About This Chapter: This chapter introduces the fundamental concepts of acids and bases, their properties (taste, feel, and behaviour with indicators), and the different types of indicators used to identify them. It covers natural indicators (litmus, turmeric, red cabbage), synthetic indicators (phenolphthalein, methyl orange), and olfactory indicators (onion, vanilla, clove oil) that work by smell change. The chapter emphasizes that acids are sour, bases are bitter and soapy, and neutral substances like distilled water have no effect on indicators. It also introduces the neutralization concept where acids and bases cancel each other's properties. These foundational ideas are essential for understanding the pH scale, salt formation, and the practical applications of acids and bases in daily life, such as treating acidity, dealing with insect stings, and preparing remedies for various conditions.
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Q1. Calcium hydroxide is chemically represented as:
Calcium hydroxide has one calcium atom (Ca²⁺) and two hydroxide ions (OH⁻), so its formula is Ca(OH)₂. The parentheses around OH indicate two hydroxide groups. CaCO₃ is calcium carbonate, CaCl₂ is calcium chloride, and CaO is calcium oxide (quicklime).
Q2. Which of the following substances is classified as an acid?
Hydrochloric acid (HCl) is a strong acid that dissociates completely in water to produce H⁺ ions. NaOH is sodium hydroxide (a base), Ca(OH)₂ is calcium hydroxide (a base), and NH₄OH is ammonium hydroxide (a base). Acids are substances that produce hydrogen ions (H⁺) in aqueous solution.
Q3. Which indicator works by detecting a change in smell?
Onion is an olfactory indicator because its characteristic smell changes when exposed to acids or bases. In basic solutions, the smell of onion disappears, while in acidic solutions, it remains. Methyl orange and litmus work by colour change, and turmeric is a natural colour indicator that turns reddish-brown in bases.
Q4. Acids and bases neutralise each other because:
When an acid reacts with a base, the H⁺ ions from the acid combine with the OH⁻ ions from the base to form water (H₂O). This process neutralizes both the acidic and basic properties, resulting in a salt and water. The acid's sourness and the base's bitterness are both cancelled out, forming a neutral solution.
Q5. Magnesium hydroxide is correctly represented as:
Magnesium hydroxide contains magnesium (Mg²⁺) and two hydroxide ions (OH⁻), giving the formula Mg(OH)₂. MgO is magnesium oxide, MgCl₂ is magnesium chloride, and MgSO₄ is magnesium sulphate. Magnesium hydroxide is commonly known as milk of magnesia and is used as an antacid.
Q6. The process by which acids and bases cancel each other's effect is called:
Neutralisation is the chemical reaction between an acid and a base that produces salt and water. The H⁺ ions from the acid combine with the OH⁻ ions from the base to form water. This process neutralizes both substances. Reduction involves gain of electrons, oxidation involves loss of electrons, and displacement involves one element replacing another in a compound.
Q7. Which substance is used as an olfactory indicator?
Vanilla essence is an olfactory indicator because its smell disappears in basic solutions but remains in acidic solutions. Phenolphthalein and methyl orange are colour indicators, and litmus is a natural colour indicator extracted from lichens. Olfactory indicators are substances whose odour changes in acidic or basic media.
Q8. The colour of neutral litmus solution is:
Litmus solution naturally has a purple colour in neutral conditions. When exposed to acids, it turns red, and when exposed to bases, it turns blue. The purple colour of litmus in neutral solutions is an important reference point for testing whether a substance is acidic, basic, or neutral.
Q9. Acids generally have which taste?
Acids have a characteristic sour taste. This is why sour foods like lemon, vinegar, and citrus fruits contain acids. For example, citric acid in lemons and acetic acid in vinegar give them their sour taste. Bases, on the other hand, have a bitter taste and are soapy to touch.
Q10. Indicators determine whether a substance is acidic or basic by showing:
Indicators are substances that change colour when they come in contact with acids or bases. For example, litmus turns red in acids and blue in bases, phenolphthalein turns pink in bases and remains colourless in acids, and methyl orange turns red in acids and yellow in bases. The colour change provides a visual way to identify the nature of a solution.
Q11. Litmus is best described as a:
Litmus is a natural dye extracted from lichens (a symbiotic association of algae and fungi). It is the most commonly used natural indicator for testing acids and bases. Since it is obtained from natural sources rather than synthesized in a laboratory, it is classified as a natural indicator.
Q12. Bitter taste in food is due to the presence of:
Bases have a bitter taste and a soapy feel. This is why substances like baking soda (sodium bicarbonate), soap, and certain vegetables have a bitter taste. For example, bitter gourd contains alkaline compounds that give it a bitter taste. Acids have a sour taste, while salts have a salty taste.
