Acids, Bases and Neutral Substances: 18 MCQs with answers

$H^+$

Acids release hydrogen ions ($H^+$) in water. Bases release hydroxide ions ($OH^-$).

The liquid is acidic.

Blue litmus turning red is the standard test for an acid. Bases turn red litmus blue, and neutral substances cause no change.

Pure water

Pure water has a pH of exactly 7, making it neutral. Lemon juice and vinegar are acidic, and soap solution is basic.

Above 7

Bases have a pH above 7. Acids are below 7, and neutral substances are exactly 7.

Litmus

Litmus is a natural indicator extracted from lichens. Turmeric, china rose, and red cabbage are other natural indicators from different sources.

True

The pH scale is logarithmic: a change of 1 unit represents a ten-fold change in hydrogen ion concentration.

100 times

Each step of 1 on the pH scale represents a ten-fold change. A difference of 2 pH units means $10 \times 10 = 100$ times more acidic.

The solution is basic.

Phenolphthalein is colourless in acids and turns pink in bases. A pink colour therefore indicates a basic solution.

Sodium chloride and water

The neutralisation reaction $HCl + NaOH \rightarrow NaCl + H_2O$ produces sodium chloride (common salt) and water.

It contains a mild base that neutralises excess acid.

Antacids contain mild bases like magnesium hydroxide that neutralise the excess hydrochloric acid in the stomach, reducing the acidity.

Quicklime

Quicklime (calcium oxide) is a base that neutralises acidic soil and raises its pH. Vinegar, lemon juice, and hydrochloric acid would make the soil more acidic.

A strong acid ionises completely in water; a weak acid ionises partially.

The key difference is the extent of ionisation. Strong acids like hydrochloric acid ionise completely, while weak acids like acetic acid ionise only partially.

Soap is basic in nature.

Turmeric turns reddish-brown in bases and stays yellow in acids. Since the stain turned reddish-brown, soap must be basic.

Both A and R are true, and R correctly explains A.

A neutral substance has pH 7 because the acid and base properties are balanced — neither $H^+$ nor $OH^-$ ions dominate. The reason correctly explains the assertion.

I and II only

Statements I and II are correct. Statement III is wrong because neutral substances cause no colour change in either blue or red litmus.

The $H^+$ ion concentration decreased by a factor of 100.

Each pH unit represents a ten-fold change. Going from pH 2 to pH 4 is a change of 2 units, so the $H^+$ concentration decreased by $10 \times 10 = 100$ times. The solution is still acidic.

Calcium oxide, resulting in the formation of calcium hydroxide and water

The correct option is calcium oxide. When calcium oxide (quicklime) is added to acidic soil, it reacts with the excess $H^+$ ions to form calcium hydroxide, which raises the pH towards neutral.

Lemon juice is acidic, baking soda is basic, and salt solution is neutral

The correct option is that lemon juice is acidic, baking soda is basic, and the salt solution is neutral. The color change indicates the pH levels: pink for acid (lemon juice), blue for base (baking soda), and no change for neutral (salt solution).