All questions of Acids, Base and Salts for Class 7 Exam

The common name of H₂SO₄ is oil of vitriol.....

Option a is the answer because Each cell of our body contains acid and it is also present in our proteins and fats means it is acid

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.

B)2HCl + Na2CO3
c)2H2O2 -> 2H2O + O2
d)2Na + Cl2 -> 2NaCl

The reaction between 2NaOH and MgSO4 is a double displacement reaction, also known as a precipitation reaction. The balanced chemical equation for the reaction is:

2NaOH + MgSO4 → Mg(OH)2 + Na2SO4

In this reaction, sodium hydroxide (NaOH) reacts with magnesium sulfate (MgSO4) to form magnesium hydroxide (Mg(OH)2) and sodium sulfate (Na2SO4).

Magnesium hydroxide is a white solid that precipitates out of the solution, while sodium sulfate remains in the solution as an aqueous (dissolved) compound.

Limewater is the common name for a diluted solution of calcium hydroxide. Calcium hydroxide, Ca(OH)2​, is sparsely soluble in water. Pure limewater is clear and colorless, with a slight earthy smell and an alkaline bitter taste of calcium hydroxide. Limewater is prepared by stirring calcium hydroxide in pure water, and filtering off the excess undissolved Ca(OH)2​. When excess calcium hydroxide is added to limewater, a suspension of calcium hydroxide particles results, giving it a milky aspect, in which case it has the common name of milk of lime.

Explanation:

Amla and lemon both are citrus fruits that are rich in vitamin C. The acidity in these fruits is due to the presence of citric acid. Citric acid is a weak organic acid that is found naturally in many fruits and vegetables, especially in citrus fruits such as lemons and oranges.

Properties of Citric Acid:
- Citric acid is a weak organic acid that has a sour taste.
- It is a natural preservative and is commonly used in the food industry to add flavor and acidity to foods and beverages.
- Citric acid is also used in cleaning products and personal care products due to its ability to remove stains and scale.

Uses of Amla and Lemon:
- Amla and lemon are used in many traditional remedies due to their high vitamin C content.
- Amla is believed to have many health benefits, including boosting immunity, improving digestion, and reducing inflammation.
- Lemon is also used for its health benefits, such as aiding in digestion, promoting hydration, and improving skin quality.

Conclusion:

In conclusion, the acid present in amla and lemon is citric acid. This weak organic acid is responsible for the sour taste of these fruits and is also responsible for many of their health benefits.

Explanation:
Salt is formed when a base reacts with an acid. This is known as a neutralization reaction. During a neutralization reaction, the hydrogen ions from the acid react with the hydroxide ions from the base to form water. At the same time, the cations from the base combine with the anions from the acid to form a salt.
Here is a detailed explanation of each option and why it is incorrect or correct:
A: Metals react with water - This is incorrect because when metals react with water, they typically form metal oxides or metal hydroxides, not salt.
B: Metals react with oxygen - This is incorrect because when metals react with oxygen, they typically form metal oxides, not salt.
C: Base reacts with acid - This is correct. When a base reacts with an acid, a neutralization reaction occurs and salt is formed.
D: None of these - This is incorrect because the correct answer is C, as explained above.
To summarize, the correct answer is C: base reacts with acid.

Introduction:
An indicator is a substance that is used to detect the presence of a chemical substance. It is widely used in various scientific experiments and analytical techniques. Indicators work by undergoing a physical or chemical change when they come into contact with the target substance. One of the most common ways an indicator shows the presence of a chemical substance is by changing color.
Explanation:
Indicators are used in various fields, including chemistry, biology, and environmental science, to determine the presence or absence of specific substances. Here, we will discuss in detail how indicators work and why changing color is the most common way they indicate the presence of a chemical substance.
Working of an Indicator:
When an indicator is exposed to a specific chemical substance, it undergoes a change in its physical or chemical properties. This change can be observed and used to determine the presence or absence of the target substance. The change can occur due to various factors such as pH, temperature, or the formation of a complex between the indicator and the target substance.
Why changing color is a common indicator:
The most common way an indicator shows the presence of a chemical substance is by changing its color. This is because:
1. Selectivity: Indicators are designed to be selective towards specific substances. They react only with the target substance and undergo a change in color, making it easier to identify the presence of that particular substance.
2. Visual Observation: Color change is easily visible to the naked eye. It provides a clear and immediate indication of the presence of the target substance without the need for complex instruments or equipment.
3. Sensitivity: Indicators can exhibit different shades or intensities of color change depending on the concentration of the target substance. This allows for the detection of even small amounts of the substance.
4. Versatility: Indicators can be used in various forms such as solutions, paper strips, or solid powders. This makes them adaptable to different experimental setups and analytical techniques.
Conclusion:
Indicators are crucial tools in chemical analysis and scientific research. Their ability to change color in the presence of specific substances makes them highly effective in detecting and identifying the presence of chemicals. By understanding the principles behind indicator reactions, scientists can leverage their properties to conduct accurate and reliable experiments.

Definition:
An indicator is a substance that undergoes a distinct observable change when conditions in its surroundings change. It is used to detect the presence, absence, or concentration of specific substances in a solution.
Given options:
A: Turmeric
B: Litmus
C: HCl
D: Methyl orange
Explanation:
To determine which option is not an indicator, we need to understand the properties of each substance.
- Turmeric: Turmeric is a spice commonly used in cooking. It does not have any specific indicator properties.
- Litmus: Litmus is a natural dye extracted from lichens. It is widely used as an indicator to determine whether a substance is acidic or basic.
- HCl: Hydrochloric acid (HCl) is a strong acid. It is not an indicator but rather a substance commonly used in chemical reactions and laboratory experiments.
- Methyl orange: Methyl orange is a pH indicator that changes color in acidic and basic solutions. It is frequently used in titration experiments.
Conclusion:
Based on the given options, the substance that is not an indicator is HCl (hydrochloric acid). It is a strong acid used in various chemical processes but does not possess indicator properties.

To determine the chemical formula of baking soda, we need to analyze its composition. Baking soda is a common household chemical compound used in baking and cleaning. Its chemical name is sodium bicarbonate.
Chemical formula of baking soda:
- Baking soda is chemically represented as NaHCO₃.
Explanation:
- Sodium bicarbonate is composed of the elements sodium (Na), hydrogen (H), carbon (C), and oxygen (O).
- The chemical formula NaHCO₃ indicates that there is one sodium atom (Na), one hydrogen atom (H), one carbon atom (C), and three oxygen atoms (O) in each molecule of baking soda.
- The presence of sodium (Na) in the formula indicates that the compound is a sodium salt.
- The carbonate ion (CO₃) in the formula indicates the presence of a bicarbonate group.
Summary:
- Baking soda is chemically represented by the formula NaHCO₃.
- The formula indicates the presence of sodium (Na), hydrogen (H), carbon (C), and oxygen (O) in the compound.

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):="" />
- 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.

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.

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.