Q13. Clove oil can act as which type of indicator?
Clove oil is an olfactory indicator because its characteristic smell changes when exposed to acids or bases. In acidic solutions, the smell of clove oil remains, while in basic solutions, it changes. Olfactory indicators are useful for testing substances when colour indicators are not suitable, especially for colourless solutions.
Q14. Natural substances that indicate the presence of acids or bases are called:
Indicators are substances that show a change in colour or smell when they come in contact with acids or bases. Natural indicators include litmus (from lichens), turmeric (from turmeric roots), and red cabbage leaves. These substances are used to identify whether a solution is acidic, basic, or neutral.
Q15. Litmus solution is naturally which colour?
Litmus solution is naturally purple in its neutral state. When added to an acidic solution, it turns red, and when added to a basic solution, it turns blue. This purple colour serves as the reference point for neutral solutions. The dye extracted from lichens produces a purple solution that changes colour depending on the pH of the solution it is added to.
Q16. Red litmus paper turns blue in which type of solution?
Red litmus paper turns blue in basic solutions because bases contain OH⁻ ions that react with the litmus dye. In acidic solutions, red litmus remains red. In neutral solutions, red litmus shows no change. This property makes red litmus paper useful for detecting the presence of bases.
Q17. Cloth strips soaked with onion extract are used to test:
Onion extract is an olfactory indicator used to test acids and bases. Cloth strips are soaked in onion extract and allowed to dry. When these strips are exposed to acidic or basic solutions, the smell of onion changes. In basic solutions, the smell disappears, while in acidic solutions, it remains unchanged.
Q18. Which indicator appears purple in a neutral solution?
Litmus solution is purple in neutral conditions. It turns red in acidic solutions and blue in basic solutions. Phenolphthalein is colourless in neutral and acidic solutions but turns pink in basic solutions. Methyl orange is orange in neutral solutions, red in acidic solutions, and yellow in basic solutions. Turmeric is yellow in neutral and acidic solutions and reddish-brown in basic solutions.
Q19. Litmus solution is extracted from which organism?
Litmus is a natural indicator extracted from lichens, which are symbiotic associations of algae and fungi. The lichen species used for litmus production is typically Roccella tinctoria. The dye extracted from these lichens produces a purple solution that changes colour depending on the pH of the solution it is added to.
Q20. Distilled water is:
Distilled water has equal concentrations of H⁺ and OH⁻ ions, making it neutral with a pH of exactly 7. It does not contain dissolved minerals or impurities. When tested with litmus paper, distilled water shows no colour change because it has no effect on indicators, confirming its neutral nature.
Q21. Lichen belongs to which division of plants?
Lichens belong to the division Thallophyta because they have a simple body structure called a thallus, which lacks true roots, stems, and leaves. Lichens are symbiotic associations between algae and fungi. Unlike angiosperms (flowering plants), bryophytes (mosses), and pteridophytes (ferns), lichens have a very simple plant body.
Q22. The smell of onion disappears in which medium?
The characteristic smell of onion disappears when exposed to basic solutions because the compounds responsible for the onion smell react with the OH⁻ ions in bases. This property makes onion an effective olfactory indicator. In acidic solutions and neutral solutions, the smell of onion remains unchanged.
Q23. Ammonium hydroxide has the chemical formula:
Ammonium hydroxide has one ammonium ion (NH₄⁺) and one hydroxide ion (OH⁻), giving the formula NH₄OH. It is a weak base formed when ammonia gas dissolves in water. NaOH is sodium hydroxide, NH₄Cl is ammonium chloride, and NH₃ is ammonia gas. Ammonium hydroxide is commonly used in cleaning products.
Q24. Which flower petals can act as natural indicators?
The petals of hydrangea, petunia, and geranium flowers contain natural pigments called anthocyanins that change colour in acidic and basic solutions. These flowers can be crushed to extract their pigments, which can then be used as natural indicators to test whether a solution is acidic or basic. Lotus, sunflower, and rose petals do not contain these pH-sensitive pigments.
Q25. A curry stain turns reddish-brown when scrubbed with soap because soap is:
Soap is basic in nature due to the presence of alkali (like sodium hydroxide or potassium hydroxide) in its manufacturing. Turmeric, which is present in curry, is a natural indicator that turns reddish-brown in basic solutions. When soap is applied to a curry stain, the basic nature of soap reacts with the turmeric, causing the stain to turn reddish-brown.
