ligands with copper (II) ions.
d) Calcium ions: Calcium ions (Ca2+) are not involved in the reaction. The formation of the complex is specific to copper (II) ions.

In conclusion, when a salt is treated with excess ammonium hydroxide, it forms a complex tetraammine copper (II) ions. The starting salt contains cupric ions, and the correct answer is option 'C'.

B)Formic acid
c)Citric acid
d)Oxalic acid

Understanding Acid-Base Reactions
When an acid reacts with a base, the primary result is the formation of salt and water. This reaction is commonly known as a neutralization reaction.
Key Components of the Reaction
- Acids: Substances that donate protons (H+) in a solution.
- Bases: Substances that accept protons (H+) or donate hydroxide ions (OH-) in a solution.
Neutralization Reaction
- When an acid and a base interact, they undergo a neutralization process.
- The acidic component (H+) from the acid combines with the basic component (OH-) from the base to form water (H2O).
Formation of Salt
- The remaining ions from the acid and base combine to form a salt.
- For example, if hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), the products will be sodium chloride (NaCl) and water.
General Reaction Equation
- The general form of the neutralization reaction can be expressed as:
Acid + Base → Salt + Water.
Conclusion
In conclusion, the correct answer to the question is option 'D' because the primary products of an acid-base reaction are indeed salt and water. This fundamental concept is crucial in understanding many chemical reactions and applications in various fields such as chemistry, biology, and environmental science.

Answer:

Washing soda, also known as sodium carbonate decahydrate (Na2CO3.10 H2O), is a hydrate compound. This means that it contains water molecules as part of its crystal structure. When washing soda is exposed to air, it undergoes a process called efflorescence, where the water molecules in the compound are lost to the surrounding atmosphere.

Explanation:

Efflorescence
When a hydrate compound, such as washing soda, is exposed to air, the water molecules present in the compound can evaporate. This process is known as efflorescence. As a result of efflorescence, the compound loses some of its water molecules and its chemical formula changes.

Chemical formula of washing soda
The chemical formula of washing soda, Na2CO3.10 H2O, indicates that it contains 10 water molecules. However, on exposure to air, some of these water molecules are lost.

Changes in the chemical formula
As the water molecules evaporate, the chemical formula of the washing soda changes. The number of water molecules decreases and the chemical formula becomes Na2CO3.x H2O, where 'x' represents the remaining number of water molecules.

Correct answer
In this case, the correct answer is option 'D', Na2CO3.H2O. This implies that on exposure to air, washing soda loses all of its water molecules except for one. Therefore, the chemical formula becomes Na2CO3.H2O.

Conclusion
When washing soda (Na2CO3.10 H2O) is exposed to air, it undergoes efflorescence and loses some of its water molecules. The correct chemical formula for washing soda after exposure to air is Na2CO3.H2O.

When sodium hydroxide (NaOH) is added to ammonium carbonate (NH4)2CO3 salt and a glass rod dipped in dilute hydrochloric acid (HCl) is brought near the test tube, we observe dense white fumes.

Explanation:

The reaction between sodium hydroxide and ammonium carbonate can be represented by the following chemical equation:

(NH4)2CO3 + 2NaOH → 2NH3 + 2H2O + Na2CO3

In this reaction, sodium hydroxide reacts with ammonium carbonate to produce ammonia gas (NH3), water (H2O), and sodium carbonate (Na2CO3). The ammonia gas is released as a result of the reaction.

When a glass rod dipped in dilute hydrochloric acid is brought near the test tube, it reacts with the ammonia gas produced in the previous step. The reaction between hydrochloric acid and ammonia can be represented as follows:

NH3 + HCl → NH4Cl

In this reaction, ammonia reacts with hydrochloric acid to form ammonium chloride (NH4Cl). This reaction is highly exothermic and releases heat.

The dense white fumes observed in this reaction are actually ammonium chloride (NH4Cl) being formed as a white solid. The fumes are formed due to the reaction between ammonia and hydrochloric acid. The white solid of ammonium chloride is formed as a result of the combination of ammonia gas and hydrochloric acid vapor.

The reaction between ammonia and hydrochloric acid is a classic example of a neutralization reaction. In this reaction, an acid (HCl) reacts with a base (NH3) to form a salt (NH4Cl) and water (H2O).

In summary, when sodium hydroxide is added to ammonium carbonate and a glass rod dipped in dilute hydrochloric acid is brought near the test tube, we observe dense white fumes. These fumes are formed due to the reaction between ammonia gas produced from the reaction between sodium hydroxide and ammonium carbonate, and hydrochloric acid. The reaction between ammonia and hydrochloric acid leads to the formation of ammonium chloride, which appears as dense white fumes.

Calcium phosphate is present in tooth enamel and its nature is basic.

Explanation:
1. Composition of Tooth Enamel:
Tooth enamel is the hard, outermost layer of the tooth that covers and protects the underlying dentin and pulp. It is primarily composed of hydroxyapatite, which is a crystalline form of calcium phosphate. Hydroxyapatite is a mineral that gives tooth enamel its strength and hardness.

