Chapter 12: Introduction to Acid and Base
Solved textbook questions with step-by-step explanations.
Grade 8 Q&A: Chapter 12: Introduction to Acid and Base
Welcome to the Questions and Answers section for Grade 8 Science, Chapter 12: "Introduction to Acid and Base." This chapter covers the fundamental properties of acids and bases, how to identify them using various indicators, the significance of the pH scale, the concept of neutralization reactions, and the importance of pH in our daily lives.
Important Questions and Answers
Q1: List three common properties of acids.
Answer:
- Taste sour.
- Are corrosive.
- Turn blue litmus paper red.
- React with active metals to produce hydrogen gas.
- React with metal carbonates to produce carbon dioxide gas.
Q2: List three common properties of bases.
Answer:
- Taste bitter.
- Feel soapy or slippery to touch.
- Turn red litmus paper blue.
- Are corrosive (strong bases).
- React with nonmetal oxides to form salt and water.
Q3: Give two examples of common acids found in daily life.
Answer: Acetic acid (in vinegar), Citric acid (in lemons/oranges), Lactic acid (in curd), Hydrochloric acid (in stomach acid).
Q4: Give two examples of common bases found in daily life.
Answer: Sodium hydroxide (in soap), Calcium hydroxide (lime water), Ammonium hydroxide (in window cleaners).
Q5: What is an 'Indicator'? Give two examples.
Answer: Indicators are substances that show a change in color when added to an acidic or basic solution, helping us identify their nature. Examples: Litmus paper, Methyl Orange, Phenolphthalein, Turmeric.
Q6: How does litmus paper react in acidic and basic solutions?
Answer: Blue litmus paper turns red in acidic solutions. Red litmus paper turns blue in basic solutions. In neutral solutions, there is no color change.
Q7: Describe the color change of Phenolphthalein in acidic and basic solutions.
Answer: Phenolphthalein remains colorless in acidic solutions and turns pink in basic solutions.
Q8: Describe the color change of Methyl Orange in acidic and basic solutions.
Answer: Methyl Orange turns red in acidic solutions and turns yellow in basic solutions.
Q9: What is the 'pH scale'? What do pH values of 3, 7, and 10 indicate?
Answer: The pH scale is a scale (0-14) used to measure the acidity or alkalinity (basicity) of a solution.
- pH = 3 indicates an acidic solution (strong acid).
- pH = 7 indicates a neutral solution.
- pH = 10 indicates a basic (alkaline) solution (strong base).
Q10: What is a 'Universal Indicator'?
Answer: A Universal Indicator is a mixture of several different indicators that shows different colors at various pH values. It provides a more precise measure of the acidity or basicity of a solution compared to single indicators.
Q11: Define 'Neutralization Reaction' with a chemical equation.
Answer: A neutralization reaction is the reaction between an acid and a base to form salt and water. This reaction typically releases heat.
Example: HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l) + Heat
Q12: What are 'Salts'? Give an example.
Answer: Salts are ionic compounds formed when an acid reacts with a base during a neutralization reaction. They are typically neutral, but some can be acidic or basic. Example: Sodium chloride (NaCl - common table salt).
Q13: How do acids react with active metals? What gas is produced?
Answer: Acids react with active metals to produce hydrogen gas and a salt. The hydrogen gas can be tested by a 'pop' sound when a burning splinter is brought near it.
Example: Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)
Q14: How do acids react with metal carbonates? What gas is produced?
Answer: Acids react with metal carbonates and bicarbonates to produce carbon dioxide gas, water, and a salt.
Example: Na₂CO₃ (s) + 2HCl (aq) → 2NaCl (aq) + H₂O (l) + CO₂ (g)
Q15: Explain the importance of pH in digestion.
Answer: The stomach produces hydrochloric acid (HCl) with a low pH (1.5-3.5), which is essential for activating digestive enzymes and killing harmful bacteria in food. If excess acid is produced, it leads to acidity and indigestion, which can be relieved by antacids (mild bases).
Q16: Why is the pH of soil important for plant growth?
Answer: Plants require a specific pH range in the soil for optimal growth and nutrient absorption. If the soil is too acidic or too basic, it affects the availability of nutrients to plants and can hinder crop yield. Farmers often adjust soil pH by adding substances like lime (basic) to acidic soil.
