Olympiad Test: Acids, Bases And Salts - 2 - Class 7 MCQ

The acid used in the making of vinegar is acetic acid.
Explanation:
- Vinegar is a liquid consisting mainly of acetic acid and water. It is produced through the fermentation of ethanol by acetic acid bacteria.
- Acetic acid is a weak acid with the chemical formula CH3COOH. It is also known as ethanoic acid.
- Acetic acid gives vinegar its characteristic sour taste and pungent odor.
- It is widely used in food preparation, as a condiment, and as a preservative.
- Acetic acid is also used in various industries, including the production of dyes, solvents, and plastics.
- Other acids like nitric acid, formic acid, and sulphuric acid are not used in the making of vinegar.

KOH (Potassium hydroxide) is the common name of caustic potash. 

Properties of a base:
- Bases taste bitter: When a base is dissolved in water, it can taste bitter.
- Bases turn red litmus paper blue: A base has the ability to change the color of red litmus paper to blue.
- Bases feel soapy to touch: Bases have a slippery or soapy texture when touched.
Therefore, the correct answer is option D: All of the above.

When acid reacts with metal to form salt + H₂

The base used in making antacids is magnesium hydroxide.

Explanation:

• Antacids: Antacids are medications that help neutralize the excess acid in the stomach, providing relief from heartburn, indigestion, and other related symptoms.


• Base: A base is a substance that can accept hydrogen ions (protons) or donate hydroxide ions in a chemical reaction.


• Magnesium hydroxide: Magnesium hydroxide is a common base used in antacids. It is a white, inorganic compound with the chemical formula Mg(OH)2. It is also known as milk of magnesia.


• Neutralization: When an antacid containing magnesium hydroxide reacts with excess stomach acid (hydrochloric acid), it undergoes a neutralization reaction. The hydroxide ions from magnesium hydroxide combine with the hydrogen ions from hydrochloric acid to form water (H2O), effectively reducing the acidity of the stomach.


• Other antacid bases: While magnesium hydroxide is commonly used, other bases such as calcium hydroxide (lime water) and aluminum hydroxide are also used in antacids.

Therefore, the correct answer is magnesium hydroxide (Option D).

Reaction in Fire Extinguishers:
- Fire extinguishers are devices used to control or extinguish fires by removing one or more elements of the fire triangle: heat, fuel, and oxygen.
- Some fire extinguishers use a chemical reaction between a carbonate compound and an acid to produce carbon dioxide (CO2) gas, which helps to eliminate the oxygen element of the fire triangle.
Carbonate and Acid Reaction:
- When a carbonate compound, such as sodium bicarbonate (NaHCO3), comes into contact with an acid, such as sulfuric acid (H2SO4) or hydrochloric acid (HCl), a chemical reaction occurs.
- The reaction can be represented by the following equation:
NaHCO3 + HCl → NaCl + H2O + CO2
Product of the Reaction:
- The primary product of this reaction is carbon dioxide (CO2) gas.
- The carbon dioxide gas is released in the form of small bubbles, creating a smothering effect on the fire.
- The carbon dioxide displaces the surrounding oxygen, thus reducing the oxygen concentration and suppressing the combustion process.
Importance of Carbon Dioxide:
- Carbon dioxide is a non-flammable gas and does not support combustion.
- It has a higher density than air, which allows it to displace oxygen effectively.
- By reducing the oxygen concentration in the vicinity of the fire, the carbon dioxide helps to starve the fire of fuel and extinguish it.
Conclusion:
- The reaction between the carbonate compound and the acid in fire extinguishers produces carbon dioxide (CO2) gas.
- The carbon dioxide gas displaces oxygen, reducing the oxygen concentration and helping to suppress the fire.
- Therefore, the correct answer is B: CO2.

In acidic medium methyl orange turns red in colour. 

In basic medium methyl orange turns yellow in colour. 

Correct Answer :- D

Explanation : The oxides of non-metals are acidic. If a non-metal oxide dissolves in water, it will form an acid. The non-metal oxides can be neutralized with a base to form salt and water.

Phenolphthalein is a commonly used pH indicator that changes color based on the acidity or basicity of a solution. In acidic and neutral solutions, phenolphthalein turns colorless. This means that there is no visible color change observed when phenolphthalein is added to an acidic or neutral solution.
Here is a more detailed explanation:
Phenolphthalein as a pH indicator:
- Phenolphthalein is a weak acid that undergoes a color change depending on the concentration of hydrogen ions (H+) in a solution.
- In acidic solutions, there is a high concentration of H+ ions, which react with the phenolphthalein and cause it to become colorless.
- In basic solutions, there is a low concentration of H+ ions, allowing the phenolphthalein to remain in its colored form (pink or red, depending on the pH).
Color changes of phenolphthalein:
- Acidic solution (pH < 7): Colorless
- Neutral solution (pH = 7): Colorless
- Basic solution (pH > 7): Pink or red
Significance of phenolphthalein's color change:
- The color change of phenolphthalein can be used to determine the acidity or basicity of a solution.
- In titration experiments, phenolphthalein is often used as an indicator to signal the endpoint of a reaction when the solution reaches a certain pH.
- Its colorless nature in acidic and neutral solutions makes it a suitable indicator for these pH ranges.
Conclusion:
Phenolphthalein is colorless in both acidic and neutral solutions. Its color change to pink or red occurs in basic solutions. By observing the color change of phenolphthalein, one can determine the pH of a solution.