Acids, Bases, and Salts

Marble contains calcium carbonate, CaCO3, a carbonate salt. Acids react with carbonates to form a salt, water, and carbon dioxide; in general, acid + carbonate produces salt + H2O + CO2. Repeated acid-rain contact can therefore dissolve or roughen the marble surface and damage structures. Option A correctly states the governing reaction, while the other claims contradict it.

After meals, bacteria may break down sugars and produce acids. These acids lower the mouth’s pH and can gradually demineralise the calcium-containing enamel. Rinsing with water helps remove food residues and dilute or reduce the acidic material, limiting its contact with enamel. Hence option A is correct. The other choices either reverse the effect or make scientifically impossible claims.

Tooth enamel is damaged more readily when the mouth becomes acidic. A pH below about 5.5 is commonly taken as the critical level at which enamel minerals begin to dissolve more significantly, increasing the risk of decay. Therefore, option B is correct. A neutral pH near 7 and absence of acid are less favourable for decay, while pH 14 is strongly alkaline and not the stated condition.

Excess hydrochloric acid in the stomach can cause a burning sensation and indigestion. Antacids contain mild bases, such as magnesium hydroxide or other suitable basic compounds, which react with and neutralise part of the excess acid. This raises the pH toward a less acidic level and provides relief. Thus option A is correct; the other options do not describe neutralisation.

In school science, an ant sting is considered to contain formic acid. Baking soda, or sodium hydrogen carbonate, is mildly basic. It reacts with the acid in a neutralisation reaction and reduces its acidic effect, so option B is correct. Option A is wrong because baking soda is basic, not acidic. Exam tip: A suitable mild base is used to reduce the effect of an acid.

A very low pH indicates strongly acidic soil. Such acidity can change the availability of mineral nutrients, reduce the uptake of some essential elements, and sometimes increase the solubility of substances harmful to roots. Consequently, plant growth may be reduced or otherwise disturbed. Option A is therefore correct. The other options ignore the biological and chemical effects of soil acidity.

Soil pH indicates whether soil is acidic, neutral, or basic. Testing it helps farmers select suitable crops and decide whether an amendment is needed, such as lime for excessive acidity or suitable organic matter and management for other conditions. Therefore, option A is correct. Testing does not transform soil, supply water, or change plants into metals, as the distractors claim.

Turmeric contains the natural pigment curcumin, which acts as an acid-base indicator. It remains yellow in an acidic or neutral medium but changes to a reddish-brown colour in a basic medium, such as a soap solution. Therefore, turmeric in option A can identify a basic solution. Common salt, sugar, and pure water do not show this characteristic indicator colour change in the stated test.

Hibiscus extract contains natural colour-producing substances that behave as an acid–base indicator. When the extract is added to an acidic medium and then to a basic medium, its colour can change differently, providing a visible clue about the solution. Thus, option A is correct. Options B, C and D are unrelated to indicator action and do not explain how acids or bases are identified.

An olfactory indicator is identified through a change in smell rather than a colour change. Onion juice has a characteristic smell, and in a basic medium that smell may become weaker or change, giving evidence that the substance is basic. Therefore, option A is correct. The claims about gold, water, or impossible detection do not describe olfactory-indicator behaviour.

Unknown laboratory solutions must never be identified by touching them. They may contain acids, bases, or other harmful chemicals that can irritate, burn, or damage skin. A suitable indicator, test method, protective equipment, and teacher’s instructions should be used instead. Option A is correct; the other choices make scientifically false claims about safety, pH, or metal formation.

Red and blue litmus provide only a broad classification and may show little or no change for a nearly neutral solution or a weak substance. A universal indicator gives a range of colours corresponding to approximate pH values, so it provides more information. Option A is correct. Tasting, rubbing, or putting an unknown solution in the eye is unsafe and scientifically invalid.

Diluting a concentrated acid with water is generally exothermic, meaning that heat is released. If water is poured rapidly into a small amount of concentrated acid, the water can heat suddenly and boil locally, causing dangerous acid droplets to splash. Therefore acid should be added slowly to a larger volume of water with stirring and protection. Option A is correct.

When water is added directly to concentrated acid, the small amount of water may absorb heat very rapidly. It can become hot or boil suddenly, forcing concentrated acid out of the container as droplets. This can cause severe burns, so the safer rule is to add acid slowly to water while stirring. Option A correctly explains the accident; the other statements contradict the exothermic process.

Neutralisation is the reaction between an acid and a base, commonly producing salt and water. In many such reactions, energy is released as heat, so the reaction mixture or its container becomes warmer. This observation supports option A. Freezing, formation of no substance, and production of only metal are not the expected results of acid–base neutralisation, so options B, C and D are incorrect.

On the pH scale, 7 represents neutrality, values below 7 indicate acidity, and values above 7 indicate basicity. A fall from pH 7 to pH 3 therefore changes the solution from neutral to acidic. Because pH is logarithmic, pH 3 also represents a much greater hydrogen-ion concentration than pH 7. Hence option A is correct; the other choices reverse or deny the pH relationship.

A pH of 7 indicates a neutral solution at ordinary school-level conditions. When pH rises above 7, the solution is classified as basic or alkaline because its hydrogen-ion concentration is lower relative to hydroxide-ion concentration. Since pH 11 is well above 7, option B is correct. Options A and C describe acidity, while option D incorrectly treats all pH values as neutral.

Lemon juice contains citric acid, so it is acidic. Soap solution is generally alkaline because soap reacts with water to produce a basic solution. Thus the pair in option A has an acidic first substance and a basic second substance. In option B the order is reversed, option C contains neutral water followed by an acid, and option D contains two commonly basic substances rather than the requested acid–base order.

The governing concept is that bases produce hydroxide ions in aqueous solution and show alkaline properties. Soap solution is basic because soap undergoes hydrolysis in water, while baking soda solution is mildly basic because sodium bicarbonate reacts with water to produce some hydroxide ions. Vinegar, lemon juice, tamarind, and tomato juice contain acids, so option B is the only pair in which both substances are basic.

The governing concept is the acidic nature of substances in water. Lemon juice contains citric acid, and vinegar contains acetic acid, so both can provide hydrogen ions and turn blue litmus red. Lime water, soap solution, baking soda, and washing soda are basic, while pure water and common salt solution are not an acidic pair. Therefore, option A is correct.