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All alkalis are _______ but all ______are not alkalis.
A. Acids
B. Bases
C. Acids, bases
D. Bases, acids

Last updated date: 20th Jun 2024
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Hint: Try to recall that bases are those substances that neutralize acid whereas alkali are those bases which are soluble in water.

Complete step by step solution:
There is a minor difference between base and alkali, but there is one.
Now, some points about base and alkalis which are as follows:

A base is a substance which reacts with an acid to produce salt and water.
Bases that form a neutral product with acids are mostly metal oxides, metal hydroxides, metal carbonates and metal hydrogen carbonates.
They are bitter in taste and slippery to touch in nature.
They are corrosive, if they are concentrated.
They turn red litmus paper to blue.
They may either be soluble or insoluble in water.
Examples of base: $NaOH, N{H_3}, ZnO, CuO$ etc.

Alkalis are those special class of bases which are soluble in water
Only dissolving in water, they produce hydroxide ions i.e. $O{H^ - }$ ion.
These are all Group 1 metal hydroxides, plus calcium hydroxides (which is only soluble in water) plus ammonium hydroxide. Example: $NaOH,CaOH,N{H_4}OH$etc.
On adding alkali to acid, the pH of the mixture increases whereas on adding acid to the alkali, the pH decreases due to the removal of alkali.
Therefore, from above it can be said that all alkalis are bases but only water-soluble bases are alkalis or we can also say that all alkalis are bases but all bases are not alkalis.

Hence, it can be concluded that option B is the correct answer.

Note: It should be remembered that alkali is a subset of base.
It should be also kept in mind that insoluble bases which are not alkalis, are all metal oxides and insoluble metal hydroxides.