Q26. The correct remedy for acidity after overeating is:
Baking soda (sodium hydrogen carbonate, NaHCO₃) is basic in nature and neutralizes the excess hydrochloric acid produced in the stomach after overeating. The reaction produces sodium chloride, water, and carbon dioxide, providing relief from acidity. Fruit juice, lemon juice, and vinegar are all acidic and would worsen the condition.
Q27. Which of the following substances is a base?
Sodium hydroxide (NaOH) is a strong base that dissociates completely in water to produce OH⁻ ions. HNO₃ is nitric acid, H₂SO₄ is sulphuric acid, and CH₃COOH is acetic acid, all of which are acids. Bases are substances that produce hydroxide ions (OH⁻) in aqueous solution.
Q28. If red litmus paper shows no colour change, the solution may be:
Red litmus paper shows no colour change in acidic and neutral solutions. However, it turns blue in basic solutions. If red litmus shows no change, the solution is either acidic or neutral. Since the question mentions the solution "may be" neutral (and not acidic), the most appropriate answer is neutral. Red litmus cannot distinguish between acidic and neutral solutions.
Q29. Sour taste in food is due to the presence of:
Acids are responsible for the sour taste in many foods. For example, citric acid in lemons and oranges, acetic acid in vinegar, lactic acid in curd, and malic acid in apples all contribute to the sour taste. Bases, on the other hand, have a bitter taste and are not typically found in foods as sour agents.
Q30. Baking soda solution is used as a remedy for acidity because it is:
Baking soda (sodium hydrogen carbonate, NaHCO₃) is basic in nature, with a pH greater than 7. When consumed for acidity, it neutralizes the excess hydrochloric acid in the stomach through a neutralization reaction. The reaction produces sodium chloride (common salt), water, and carbon dioxide gas, providing relief from the burning sensation caused by acid reflux.
Q31. Substances whose odour changes in acidic or basic media are called:
Olfactory indicators are substances whose smell changes when exposed to acidic or basic solutions. Examples include onion, vanilla essence, and clove oil. These indicators are particularly useful for testing substances where colour indicators are not effective, such as colourless solutions. Synthetic indicators work by colour change, natural indicators are extracted from plants, and neutral indicators is not a standard classification.
Q32. Bases change red litmus paper to which colour?
Bases change red litmus paper to blue. This is because the OH⁻ ions present in bases react with the litmus dye, causing a colour change from red to blue. This is the opposite of acids, which turn blue litmus paper red. The litmus colour change is a basic test for identifying acids (red) and bases (blue).
Q33. The smell of vanilla essence disappears in which medium?
Vanilla essence is an olfactory indicator whose characteristic smell disappears in basic solutions. The compounds responsible for the vanilla smell react with the OH⁻ ions in bases, causing the aroma to vanish. In acidic solutions, the smell of vanilla essence remains unchanged. This property makes vanilla essence useful for testing the nature of unknown solutions.
Q34. To test acids and bases in the laboratory, substances are placed on:
A watch glass is a shallow circular glass dish used in laboratories for holding small amounts of substances for testing. When testing acids and bases with indicators, the test substance is placed on a watch glass, and a few drops of the indicator are added to it. This method is safe, requires minimal amounts of substances, and allows for easy observation of colour changes.
Q35. Indicators help us identify whether a substance is:
Indicators are substances that change colour or smell when they come in contact with acids or bases. They help determine whether a substance is acidic, basic, or neutral. Indicators do not help identify whether a substance is heavy or light, solid or liquid, or metal or non-metal. Their purpose is exclusively to test the acidic or basic nature of solutions.
Q36. Turmeric is an example of which type of indicator?
Turmeric is a natural indicator obtained from the turmeric plant (Curcuma longa). It turns reddish-brown in basic solutions and remains yellow in acidic and neutral solutions. Turmeric is commonly used in everyday life as a natural indicator, for example, to test the basic nature of soap. It is classified as a natural indicator because it is obtained from natural sources.
Q37. Phenolphthalein remains colourless in which type of solution?
Phenolphthalein is a synthetic indicator that remains colourless in acidic solutions. It turns pink in basic solutions and remains colourless in neutral solutions as well. The colour change occurs because the OH⁻ ions in basic solutions react with the phenolphthalein molecule, causing a structural change that produces a pink colour. In acids, no such reaction occurs.
Q38. Which of the following is NOT an indicator?
Sugar (sucrose) is a carbohydrate that does not change colour or smell in acidic or basic solutions. It is not an indicator. Phenolphthalein, litmus, and turmeric are all indicators that change colour in the presence of acids or bases. Phenolphthalein is a synthetic indicator, while litmus and turmeric are natural indicators.