Q17: What is 'Acid Rain' and how does it affect the environment?
Answer: Acid rain is rainwater with a pH less than 5.6, caused by air pollutants like sulfur dioxide and nitrogen oxides. It harms aquatic life in lakes and rivers, damages historical buildings (e.g., marble structures), and can affect soil fertility and plant growth.
Q18: How does pH relate to tooth decay? How can it be prevented?
Answer: Tooth decay occurs when the pH inside the mouth falls below 5.5. This happens because bacteria in the mouth break down sugar and produce acids, which corrode tooth enamel. Using basic toothpaste helps neutralize these acids, preventing tooth decay.
Q19: How can baking soda be used to treat an ant sting?
Answer: Ant stings contain formic acid, which causes pain and irritation. Rubbing baking soda (sodium bicarbonate), which is a mild base, on the affected area can neutralize the acid and provide relief from the sting.
Q20: What is the pH of pure water? Is it acidic, basic, or neutral?
Answer: The pH of pure water is 7, which indicates that it is a neutral solution.
Q21: Give an example of a natural indicator. How does it react with a base?
Answer: Turmeric is a natural indicator. It remains yellow in acidic or neutral solutions but turns reddish-brown in basic solutions. Red cabbage juice is another natural indicator.
Q22: Why are strong acids and bases corrosive?
Answer: Strong acids and bases are corrosive because they are highly reactive and can cause chemical burns to living tissues and damage materials by reacting with them. This is due to their ability to donate or accept protons (acids) or hydroxide ions (bases) very readily.
Q23: What is the 'pop' sound test used for?
Answer: The 'pop' sound test is used to confirm the presence of hydrogen gas (H₂). When a burning splinter or matchstick is brought near a gas that produces a 'pop' sound, it indicates the presence of hydrogen gas, typically evolved when an active metal reacts with an acid.
Q24: What type of oxide do nonmetals typically form, and what is their nature?
Answer: Nonmetals typically react with oxygen to form nonmetal oxides, which are generally acidic in nature. These oxides dissolve in water to form acids (e.g., CO₂ + H₂O → H₂CO₃).
Q25: If a solution changes phenolphthalein to pink, what can you conclude about its nature?
Answer: If a solution changes phenolphthalein to pink, you can conclude that the solution is basic (alkaline).
Exercise Solutions (From Screenshot Page 88)
Q1: Fill in the blanks.
- Acids generally taste sour.
- Bases generally taste bitter.
- Acids turn blue litmus paper red.
- Bases turn red litmus paper blue.
- Phenolphthalein remains colorless in acidic solutions.
- Phenolphthalein turns pink in basic solutions.
- Methyl orange turns red in acidic solutions.
- Methyl orange turns yellow in basic solutions.
- The pH of a neutral solution is 7.
- The pH of an acidic solution is less than 7.
- The pH of a basic solution is greater than 7.
- The reaction between an acid and a base is called neutralization.
- Our stomach produces hydrochloric acid.
- Tooth decay occurs when the pH in the mouth falls below 5.5.
Q2: Match the pairs.
(Note: As an AI, I cannot create interactive matching. I will provide the correct pairs.)
- Lemon juice - Acidic
- Soap solution - Basic
- Pure water - Neutral
- Litmus paper - Indicator
- Stomach acid - Hydrochloric acid
- Vinegar - Acetic acid
- Curd - Lactic acid
- Antacid - Base
Q3: Give scientific reasons.
- Antacids are used to treat acidity.
Reason: Antacids are basic substances. Acidity in the stomach is caused by the excessive production of hydrochloric acid. When antacids are consumed, they neutralize the excess stomach acid through a neutralization reaction, forming salt and water, thereby relieving the symptoms of acidity and indigestion. - Toothpaste is generally basic.
Reason: Tooth decay occurs when bacteria in the mouth break down food particles, especially sugars, and produce acids. These acids lower the pH in the mouth (below 5.5), which starts to corrode tooth enamel. Toothpaste is generally basic (alkaline) to neutralize these acids, prevent enamel corrosion, and thus protect teeth from decay. - Farmers add lime to acidic soil.