2. Nature of Calcium Phosphate:
Calcium phosphate is an inorganic compound that contains calcium ions (Ca2+) and phosphate ions (PO43-). It is classified as a basic compound because it can accept protons (H+) and increase the concentration of hydroxide ions (OH-) in solution.

3. Acid-Base Properties:
Acidic compounds are substances that release hydrogen ions (H+) in solution, while basic compounds release hydroxide ions (OH-). Neutral compounds have an equal concentration of hydrogen ions (H+) and hydroxide ions (OH-). Amphoteric compounds can act as both acids and bases depending on the pH of the solution.

4. Tooth Enamel and pH:
Tooth enamel is constantly exposed to various acids produced by bacteria in the mouth, as well as acidic foods and drinks. The acids can demineralize the enamel, leading to tooth decay and cavities. However, tooth enamel also has the ability to remineralize itself to some extent through the uptake of minerals like calcium and phosphate ions.

5. Role of Calcium Phosphate:
Calcium phosphate plays a crucial role in maintaining the integrity and strength of tooth enamel. When the pH in the mouth becomes acidic, calcium phosphate in the enamel can release calcium ions and phosphate ions. These ions can combine with hydroxide ions (OH-) from saliva to form hydroxyapatite crystals, which remineralize the enamel and restore its hardness.

6. Conclusion:
In conclusion, calcium phosphate is present in tooth enamel and its nature is basic. It plays a vital role in maintaining the strength and integrity of tooth enamel by remineralizing it in acidic conditions.

Acidic Strength of Solutions

Introduction:
The pH scale is used to measure the acidity or alkalinity of a solution. It ranges from 0 to 14, with 7 being considered neutral. Solutions with a pH less than 7 are acidic, and solutions with a pH greater than 7 are alkaline or basic.

Given Information:
- Solution A has a pH of 3.
- Solution B has a pH of 4.
- Solution C has a pH of 6.
- Solution D has a pH of 8.

Determining the Most Acidic Solution:
To determine the most acidic solution among A, B, C, and D, we need to compare their pH values. The lower the pH value, the more acidic the solution is.

- Solution A has a pH of 3.
- Solution B has a pH of 4.
- Solution C has a pH of 6.
- Solution D has a pH of 8.

Comparison:
- Solution A has the lowest pH value of 3, making it the most acidic solution among the given options.
- Solution B has a pH of 4, making it less acidic than solution A.
- Solution C has a pH of 6, making it less acidic than solutions A and B.
- Solution D has the highest pH value of 8, making it the least acidic solution among the given options.

Conclusion:
Based on the pH values provided, solution A with a pH of 3 has the highest acidic strength among solutions A, B, C, and D.

**Explanation:**

The statement that is not correct is option C: All metal oxides react with water to give a salt and acid.

**Metal Oxides and Water:**

Metal oxides are compounds formed by the reaction of metals with oxygen. When metal oxides react with water, they generally form metal hydroxides.

For example:
- Sodium oxide (Na2O) reacts with water (H2O) to form sodium hydroxide (NaOH):
Na2O + H2O → 2NaOH
- Magnesium oxide (MgO) reacts with water (H2O) to form magnesium hydroxide (Mg(OH)2):
MgO + H2O → Mg(OH)2

**Acid-Base Reactions:**

Acid-base reactions occur when an acid reacts with a base to form a salt and water. Metal oxides can react with acids to form salts and water, but not the other way around. Metal oxides are considered basic in nature.

For example:
- Calcium oxide (CaO) reacts with hydrochloric acid (HCl) to form calcium chloride (CaCl2) and water:
CaO + 2HCl → CaCl2 + H2O

**Correct Statements:**

a) All metal carbonates react with acid to give a salt, water, and carbon dioxide.
- This statement is correct. When metal carbonates react with acids, they produce a salt, water, and carbon dioxide gas. For example, when calcium carbonate (CaCO3) reacts with hydrochloric acid (HCl), it forms calcium chloride (CaCl2), water (H2O), and carbon dioxide (CO2):
CaCO3 + 2HCl → CaCl2 + H2O + CO2

b) Some metals react with acids to give salt, water, and hydrogen.
- This statement is correct. Some metals, such as magnesium and zinc, react with acids to produce a salt, water, and hydrogen gas. For example, when magnesium (Mg) reacts with hydrochloric acid (HCl), it forms magnesium chloride (MgCl2), water (H2O), and hydrogen gas (H2):
Mg + 2HCl → MgCl2 + H2

d) Some non-metal oxides react with water to form an acid.
- This statement is correct. Non-metal oxides, such as sulfur dioxide (SO2) and nitrogen dioxide (NO2), can react with water to form acidic solutions. For example, when sulfur dioxide (SO2) dissolves in water, it forms sulfurous acid (H2SO3):
SO2 + H2O → H2SO3

C option dilution because it is a process in which less concentration of water is added into an acid.