Acids, Bases, and Salts

Tooth enamel is a hard mineral layer, mainly containing calcium phosphate. Bacteria in dental plaque use sugars and produce acids. When the pH at the enamel surface falls sufficiently, the mineral dissolves more readily; repeated demineralisation can produce cavities. Thus acid promotes enamel erosion and tooth decay, making option A correct. It does not turn enamel into gold, strengthen it, or lack any connection with dental damage.

pH is an inverse measure of hydrogen-ion concentration, so a value below 5 indicates a strongly acidic environment. Tooth enamel begins to be threatened when the mouth becomes sufficiently acidic, because its mineral can dissolve by demineralisation. Therefore a pH below 5 represents a harmful acidic condition and increases the risk of tooth decay. It is not basic, neutral, or chemically harmless, so option A is correct.

Electrical conduction in an aqueous solution requires charged particles that can move. Sodium chloride dissociates in water into Na⁺ and Cl⁻ ions, and these ions migrate towards opposite electrodes when a potential difference is applied. The solution is neutral overall because the positive and negative charges are equal, but neutrality does not mean absence of ions. Coloured particles, lack of water, or conversion into an acid are not the reasons; therefore option B is correct.

Turmeric contains curcumin, which acts as a natural indicator. In an acidic or neutral medium it generally remains yellow, whereas in a basic medium it develops a reddish-brown colour. Observing this colour change therefore indicates that the tested solution is basic. The result does not show metallic character, and it is inconsistent with a purely neutral or acidic solution under the stated indicator test. Hence option C is correct.

Indicators provide the governing evidence about the solution’s acid-base nature. Blue litmus turns red in an acidic medium, while phenolphthalein remains colourless in acidic and neutral media and turns pink in a sufficiently basic medium. The two observations are therefore consistent with acidity. They do not prove that the solution contains no salt, and a solution is not itself metallic. Thus option A is the valid conclusion.

Hard water contains dissolved calcium and magnesium ions, which can interfere with soap and form deposits. Washing soda, sodium carbonate (Na₂CO₃), supplies carbonate ions that react with Ca²⁺ and Mg²⁺ to form insoluble carbonates. These precipitates can be removed, lowering the concentration of hardness-causing ions and softening the water. Baking soda, vinegar, and common salt do not perform this standard softening action, so option B is correct.

The governing concept is neutralisation. In water, an acid supplies hydrogen ions, H⁺, while a base supplies hydroxide ions, OH⁻. These ions combine in the net ionic equation H⁺ + OH⁻ → H₂O, producing water and reducing the acidic and basic effects. Sodium and chloride ions may remain as spectator ions in some reactions, while metal–oxygen or carbon–nitrogen combinations are unrelated. Therefore option B is correct.

The pH scale is logarithmic: pH = −log[H⁺]. A tenfold decrease in hydrogen-ion concentration raises pH by one unit. Making the solution 100 times, or 10² times, less acidic means [H⁺] becomes 100 times smaller, so the pH rises by log₁₀(100) = 2 units. Starting from pH 3, the approximate new pH is 3 + 2 = 5. Therefore option C is correct.

The key qualitative test is that carbon dioxide turns limewater milky by forming insoluble calcium carbonate: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. An acid reacts with a carbonate ion to release carbon dioxide, for example CO₃²⁻ + 2H⁺ → CO₂ + H₂O. Therefore the unknown solid may contain carbonate ions. Chloride and sulphate do not normally produce this gas in the stated test, and hydroxide neutralises acid without releasing CO₂, so option C is correct.

The governing concept is acid–base neutralisation. Most metal oxides are basic oxides, so they react with acids to produce a salt and water. For example, copper(II) oxide reacts with hydrochloric acid: CuO + 2HCl → CuCl₂ + H₂O. The formation of salt and water identifies option A. Photosynthesis, evaporation, and condensation do not describe this chemical change.

The governing concept is the reaction between acidic and basic oxides. Carbon dioxide is a non-metal oxide with acidic character, while lime water contains calcium hydroxide, a base. They react as CO₂ + Ca(OH)₂ → CaCO₃ + H₂O, producing a white precipitate that makes lime water milky. Therefore option A is correct. Carbon dioxide is not a metal, basic oxide, or neutral salt.

The governing safety principle is that dilution of concentrated acid is exothermic. If a small amount of water is poured into a larger volume of acid, the water can heat suddenly, boil locally, and eject corrosive acid droplets. The safe laboratory practice is to add acid slowly to a larger quantity of water while stirring. Therefore option A explains the increased risk; the other choices describe impossible or chemically unrelated effects.

The governing concept is the chemical reactivity of food acids with metals. Sour foods commonly contain acids such as acetic, citric, or lactic acid. During prolonged contact, these acids may react with copper or its surface compounds and introduce copper salts into the food. Some copper compounds are harmful in excessive amounts, so such storage is avoided. Option A is correct; the other statements contradict the acidic nature of sour foods or make unsupported claims.

The governing concept is the logarithmic definition of pH: pH = −log₁₀[H⁺]. If the hydrogen-ion concentration changes from [H⁺] to 10[H⁺], the new value is −log₁₀(10[H⁺]) = −1 − log₁₀[H⁺], so the pH falls by exactly one unit. Acidity therefore increases. Option A is correct; a tenfold concentration change does not produce a ten-unit or positive pH change.

The governing concept is neutralisation and control of soil pH. Lime, commonly containing calcium carbonate or calcium hydroxide, is basic and is added in measured amounts to reduce excess acidity. If too much is applied, the soil pH can rise above the suitable range and the soil becomes excessively alkaline. This may reduce nutrient availability and harm plant growth. Therefore option A is correct; the other options are not consequences of adding lime.

The governing classification is based on solubility. A base is a substance that can neutralise an acid, but not every base dissolves appreciably in water. A base that dissolves in water is specifically called an alkali; its aqueous solution contains hydroxide ions, such as sodium hydroxide solution. Therefore option C is correct. A salt is a product of many neutralisation reactions, an acid is chemically different, and an indicator only changes colour to show acidity or basicity.

The governing concept is electrolysis of brine in the chlor-alkali process. Common salt is sodium chloride, NaCl. When its aqueous solution is electrolysed, chlorine gas forms at the anode, hydrogen gas forms at the cathode, and sodium hydroxide remains in solution: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Hence option A is correct. The other statements confuse salt chemistry or contradict its known solubility.

The governing concept is the colour response of acid–base indicators. Methyl orange is red in an acidic medium and becomes yellow in a basic medium; phenolphthalein is colourless in acidic or neutral conditions and turns pink in a basic solution. The observed combination, red methyl orange and colourless phenolphthalein, therefore indicates an acidic solution. Option C is correct. A basic solution would make phenolphthalein pink, so option A is contradicted.

The governing concept is the nature of salts formed from strong acids and strong bases. Sodium chloride is produced from hydrochloric acid and sodium hydroxide, so its aqueous solution is generally neutral, with pH close to 7. Dilution lowers the concentrations of both sodium and chloride ions but does not create a significant excess of hydrogen or hydroxide ions. Hydrochloric acid and sodium hydroxide would remain acidic or basic, while sodium carbonate is basic.

The governing concept is the chemical action of available chlorine. Bleaching powder, commonly represented as calcium oxychloride, releases chlorine or hypochlorous species that oxidise coloured substances and many microorganisms. Thus it is used for bleaching cloth and paper and for disinfecting drinking water and surfaces. Gypsum is mainly used in plaster products, washing soda is a cleaning and softening agent, and sodium chloride is common salt rather than a bleaching disinfectant.