Acids, Bases, and Salts

In the chlor-alkali process, concentrated brine is electrolysed. Chloride ions give chlorine gas at the anode, water is reduced at the cathode to produce hydrogen gas, and sodium ions combine with hydroxide ions in solution to form sodium hydroxide. Therefore, the three main products are chlorine, hydrogen, and sodium hydroxide. The other combinations do not describe brine electrolysis, so option A is correct.

The chlor-alkali process involves electrolysis of concentrated aqueous sodium chloride, or brine. At the anode, chloride ions lose electrons and are oxidised: 2Cl⁻ → Cl₂ + 2e⁻. Therefore, chlorine gas is released at the anode. Hydrogen is produced at the cathode, while sodium hydroxide remains in solution; oxygen and carbon dioxide are not the expected products of this process.

In the chlor-alkali process, brine is electrolysed. Water molecules gain electrons at the cathode and are reduced, producing hydrogen gas and hydroxide ions: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Hence hydrogen is formed at the cathode. Chlorine is produced at the anode, whereas nitrogen and sulphur dioxide are not products of brine electrolysis. Hydrogen may be identified by its characteristic pop sound.

Tooth enamel is mainly mineral matter and can be damaged when the mouth becomes sufficiently acidic. Bacteria use food sugars and produce acids, lowering the pH around the teeth. When the pH falls below about 5.5, enamel dissolution becomes more likely and the risk of dental decay rises. A pH of 7 is neutral, while 10 and 14 are alkaline, so they do not represent the critical acidic limit asked here.

Bacteria in the mouth can convert food particles, especially sugars, into acids. These acids lower the local pH and may attack tooth enamel. Basic toothpaste contains mildly alkaline substances that react with and neutralise excess acid, helping to reduce acidity and protect enamel. It does not dissolve enamel, make the pH zero, or increase acid. Therefore option A correctly describes neutralisation.

Acidic soil has a pH lower than the range suitable for many crops. Slaked lime, calcium hydroxide, Ca(OH)₂, is a basic substance. It reacts with excess acidic substances in the soil and neutralises them, thereby raising the soil pH toward a more suitable value. It is not added to create gas or sugar, and it does not increase acidity. Thus option A correctly states its agricultural purpose.

Curd contains lactic acid and is therefore acidic. If it is stored for a considerable time in a copper vessel, the acid may react with the metal. Such a reaction can form copper compounds that contaminate the food and may be harmful when consumed in excess. Copper does not convert curd into water or sweetness, and the statement that curd has no acid is incorrect. Hence option A is the safe and chemically relevant explanation.

Laboratory chemicals may be corrosive, poisonous, or otherwise harmful, even when only a small quantity is present. Tasting an unknown acid or base can cause burns, poisoning, or injury to the mouth and digestive system, so it is never an acceptable identification method. Safe testing uses suitable indicators, pH paper, or a teacher-approved procedure. The other options are false: acids and bases are not always sweet, may be colourless, and can have strong effects.

The governing concept is neutralisation. After food remains in the mouth, bacteria may break down sugars and produce acids. These acids can attack tooth enamel and contribute to tooth decay. Basic toothpaste contains mildly alkaline substances that react with and reduce the effect of the acids. Therefore option A is correct. The other options are incorrect because toothpaste does not convert teeth into acid, stop saliva completely, or protect teeth by depositing ordinary salt.

The governing concept is acid-base neutralisation. Acidic soil has an excess of acidic substances, so a suitable treatment should be mildly basic rather than another acid. Slaked lime, calcium hydroxide, is basic and can react with excess acidity, helping raise the soil pH toward a more suitable range. Thus option A is correct. Vinegar, lemon juice, and dilute hydrochloric acid are acidic, so adding them would generally increase rather than reduce soil acidity.

This is a neutralisation reaction between a base and an acid. The balanced equation is NaOH + HCl → NaCl + H₂O. The sodium ion comes from sodium hydroxide, while the chloride ion comes from hydrochloric acid; together they form sodium chloride, and hydrogen ions combine with hydroxide ions to form water. Hence option A is correct. The other salts require carbonate, calcium, or sulphate ions, none of which are present in the reactants.

Drinking water can contain disease-causing bacteria and other microorganisms. Bleaching powder releases active chlorine, which disinfects water by damaging or killing many such microbes when used in an appropriate controlled amount. Thus option A correctly explains its use in water treatment. It does not sweeten water, convert it into milk, or remove dissolved sodium chloride; those claims are unrelated to disinfection.

