Acids, Bases, and Salts

The name describes two important products of brine electrolysis. Chlorine gas is released at the anode, while sodium hydroxide, which is an alkali, is obtained in the solution; hydrogen is also produced at the cathode. Therefore, option A is correct because “chlor” refers to chlorine and “alkali” refers to sodium hydroxide. The other options incorrectly say that only salt, only water, or no chemical product is formed.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, whereas baking soda is sodium hydrogen carbonate, NaHCO₃. The hydrate contains ten molecules of crystallisation water, while the hydrogen carbonate is a different salt with different uses. Therefore, option B is correct. Option A reverses their identities, and options C and D name unrelated salts: sodium chloride and calcium sulphate.

Anhydrous copper sulphate, CuSO₄, is white because it contains no water of crystallisation. When water is added, hydrated copper sulphate, commonly represented as CuSO₄·5H₂O, forms and the solid becomes blue. This characteristic colour change is therefore used as a simple test for water, making option A correct. Chlorine, hydrogen, and nitrogen do not cause this specific white-to-blue change in anhydrous copper sulphate.

Acidic soil contains excess acidic substances that can harm plant growth. Lime, commonly calcium oxide, calcium hydroxide, or related carbonate material, is basic and reacts with the acids in the soil. This acid–base reaction reduces acidity and produces salt and water or related neutral products; the process is called neutralisation. Evaporation, condensation, and magnetisation do not involve an acid reacting with a base. Therefore, option A is correct.

The governing concept is the common property of acids: many naturally occurring acids have a sour taste, although substances must never be tasted in a laboratory. Lemon juice contains citric acid, so option B is the suitable answer. Soap solution, lime water, and sodium hydroxide solution are basic rather than acidic; their identification should be made with indicators and safe laboratory procedures.

The governing concept is the characteristic behaviour of bases. Aqueous bases may feel slippery or soapy because they interact with oils on the skin, but direct contact is unsafe. Sodium hydroxide solution is a strong base, so option C is correct. Vinegar, tamarind juice, and lemon juice contain acids and are therefore plausible acidic distractors, not the expected basic substance.

The governing concept is that indicators are substances used to test the acidic or basic nature of a medium. Litmus and several other indicators show colour changes, while olfactory indicators show changes in smell. Thus option A gives the most complete correct statement. Indicators do not identify only metals, neutralise every solution, or cause a gas-to-solid conversion, so B, C, and D are false.

The governing concept is laboratory safety. Acids and bases can irritate or damage body tissues, and concentrated substances may be corrosive or poisonous. Therefore directly tasting them is unsafe, making option A correct. The other choices are scientifically wrong: substances can have tastes, acids are not all sweet, and bases are not all sour. Indicators should be used instead.

The governing concept is dilution. Adding water increases the total volume of an acidic solution while the amount of acid already present does not increase. Consequently, the concentration, or number of hydrogen ions per unit volume, decreases. The solution becomes less acidic and its pH generally rises toward 7, although it does not necessarily become neutral. Thus option A is correct; the other choices contradict dilution or describe impossible changes.

The governing concept is dilution of a base. When water is added, the total volume increases, but no extra hydroxide-producing solute is supplied. Therefore, the number of hydroxide ions per unit volume decreases. The solution becomes less basic, and its pH generally moves toward 7 rather than instantly becoming acidic. Option B correctly states the concentration change; the other options claim an increase or impossible chemical transformations.

The governing concept is the colour scale of a universal indicator. It displays different colours over a range of pH values: acidic solutions are generally red to orange, strongly basic solutions are blue to violet, and green is associated with a pH close to 7. Thus the observed green colour indicates that the solution is approximately neutral. It does not prove that the solution is metallic or strongly acidic/basic, so option B is correct.

The governing concept is the ionisation of acids in aqueous solution. Hydrogen chloride, HCl, dissolves in water and forms hydronium ions, represented at school level as hydrogen ions: HCl + H₂O → H₃O⁺ + Cl⁻. These aqueous hydrogen or hydronium ions are responsible for acidic properties. Sodium hydroxide supplies hydroxide ions, while calcium oxide and sodium carbonate produce basic or alkaline solutions in water. Therefore option B is correct.

The governing concept is the aqueous behaviour of bases. A base produces hydroxide ions in water, either directly, as sodium hydroxide dissociates into Na⁺ and OH⁻, or through reaction with water. These ions give the solution basic properties; its pH is generally greater than 7. Acids provide hydrogen or hydronium ions, bases turn red litmus blue rather than blue litmus red, and a pH below 7 indicates acidity. Therefore option C is correct.

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.

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.

The governing concept is the litmus indicator test. An acidic solution changes blue litmus to red because hydrogen ions interact with the indicator. Red litmus does not show a further colour change because it already has the acid-side colour. A neutral solution changes neither strip, and a base would turn red litmus blue. Therefore, option C is correct.

The governing concept is the colour response of litmus in basic media. A base changes red litmus to blue because hydroxide ions create alkaline conditions. Blue litmus remains blue in the same solution, so the two observations agree. An acid would produce the opposite change, while a neutral solution would change neither strip. Thus, option D, basic, is correct.

An acid produces hydrogen ions, or hydronium ions in water, and commonly has a sour character. It changes blue litmus paper to red because the indicator responds to the acidic medium. Thus option A is correct. A base generally turns red litmus blue, a metal is a broad class of element rather than the indicated solution category, and a neutral gas does not explain both observations. In practice, chemicals must never be tasted; the statement describes properties only.

A basic solution can feel slippery or soapy because alkaline substances react with oils on the skin, although an unknown chemical must never be touched directly. The decisive test here is that a base changes red litmus to blue. Therefore, option B is correct. Acids generally turn blue litmus red, neutral solutions show no such characteristic change, and carbonate gas is not a suitable category for a solution displaying this indicator response.

The governing concept is the characteristic reaction of acids with carbonates. A pH below 7 indicates that the solution is acidic, meaning it has a higher hydrogen-ion concentration than a neutral solution. Acids react with metal carbonates to form a salt, water and carbon dioxide gas, causing effervescence. Therefore option A is correct; a base or neutral solution does not give this combined evidence, and solid salt is not a solution.