Acids, Bases, and Salts

Sodium hydroxide is a strong base, while the residue in an acid bottle may contain an acid. If the two come into contact, neutralisation occurs: acid plus base forms salt and water, and the process releases heat. A concentrated mixture may heat rapidly and cause splashing, burns, or damage to the container. Thus option A identifies the real laboratory hazard; the other choices are chemically false.

A very low pH indicates that the soil is acidic. Lime, commonly containing calcium carbonate or calcium oxide, is basic and is added to neutralise excess acidity. However, the amount must be controlled. If too much lime is applied, the neutralisation can go beyond the suitable range and raise the pH excessively, making the soil alkaline. Therefore option A is correct; the other choices do not describe this reaction.

Dental plaque contains bacteria that metabolise sugars left on the teeth. During this process they produce organic acids. The added hydrogen ions lower the pH at the tooth surface. If the pH remains sufficiently low, minerals dissolve from enamel and decay can begin. Hence option A is correct. Teeth do not manufacture bases, water does not become hydrogen gas, and toothpaste is generally used to help reduce acidity rather than increase it.

Carbonate rocks such as limestone contain carbonate ions, which react with acids. In a simplified reaction, an acid and a carbonate produce a salt, water, and carbon dioxide; for example, carbonate minerals can slowly dissolve in acidic rainwater. Repeated dissolution weakens and erodes the rock surface. Therefore option A is correct. Acid rain does not strengthen all carbonates, and its effects are not limited to iron.

Turning red litmus blue shows that the sample is basic. Carbonates and hydrogen carbonates also behave as bases, but when an acid is added they release carbon dioxide, producing visible effervescence. Sodium hydroxide is a strong base and reacts with acid by neutralisation to form salt and water, without producing carbon dioxide. Thus option A fits both observations; options B-D contain carbonate or hydrogen carbonate and would give CO₂.

Plaster of Paris is calcium sulfate hemihydrate, CaSO₄·½H₂O. On adding water, it rehydrates and sets into gypsum, CaSO₄·2H₂O. A correct amount of water permits this reaction and gives a workable paste. Excess water makes the mixture too dilute; after setting, the extra water leaves pores and reduces strength, while the diluted paste may set less effectively. Therefore option A is correct.

At about 25°C, a neutral aqueous solution has pH 7 because hydrogen-ion and hydroxide-ion concentrations are equal. A pH value above 7 indicates that hydroxide ions predominate relative to hydrogen ions, so the solution is basic. A solution of pH 8 is therefore only mildly alkaline, not neutral. Option A states this rule correctly; pH 8 is not a strong acid, and neutral pH is not 14.

Relief by neutralisation depends on the chemical nature of the injected substance. If a sting contains an acidic component, a suitable mild basic substance may reduce its effect; if it contains a basic component, a suitable mild acidic substance may be considered. The substances and medical risks are not necessarily identical for every sting, so one remedy cannot be assumed universally safe. Thus option A is correct.

Diluting a concentrated acid is exothermic: heat is released when acid mixes with water. If a small amount of water is poured into a larger amount of concentrated acid, that water can heat suddenly, boil locally, and eject acid droplets. The safe laboratory rule is to add acid slowly to a larger volume of water while stirring and wearing protection. Therefore option A identifies the mistake and its hazard.

The governing concept is the characteristic test for hydrogen. When a reactive metal reacts with an acid, hydrogen gas is commonly released, for example: Zn + 2HCl → ZnCl₂ + H₂. Hydrogen is highly combustible and, on bringing a flame near the collected gas, burns rapidly with a distinct pop sound. Carbon dioxide would extinguish a flame, while nitrogen is not combustible. Therefore, option A is correct.

Heating baking soda, sodium hydrogen carbonate, produces sodium carbonate, carbon dioxide, and water: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Sodium carbonate is a salt of a strong base and a weak acid; its aqueous solution undergoes hydrolysis and contains excess hydroxide ions. Hence the solution, and the substance in the school-level sense asked here, is basic. Option A is correct; it is not acidic, neutral, or a description of metallic nature.

The governing concept is water of crystallisation: a fixed number of water molecules incorporated into the crystal structure of some salts. On heating, this water may be released as droplets or vapour, and removal can alter the crystal structure and colour. For example, hydrated copper sulphate is blue and becomes white on losing water. A salt cannot be defined as ion-free or always gaseous. Thus option A is correct.

Lemon juice contains citric acid, so it is acidic rather than basic. Acids can react with metals under suitable conditions; prolonged contact may allow copper to dissolve slowly and form copper-containing salts or other contaminants. This is why acidic foods are not generally stored in unlined copper vessels. The claim that copper never reacts with acids is false, and the other alternatives describe impossible or irrelevant changes. Therefore option A is correct.

Two observations support the same conclusion. Blue litmus turning red indicates an acidic solution. Chalk is mainly calcium carbonate, CaCO₃. Acids react with carbonates to produce a salt, water, and carbon dioxide: acid + CaCO₃ → salt + H₂O + CO₂. The escaping CO₂ causes bubbles. Hydrogen and oxygen are not the expected gases in this reaction, so option A is correct.

Calcium hydroxide, Ca(OH)₂, is a base and can react with an acidic oxide such as carbon dioxide. For example, Ca(OH)₂ + CO₂ → CaCO₃ + H₂O; calcium carbonate is the salt formed, and the limewater may turn milky. Vinegar, hydrochloric acid, and lemon juice are acidic substances, so they do not normally neutralise an acidic gas to produce a salt in this context. Hence option A is correct.

On the usual school pH scale, values above 7 indicate basic solutions, while 7 is neutral. A pH of 11 therefore represents a basic solution. Adding water dilutes the hydroxide ions, reducing their concentration and moving the pH toward 7. Although the pH falls to 9, it remains above 7, so the solution is still basic but less strongly basic. Thus option A is correct; it has not become acidic or neutral.

The governing idea is salt hydrolysis. A salt produced from a strong base and a weak acid generally gives a basic aqueous solution because the anion of the weak acid reacts with water and generates some OH⁻ ions. Basic solutions turn red litmus blue. The exact strength depends on concentration and hydrolysis, but the expected school-level result is basicity. Therefore option A is correct; the other colour changes are not standard acid-base indicators.

An acidic solution contains a relatively high concentration of hydrogen ions, H⁺, and its pH is low. Added hydroxide ions react with hydrogen ions in the neutralisation reaction H⁺ + OH⁻ → H₂O. This removes free H⁺ ions from the solution, so the hydrogen-ion concentration decreases and pH rises. The solution may eventually approach neutrality, depending on the amounts added. Options B, C, and D do not describe this reaction. Therefore A is correct.

Electrical conduction in an aqueous solution requires mobile charged particles. Sodium chloride dissociates in water into Na⁺ and Cl⁻ ions; when a potential difference is applied, these ions move toward opposite electrodes and carry current. Neutrality refers to the overall acid-base condition, approximately pH 7, not to an absence of ions. Thus a neutral NaCl solution can conduct electricity. Sugar does not provide ions, colour is irrelevant, and free electrons are not the main carriers in solution. Option A is correct.

The governing concept is the reaction of carbon dioxide with lime water, which is aqueous calcium hydroxide. Initially, carbon dioxide forms insoluble calcium carbonate, Ca(OH)2 + CO2 → CaCO3 + H2O, producing milkiness. When carbon dioxide is passed in excess, the precipitate reacts further to form soluble calcium hydrogen carbonate, CaCO3 + CO2 + H2O → Ca(HCO3)2. The precipitate dissolves, so option A is correct; the other conditions do not cause this change.