All questions of Acids, Bases and Salts for Class 10 Exam

Because.... ca(OH)2+ cl2= caocl2 + h2o that's why

Assertion (A) is true but reason (R) is false.

Methyl orange is a pH indicator frequently used in titration because of its clear and distinct colour variance at different pH values. Methyl orange shows pink colour in acidic medium and yellow colour in basic medium. Because it changes colour at the pH of a mid strength acid, it is usually used in titration for acids. Unlike a universal indicator, methyl orange does not have a full spectrum of colour change, but it has a sharp end point.

This is a natural indicator which are found in the nature of the plants

Assertion (A) is true but reason (R) is false.

Calcium hydroxide is obtained by reaction of calcium oxide and water.

It is the property acid that ..... when acid react with metal carbonates ....acid + metal carbonates -------> salt +carbon dioxide + water .... so option B is correct.......

• Assertion (A): When zinc is added to dilute hydrochloric acid, hydrogen is indeed given off. This is a chemical reaction where zinc displaces hydrogen from hydrochloric acid, producing zinc chloride and hydrogen gas.
• Reason (R): The statement that "Hydrogen chloride molecules contain hydrochloric acid and hydrogen atoms" is misleading. Hydrogen chloride (HCl) is a compound consisting of hydrogen and chlorine, but it doesn't accurately explain why hydrogen gas is evolved when zinc reacts with dilute HCl.

Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Baking soda, being alkaline, neutralises the acidity in the stomach and removes it.

Assertion (A): Gas bubbles are observed when sodium carbonate is added to dilute hydrochloric acid.
Reason (R): Carbon dioxide is given off in the reaction.

The correct answer is option 'A': Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Explanation:

When sodium carbonate (Na2CO3) is added to dilute hydrochloric acid (HCl), a reaction takes place resulting in the formation of carbon dioxide (CO2) gas. This reaction is represented by the following chemical equation:

Na2CO3 + 2HCl → 2NaCl + H2O + CO2

This reaction is a type of double displacement reaction, where sodium carbonate reacts with hydrochloric acid to produce sodium chloride, water, and carbon dioxide gas.

Now, let's analyze the given assertion and reason:

Assertion (A): Gas bubbles are observed when sodium carbonate is added to dilute hydrochloric acid.
This assertion is true. When sodium carbonate is added to dilute hydrochloric acid, effervescence occurs, and gas bubbles are observed. This is evident in the reaction mentioned above, where carbon dioxide gas is produced.

Reason (R): Carbon dioxide is given off in the reaction.
This reason is also true and it explains why gas bubbles are observed. During the reaction between sodium carbonate and hydrochloric acid, carbon dioxide gas is given off as a product. This gas evolution leads to the formation of gas bubbles.

Hence, both the assertion and reason are true, and the reason correctly explains the occurrence of gas bubbles. Therefore, the correct answer is option 'A': Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

**Turmeric as a Natural Indicator**

Turmeric, a commonly used spice in cooking, can also be used as a natural indicator to detect the presence of bases. The active compound in turmeric responsible for this property is called curcumin. When curcumin comes into contact with bases, it undergoes a chemical reaction that results in a change in color. In the presence of bases, turmeric turns bright red.

**Explanation**

Turmeric contains a class of compounds known as polyphenols, which are responsible for its vibrant yellow color. Curcumin, one of the polyphenols present in turmeric, acts as a pH indicator. pH indicators are substances that change color depending on the acidity or alkalinity (basicity) of a solution.

Bases are substances that can accept protons or donate hydroxide ions (OH-) in a chemical reaction. When a base is added to a solution of turmeric, the curcumin molecules react with the base, resulting in the formation of a new compound. This new compound has a different structure and absorbs light in a different range of wavelengths, giving rise to a change in color.

In the case of turmeric, the reaction with bases leads to the formation of a red compound. This compound is responsible for the bright red color observed when turmeric comes into contact with bases. The exact mechanism of this reaction is complex and involves multiple steps, including the deprotonation of curcumin and the formation of a conjugated system.

**Conclusion**

In conclusion, turmeric can be used as a natural indicator to detect the presence of bases. When turmeric is exposed to bases, it undergoes a chemical reaction that results in a change in color. The active compound in turmeric, curcumin, reacts with bases to form a red compound, giving turmeric a bright red color in the presence of bases.

Ammonia solution, which is alkaline, turns the red litmus paper blue.

Because H₂SO₄ is a strong acid, it readily forms hydronium ions when dissolved in water which are responsible for its corrosive action.

What Does the "p" in pH Stand For?
The "p" in pH stands for "power." This designation is derived from the German word "Potenz," which translates to "power" or "potential."
Understanding pH Scale
The pH scale is a logarithmic scale used to measure the acidity or basicity of a solution. It ranges from 0 to 14, where:
- A pH of 7 is considered neutral (pure water).
- A pH less than 7 indicates acidity (higher hydrogen ion concentration).
- A pH greater than 7 indicates basicity (lower hydrogen ion concentration).
Measurement of Hydrogen Ion Concentration
The pH scale specifically measures the concentration of hydrogen ions (H+) in a solution. This is crucial because:
- Acidic Solutions: Higher concentrations of H+ ions lead to lower pH values. For example, lemon juice has a pH of around 2, indicating high acidity.
- Basic Solutions: Lower concentrations of H+ ions correspond to higher pH values. For instance, bleach has a pH of around 12, indicating it is basic.
Importance of pH
Understanding pH is essential in various fields, including:
- Chemistry: It helps in determining the nature of chemical reactions.
- Biology: Enzyme activity and metabolic processes are pH-dependent.
- Environmental Science: pH affects aquatic life and soil health.
In summary, the correct answer is option 'B' because the pH scale effectively measures the hydrogen ion concentration in a solution, which determines its acidity or basicity.

Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Assertion: H3PO4 and H2SO4 are known as polybasic acids.
Reason: They have two or more than two protons per molecule of the acid.

The correct answer is option 'A': Both assertion (A) and reason (R) are true, and reason (R) is the correct explanation of assertion (A).

Explanation:
Polybasic acids are acids that contain more than one ionizable hydrogen atom per molecule. In other words, they can donate multiple protons (H+) when dissolved in water.

H3PO4 (phosphoric acid) and H2SO4 (sulfuric acid) are both examples of polybasic acids. Let's break down the reason given in the assertion:

1. H3PO4 (phosphoric acid):
- Phosphoric acid has three ionizable hydrogen atoms (protons) per molecule.
- In an aqueous solution, it can donate three protons (H+).
- The three ionizable hydrogens in H3PO4 are connected to three different oxygen atoms, making it a triprotic acid.

2. H2SO4 (sulfuric acid):
- Sulfuric acid has two ionizable hydrogen atoms (protons) per molecule.
- In an aqueous solution, it can donate two protons (H+).
- The two ionizable hydrogens in H2SO4 are connected to two different oxygen atoms, making it a diprotic acid.

Based on the above explanation, both the assertion and the reason are true. H3PO4 and H2SO4 are polybasic acids because they have more than two protons per molecule, which can be donated as H+ ions when dissolved in water.

Therefore, option 'A' is the correct answer.

Plaster of Paris when mixed with water and applied around the fractured limbs, it sets into a hard mass and keeps the bone joints in a fixed position. So, it is commonly used for setting fractured bones.

Assertion (A) is false but reason (R) is true.

• Assertion (A): pH = 7 signifies pure water.
  • This statement is true. Pure water at 25°C has a neutral pH of 7, which is a result of the equal concentrations of hydrogen ions [H+][H^+][H+] and hydroxide ions [OH−][OH^-][OH−].
• Reason (R): At this pH, [H+] = [OH-] = 10^-7.
  • This statement is also true. In pure water at 25°C, the concentration of hydrogen ions [H+][H^+][H+] and hydroxide ions [OH−][OH^-][OH−] are both 10−710^{-7}10−7 M, leading to a pH of 7.

Understanding pH of Salts from Weak Acids and Weak Bases
When a salt is formed from a weak acid and a weak base, the pH of the resulting solution cannot be definitively predicted due to the following reasons:
Nature of the Components
- Weak Acids: They do not completely dissociate in solution, resulting in a lower concentration of hydrogen ions (H+).
- Weak Bases: Similarly, they do not completely dissociate, leading to a lower concentration of hydroxide ions (OH-).
Acid-Base Reactions
- The salt will dissociate into its ions, which can either affect the pH positively or negatively depending on their relative strengths.
- The equilibrium between the weak acid's conjugate base and the weak base's conjugate acid plays a crucial role in determining the final pH.
Factors Influencing pH
- Relative Strengths: If the conjugate base of the weak acid is stronger than the conjugate acid of the weak base, the pH will be greater than 7 (basic). Conversely, if the conjugate acid is stronger, the pH will be less than 7 (acidic).
- Concentration: The concentration of the ions in solution can also shift the pH, making it more challenging to predict without specific information about the concentrations and strengths involved.
Conclusion
Due to the interplay of these factors, the pH of a salt formed from a weak acid and a weak base cannot be precisely determined. It can vary widely based on the specific characteristics of the acid and base, leading to the conclusion that option 'D' is correct: the pH cannot be predicted.

Understanding Lime Water Reaction with Carbon Dioxide
When excess carbon dioxide is passed through lime water (a saturated solution of calcium hydroxide), a series of chemical reactions occur that ultimately lead to the observed outcomes.
Initial Reaction
- Lime water is clear due to the presence of calcium hydroxide (Ca(OH)2).
- When carbon dioxide (CO2) is introduced, it reacts with calcium hydroxide to form calcium carbonate (CaCO3).
Formation of Calcium Carbonate
- The reaction can be summarized as follows:
Ca(OH)2 + CO2 → CaCO3 + H2O
- Calcium carbonate is insoluble in water, leading to the formation of a white precipitate, which can make the lime water appear milky.
Excess Carbon Dioxide Impact
- If an excess amount of carbon dioxide is passed through the lime water, the following reaction takes place:
CaCO3 + CO2 + H2O → Ca(HCO3)2
- This reaction converts the precipitate (calcium carbonate) back into calcium bicarbonate (calcium hydrogen carbonate), which is soluble in water.
Final Observation
- As a result of this reaction, the initial milkiness of the lime water disappears, leading to a clear solution again.
- This explains why the correct answer is option 'D': The milkiness of the lime water disappears when excess carbon dioxide is passed through it.
In summary, the initial reaction creates a milky solution due to calcium carbonate formation, but excess CO2 causes the precipitation to dissolve, restoring the clarity of the lime water.