Plaster of Paris is calcium sulfate hemihydrate, CaSO4·1/2H2O. It reacts with water and changes back into gypsum, CaSO4·2H2O, forming a hard solid mass. This setting reaction is useful when making casts, but unwanted moisture can make the powder set during storage and spoil it before use. Hence option A is correct; it does not become acid, sugar, or burn in moisture.

Hydrochloric acid reacts with sodium carbonate in an acid–carbonate reaction. The balanced equation is 2HCl + Na2CO3 → 2NaCl + H2O + CO2. Thus sodium chloride is the salt, and water plus carbon dioxide are the other products. Carbon dioxide produces effervescence and turns limewater milky, providing a useful test. Therefore option A is correct; hydrogen, oxygen, chlorine, and ammonia are not formed in this reaction.

The governing concept is the acid–carbonate reaction. When an acid reacts with sodium hydrogen carbonate, the products are a salt, water and carbon dioxide: acid + NaHCO₃ → salt + H₂O + CO₂. The escaping carbon dioxide forms visible bubbles, producing effervescence. Hydrogen is not released, salt vapour is not formed, and freezing water cannot explain the observation. Therefore option A is correct.

The governing concept is the aqueous definition of an acid. In water, an acid increases the concentration of hydrogen ions, more accurately forming hydronium ions, H₃O⁺. This explains acidic behaviour and indicator changes. Acids are not all safe; some are corrosive. Acids turn blue litmus red, not red litmus blue. Option D is also incorrect because acids are substances, not necessarily metals. Thus option A gives the scientifically correct general statement.

The governing concept is the basic behaviour of substances in aqueous solution. A soluble base dissociates or reacts with water to increase hydroxide-ion concentration, giving OH⁻ ions in the solution. This is associated with turning red litmus blue. Option B reverses the litmus result, while C is too absolute because some bases are soluble and D describes an acid-like taste, not a property of bases. Therefore A is the most correct statement.

The governing concept is the colour change of an acid–base indicator. Methyl orange is red in an acidic solution and changes through an orange transition range toward yellow in an alkaline solution. Because the colour depends on the solution’s pH, it helps distinguish acidic and alkaline conditions during testing or titration. Option B describes its alkaline colour incompletely, while C and D give incorrect colours and interpretations. Therefore A is correct.

Litmus is an acid-base indicator. A basic solution changes red litmus paper to blue because the indicator responds to the alkaline medium. Blue litmus turning red instead indicates an acid. Limewater becoming milky is associated with carbon dioxide reacting with calcium hydroxide, while a pop sound from a burning splint tests for hydrogen gas. Thus only option A is evidence of a possible base.

Lemon juice contains citric acid, while baking soda is sodium hydrogen carbonate, NaHCO₃. An acid reacting with a hydrogen carbonate produces a salt, water, and carbon dioxide: acid + hydrogen carbonate → salt + H₂O + CO₂. The carbon dioxide escapes as visible bubbles or fizz. No metal is formed, and the observation itself proves that a reaction occurs, so option A is correct.

Litmus is a natural acid-base indicator. In an acidic medium, blue litmus changes to red because the indicator responds to the presence of acidic ions. This red colour is a characteristic test for acidity, although it does not by itself measure the exact pH or strength of the acid. A basic solution generally turns red litmus blue, while green or no change is not the standard result for blue litmus in acid. Thus A is correct.

Red litmus paper is an indicator used to test for basicity. When placed in a basic or alkaline solution, it changes from red to blue because the indicator has a different colour in an alkaline medium. Keeping it red would not show the characteristic basic response, and yellow or black are not the standard litmus colours for this test. Therefore option B correctly describes the observation and identifies the solution as basic.

At about 25 °C, the pH scale classifies aqueous solutions: pH 7 is neutral, values below 7 are acidic, and values above 7 are basic or alkaline. A pH below 7 indicates a relatively higher concentration of hydrogen ions than in a neutral solution. Therefore the solution is acidic. Option B describes pH above 7, option C corresponds approximately to pH 7, and option D is not a pH classification.

The pH scale describes the acidic or basic nature of an aqueous solution. A pH of 7 is conventionally neutral at about room temperature, while values greater than 7 indicate a lower hydrogen-ion concentration and a higher relative hydroxide-ion concentration. Therefore, the solution is basic, so option B is correct. Acidic means pH below 7, neutral means pH 7, and “salt-free” is not a pH classification.

The governing concept is the pH scale, which expresses the hydrogen-ion concentration of an aqueous solution. Under usual school-level conditions, a neutral solution has balanced acidic and basic effects and is assigned pH 7. Thus option B, seven, is correct. A pH near 1 is strongly acidic, pH 10 is basic, and pH 14 represents a very strongly basic condition rather than neutrality.