Acids, Bases, and Salts

The governing concept is neutralisation and the pH scale. A base supplies hydroxide ions, which react with hydrogen ions from the acid: H⁺ + OH⁻ → H₂O. If only a small amount of base is added, it consumes only part of the available H⁺ ions. Their concentration therefore decreases and pH rises, but some acidity remains, so complete neutralisation has not occurred. Hence option A is correct.

The governing concept is that acid or base behaviour of many gases appears only in the presence of water. Moisture allows the gas to dissolve and form ions; those ions then interact with the litmus indicator and change its colour. Dry litmus lacks the water needed for this ion formation, so it remains unchanged. Therefore option A is correct. The other options incorrectly call the indicators defective or deny that the gas is a substance.

The governing concept is the different composition and uses of two sodium salts. Baking soda, sodium hydrogen carbonate (NaHCO₃), releases carbon dioxide on heating or reacting with an acid; the gas makes dough and cakes rise. Washing soda, sodium carbonate decahydrate (Na₂CO₃·10H₂O), removes calcium and magnesium ions and is used to soften hard water. Thus option A is correct.

The governing concept is chemical safety, which depends on a substance’s nature, strength, concentration, and exposure rather than simply whether it is an acid or a base. Concentrated sulphuric acid can cause severe burns, while strong sodium hydroxide can also damage skin and tissues. Some dilute acids and bases are safe in controlled uses, but that does not make every one harmless. Therefore option A is correct; the absolute claims in B, C, and D are false.

The governing concept is the use of characteristic tests for acids and bases. Magnesium is above hydrogen in the reactivity series and reacts with an acid to release hydrogen gas, for example: Mg + 2HCl → MgCl₂ + H₂. Separately, a solution that changes red litmus to blue is basic. Therefore the first solution is acidic and the second is basic, so option A is correct; the other choices reverse or ignore these observations.

The governing concept is the colour scale of a universal indicator. Green generally corresponds to a nearly neutral solution around pH 7, while yellow indicates a lower pH and therefore mild acidity. Adding acid increases the hydrogen-ion concentration and shifts the pH below neutral, producing the yellow colour. Hence option B is correct. The solution did not become more basic, remain unchanged, or turn solid, so A, C, and D do not fit the observation.

The governing concept is the relationship between pH and hydroxide concentration in a basic solution. A pH of 11 indicates alkalinity, so hydroxide ions are present in relatively high effective concentration. Bringing the pH closer to 7 means neutralising the base, usually by adding acid; hydrogen ions react with hydroxide ions according to H⁺ + OH⁻ → H₂O. Thus the effective amount of OH⁻ decreases, making option C correct. Chloride and nitrate ions do not determine this change.

The governing concept is the difference between concentration and acid strength. Concentration means the quantity of acid present in a given volume of solution, whereas strength describes the extent to which acid molecules ionise in water. A concentrated weak acid and a dilute strong acid are possible. Hence option B is correct; the other choices confuse these terms or make chemically false claims about pH and water.

The governing concept is water of crystallisation, which is a fixed number of water molecules incorporated into certain salt crystals. Blue hydrated copper sulphate, CuSO4·5H2O, loses this water on heating and becomes white anhydrous CuSO4. Adding water restores the hydrated blue form. Therefore option B is correct; chlorine, sodium, and carbon dioxide do not explain this reversible colour change.

The governing concept is the indicator colour change during neutralisation. Phenolphthalein is pink in a sufficiently basic medium but becomes colourless when the solution is no longer sufficiently basic. Adding acid consumes hydroxide ions and lowers the solution’s basicity. Therefore option A is correct. More basic conditions preserve or intensify the pink colour, while metal formation and guaranteed carbon dioxide are irrelevant.

The governing concept is the hydrolysis and nature of salts. A salt produced from a weak acid and a strong base generally gives a basic solution because the conjugate base of the weak acid reacts with water to produce hydroxide ions. This turns red litmus blue, so option B is correct. Strong acid–weak base salts are usually acidic, strong–strong salts are approximately neutral, and acid plus water is not a salt-forming combination.

The governing concept is electrical conduction by mobile charged particles. When an acid dissolves in water, it forms ions such as hydronium and accompanying negative ions. These ions move between electrodes and carry electric charge, so measurable current or conductivity is direct evidence of their presence. Option B is correct; smelling, cooling, or inserting glass does not demonstrate mobile ions.

The governing concept is that pH measures the acidic or basic condition of water and strongly influences biological and chemical processes. Acid rain introduces acidic substances, lowering lake pH. Excess acidity can interfere with enzyme activity, reproduction, respiration, and the survival of sensitive aquatic organisms; it may also affect dissolved minerals. Thus option A is correct, while the other statements contradict the meaning of low pH.

The governing concept is an acid–carbonate reaction. Baking soda, or sodium hydrogen carbonate, reacts with the acid and releases carbon dioxide: NaHCO₃ + H⁺ → CO₂ + H₂O. The escaping carbon dioxide gets trapped as tiny bubbles in the batter or mixture, so the food rises and becomes porous, soft, and spongy. Hydrogen does not form in this reaction, oxygen does not provide the colour, and chlorine is not involved; therefore option C is correct.

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.

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 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 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 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 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.