Q39. Red cabbage leaves can act as:
Red cabbage contains anthocyanin pigments that change colour depending on the pH of the solution they are exposed to. In acidic solutions, the pigment turns red or pink, in neutral solutions it is purple, and in basic solutions it turns green or blue. This property makes red cabbage extract a valuable natural indicator for testing the acidic or basic nature of solutions.
Q40. Neutralisation is useful in treating which condition?
Neutralisation is the process where an acid and a base react to form salt and water. In treating acidity (hyperacidity in the stomach), antacids which are basic substances are used to neutralize the excess hydrochloric acid present in the stomach. This provides relief from the burning sensation and discomfort caused by acid indigestion. Neutralisation is not used for treating infections, fractures, or burns.
Q41. Bases generally have which taste?
Bases have a bitter taste and a soapy feel. This is why substances like baking soda, soap, and certain medicinal antacids taste bitter. For example, the bitter taste of some vegetables like bitter gourd is due to the presence of basic (alkaline) compounds. Acids have a sour taste, salts are salty, and sugars are sweet.
Q42. Another synthetic indicator used to test acids and bases is:
Methyl orange is a synthetic indicator used in laboratories to test acids and bases. It turns red in acidic solutions and yellow in basic solutions. It is orange in neutral solutions. Unlike natural indicators (red cabbage, turmeric) and olfactory indicators (clove oil), methyl orange is manufactured in laboratories and is classified as a synthetic indicator.
Q43. Which of the following is a synthetic indicator?
Phenolphthalein is a synthetic indicator manufactured in laboratories. It is colourless in acidic and neutral solutions but turns pink in basic solutions. Onion is a natural olfactory indicator, litmus is a natural indicator obtained from lichens, and turmeric is a natural indicator obtained from the turmeric plant. Synthetic indicators are chemically manufactured substances used for testing acids and bases.
Q44. Onion strips are stored overnight in a refrigerator to:
Onion strips are prepared as an olfactory indicator by soaking cloth strips in onion juice and then allowing them to dry. These strips are stored overnight in a refrigerator to allow the onion juice to properly soak into the cloth and for the strips to dry completely. This preparation process ensures that the onion smell is retained in the strips and can be used effectively as an olfactory indicator for testing acids and bases.
Q45. Acetic acid is represented by the formula:
Acetic acid is chemically represented as CH₃COOH. It is the main component of vinegar and gives vinegar its characteristic sour taste and smell. H₂SO₄ is sulphuric acid, HNO₃ is nitric acid, and HCl is hydrochloric acid. Acetic acid is a weak organic acid with a carboxyl group (-COOH) attached to a methyl group (-CH₃).
Q46. Soap is basic in nature because it:
Soap is basic in nature, and this can be demonstrated using turmeric, which is a natural indicator. When soap is applied to turmeric, the turmeric turns reddish-brown, confirming that soap is basic. If soap were acidic, it would not turn turmeric reddish-brown. Additionally, soap would turn red litmus blue if it is basic, not blue litmus red.
Q47. Methyl orange turns red in which medium?
Methyl orange is a synthetic indicator that turns red in acidic solutions and yellow in basic solutions. It is orange in neutral solutions. When methyl orange is added to an acidic solution, the H⁺ ions react with the methyl orange dye, causing a colour change from orange to red. This property makes methyl orange useful for detecting acidic solutions.
Q48. The reddish-brown turmeric stain turns yellow again after washing with water because:
Turmeric turns reddish-brown when it comes in contact with a base like soap. When the stain is washed with water, the excess base (soap) is removed, and the turmeric returns to its natural yellow colour. Water is neutral, so it does not cause the colour change; instead, it washes away the basic substance that was causing the turmeric to turn reddish-brown.
Q49. Onion can be used as which type of indicator?
Onion is an olfactory indicator because its characteristic smell changes when exposed to acids or bases. In basic solutions, the smell of onion disappears, while in acidic solutions, it remains. Olfactory indicators work by smell change rather than colour change. Onion is not a synthetic indicator, and while it is natural, it is specifically classified as an olfactory indicator due to its mode of action.
Q50. Acids change blue litmus paper to which colour?
Acids change blue litmus paper to red. This is the fundamental test for identifying acids using litmus paper. The H⁺ ions present in acids react with the blue litmus dye, causing it to turn red. This colour change is the opposite of bases, which turn red litmus paper blue. The litmus test is the most basic and widely used method for identifying acids.
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