Which of the following statements is/are correct? 1. All the bases are alkali. 2. All alkalis dissolve in water. 3. Alkalis are soapy to touch, bitter in taste and corrosive in nature. Select the correct answer using the code given below:
- (a)1 only
- (b)1 and 3 only
- (c)2 and 3 only
- (d)3 only
Correct — C, 2 and 3 only. Statement 1 reverses the relationship between the two words. Bases generate hydroxide ions in water, and the textbook is explicit that all bases do not dissolve in water — an alkali is a base that dissolves in water. So every alkali is a base, but not every base is an alkali; copper hydroxide and iron hydroxide are bases that stay stubbornly insoluble. Statement 2 is true by that same definition, since dissolving in water is the very property that earns a base the name alkali. Statement 3 lists the three properties the textbook attaches to alkalis in one sentence — they are soapy to touch, bitter and corrosive — together with the warning never to taste or touch them, because they can cause harm. Statements 2 and 3 stand and statement 1 falls, which makes (c) the answer. The set-relationship is worth drawing once: alkalis form a smaller circle inside the larger circle of bases.
- (a)1 only — Keeps the single false statement and discards both true ones. All bases are not alkalis; only the water-soluble ones are.
- (b)1 and 3 only — Correct about the properties of alkalis and wrong about the set relationship. It also drops statement 2, which is simply the definition of an alkali restated.
- (d)3 only — Too cautious. Statement 2 is not an extra claim at all — a base that dissolves in water is exactly what an alkali means.
An acid produces hydrogen ions in aqueous solution and a base produces hydroxide ions. Solubility is what divides the bases into two groups: the soluble ones are called alkalis, and the sodium, potassium and calcium hydroxides belong there, while copper, iron and aluminium hydroxides do not dissolve and remain simply bases. Alkalis feel soapy, taste bitter and are corrosive, and their solutions turn red litmus blue and give a pH above seven.
The item is a set-membership test dressed as a chemistry question, and the false statement is the one that gets the direction of inclusion backwards. Asking which way round the words nest settles it: every alkali is a base because it does produce hydroxide ions, but a base needs the extra property of solubility before it can be called an alkali. Statement 3 can look suspicious because listing three properties at once feels like an invitation to find one that fails, yet all three are in the textbook sentence, and the accompanying safety warning is itself evidence that the corrosive claim is meant seriously — soapiness comes from the alkali beginning to break down the skin's own oils and proteins, which is corrosion in slow motion. Sodium hydroxide, calcium hydroxide and potassium hydroxide are the alkalis worth naming from memory.
- Bases generate hydroxide ions in water; bases which are soluble in water are called alkalis.
- All bases do not dissolve in water — copper hydroxide and iron hydroxide are bases but not alkalis.
- Alkalis are soapy to touch, bitter and corrosive, and should never be tasted or touched.
- Sodium hydroxide, potassium hydroxide and calcium hydroxide are the common alkalis; limewater is a solution of calcium hydroxide.
- An alkaline solution turns red litmus blue and has a pH above 7, rising towards 14 as the hydroxide-ion concentration increases.
- Reading 'all bases are alkalis' as equivalent to 'all alkalis are bases'; only the second is true.
- Rejecting statement 3 because it lists three properties at once; all three are standard textbook attributes of alkalis.
- Treating soapiness as harmless — it is a sign that the alkali is attacking the skin.
As a statements item on the base-versus-alkali distinction, as a properties question on soapiness, bitterness and corrosiveness, or as an identification item asking which listed hydroxide is an alkali.
CDS_GK_2021_I_Q1162021Calcium oxide reacts vigorously with water to produce slaked lime whose solution is used for white-washing walls. This slaked lime reacts with component (A) in air to form a thin layer of component (B) to give a shiny finish. What are the components (A) and (B)?
- (a) A – O2 ; B – CaCO3
- (b) A – CO2 ; B – Ca(OH)2
- (c) A – O2 ; B – Ca(OH)2
- (d) A – CO2 ; B – CaCO3
Answer(d) A – CO2 ; B – CaCO3
A worked example of the same distinction from the same paper cycle. Slaked lime is calcium hydroxide, a base whose solution — limewater — is an alkali, and the white-washing chemistry only works because that solution can be spread on a wall.
- practice — not a real PYQ
Which one of the following is a base but not an alkali?
- (a)Sodium hydroxide
- (b)Potassium hydroxide
- (c)Copper hydroxide
- (d)Calcium hydroxide
Answer(c) Copper hydroxide — it does not dissolve in water, and only water-soluble bases are called alkalis.
- practice — not a real PYQ
An aqueous solution turns red litmus blue and feels soapy to the touch. Its pH is most likely to be
- (a)1
- (b)5
- (c)7
- (d)11
Answer(d) 11 — soapiness and the litmus change both mark an alkaline solution, which sits above 7 on the pH scale.