Reason: Plants require a specific pH range in the soil for optimal growth and nutrient absorption. If the soil becomes too acidic, it can hinder plant growth. Lime (calcium hydroxide or calcium carbonate) is a basic substance. Farmers add lime to acidic soil to neutralize the excess acidity, bring the pH to a suitable range, and improve soil fertility for better crop yield. - Acid rain is harmful to aquatic life.
Reason: Acid rain, which has a pH less than 5.6 due to dissolved sulfur dioxide and nitrogen oxides, significantly lowers the pH of lakes, rivers, and other water bodies. Most aquatic organisms, including fish and plants, are very sensitive to changes in water pH. A sudden drop in pH due to acid rain can stress or kill these organisms, disrupting the aquatic ecosystem and reducing biodiversity.
Q4: Answer the following questions.
- Explain the properties of acids.
Answer:- Taste: Sour (e.g., lemon juice).
- Corrosive: Strong acids can cause severe burns and damage materials.
- Indicators: Turn blue litmus paper red. Keep red litmus paper red. Keep phenolphthalein colorless. Turn methyl orange red.
- Reaction with Metals: React with active metals to produce hydrogen gas and a salt (e.g., Zinc + HCl → Zinc chloride + Hydrogen).
- Reaction with Carbonates/Bicarbonates: React with metal carbonates and bicarbonates to produce carbon dioxide gas, water, and a salt (e.g., Sodium carbonate + HCl → Sodium chloride + Water + Carbon dioxide).
- Reaction with Bases: Undergo neutralization reaction to form salt and water.
- Explain the properties of bases.
Answer:- Taste: Bitter (e.g., soap).
- Feel: Soapy or slippery to touch.
- Corrosive: Strong bases are also corrosive.
- Indicators: Turn red litmus paper blue. Keep blue litmus paper blue. Turn phenolphthalein pink. Turn methyl orange yellow.
- Reaction with Acids: Undergo neutralization reaction to form salt and water.
- Reaction with Nonmetal Oxides: React with nonmetal oxides (which are acidic) to form salt and water (e.g., CO₂ + Ca(OH)₂ → CaCO₃ + H₂O).
- Explain the pH scale and its importance in daily life.
Answer: pH Scale: The pH scale is a numerical scale ranging from 0 to 14, used to express the acidity or alkalinity (basicity) of a solution.- pH < 7: Acidic solution (lower pH indicates stronger acid).
- pH = 7: Neutral solution (e.g., pure water).
- pH > 7: Basic (alkaline) solution (higher pH indicates stronger base).
- Digestion: Stomach acid (low pH) is crucial for digestion. Antacids (bases) are used to neutralize excess acid.
- Soil pH: Plants grow best in a specific soil pH range. Farmers adjust soil pH for optimal crop yield.
- Tooth Decay: Acids produced by bacteria in the mouth lower pH, causing tooth decay. Basic toothpaste helps neutralize these acids.
- Acid Rain: Low pH of rainwater due to pollutants harms aquatic life and structures.
- Insect Stings: pH knowledge helps in treating stings (e.g., baking soda for ant sting, vinegar for wasp sting).
- Explain neutralization reaction with an example.
Answer: A neutralization reaction is a chemical reaction in which an acid and a base react with each other to form a salt and water. This reaction is typically exothermic, meaning it releases heat. The properties of the acid and base are neutralized, resulting in a neutral solution (if the acid and base are of equal strength). Example: When hydrochloric acid (a strong acid) reacts with sodium hydroxide (a strong base), they neutralize each other to form sodium chloride (common salt) and water.
HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l) + Heat
(Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water + Heat)
Q5: Differentiate between.
- Acid and Base
Feature Acid Base Taste Sour Bitter Feel Corrosive, sometimes sticky Soapy/Slippery, corrosive (strong) Litmus Test Turns blue litmus red Turns red litmus blue pH Range Less than 7 Greater than 7 Ions in Water Produces H⁺ ions Produces OH⁻ ions - Physical Change and Chemical Change (from previous chapter, but good for revision)
Feature Physical Change Chemical Change Nature Temporary Permanent New Substance No new substance formed New substance(s) formed Composition Chemical composition remains same Chemical composition changes Reversibility Often reversible Generally irreversible
References
- Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 12: Introduction to Acid and Base.
- Maharashtra State Board 8th Standard Science Syllabus.
- Balbharati Science and Technology Textbook.