Acids, Bases, and Salts

The governing concept is acid-base neutralisation. A pH of 9 indicates that the original solution is basic. Adding acid supplies hydrogen ions, which react with hydroxide ions or other basic species and reduce the basicity. When the pH reaches 7, the solution is approximately neutral under the stated conditions. Therefore option C is correct; displacement, combination, and decomposition are not identified by this pH change.

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.

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.

When hydrogen-ion concentration is much greater than hydroxide-ion concentration, the solution is acidic and its pH is below 7. Universal indicator shows red at very low pH and orange in a moderately strong acidic range, so red or orange is the most likely colour. Blue and violet indicate alkaline conditions, while green is near neutral. The indicator is not necessarily colourless. Therefore A is correct.

Laboratory safety requires that chemicals must never be tasted because even a small quantity of an acid may be corrosive, poisonous, or contaminated. The correct approach is to use an indicator, pH paper, or an instrument under proper supervision. Therefore option A is correct. Acids may have tastes, but taste cannot safely or accurately identify them; indicators can be used for both acidic and basic solutions.

Sodium hydrogen carbonate, NaHCO3, is the chemical name of baking soda, so option A is correctly matched. Plaster of Paris is calcium sulphate hemihydrate, not calcium carbonate; washing soda is sodium carbonate decahydrate, not calcium sulphate hemihydrate; and bleaching powder is mainly calcium oxychloride rather than sodium chloride. Thus only the first pair is correct.

The governing safety principle is immediate dilution and removal of the corrosive substance. The affected skin should be washed with a large amount of running water as soon as possible, which reduces the acid concentration and carries it away. Hence option A is correct. Applying a strong base is unsafe because neutralisation may release heat and cause another chemical injury. Rubbing can spread or damage the skin, and waiting allows the acid to penetrate further; medical help should be sought when needed.

The governing concept is neutralisation: an acid reacts with a base to reduce its acidic properties, usually forming salt and water. If the irritating material from an ant sting is acidic, a suitable mild base can react with some of it and reduce the acidity at the skin surface, giving relief. Therefore option A is correct. Options B and D contradict the acid–base reaction, while C is scientifically impossible. Only a safe, mild substance should be used, not a concentrated chemical.

The governing concept is the colour response of acid–base indicators. Turmeric changes to reddish-brown in a basic medium, and phenolphthalein is colourless in acidic or neutral conditions but turns pink in a basic solution. Since both independent observations indicate alkalinity, option B is correct. An acidic solution would not produce this pair of changes, neutrality would keep phenolphthalein colourless, and “gaseous” describes a state of matter rather than the chemical nature shown by the indicators.

Methyl orange is a synthetic acid-base indicator with a transition range in which it appears red in an acidic medium and yellow in a basic or sufficiently alkaline medium. Therefore option B matches both observations. Phenolphthalein is colourless in acid and pink in base; blue litmus changes to red in acid but does not become yellow in base; turmeric usually changes from yellow to reddish-brown in a base.

The governing laboratory-safety principle is never to taste an unknown chemical. Acids and bases may be corrosive or poisonous, and even a small amount can injure the mouth or internal organs. Identification should instead use safe tests such as litmus, a universal indicator, or pH paper, following proper instructions. Thus option A is correct; taste cannot provide an exact pH and indicators are specifically designed for this purpose.

Washing soda is sodium carbonate decahydrate, with the formula Na₂CO₃·10H₂O; hence option A gives a correct common-name and chemical-name pair. Baking soda is sodium hydrogen carbonate, bleaching powder is not sodium chloride, and Plaster of Paris is calcium sulphate hemihydrate rather than calcium carbonate. These comparisons eliminate B, C, and D and confirm A unambiguously.

A slippery or soapy feel is a common physical indication of a basic solution, although it should not be tested by touching unknown chemicals in practice. Turmeric is a natural indicator: it remains yellow in acidic or neutral conditions but turns reddish-brown in the presence of a base. Both observations point to alkalinity, so option B is correct. Acidic and neutral solutions do not produce this combined response, and being salt-free is not a measure of acid-base nature.

An active metal reacting with a dilute acid commonly produces a salt and hydrogen gas, for example Zn + 2HCl → ZnCl₂ + H₂. Hydrogen is combustible and, when brought near a burning splint, burns rapidly with the characteristic ‘pop’ sound. Therefore option A is the correct conclusion. Carbon dioxide extinguishes a flame, while chlorine and nitrogen do not give this standard pop test for hydrogen.

The pH scale describes the acidic or basic character of an aqueous solution. At approximately 25°C, pH 7 represents neutrality, where hydrogen-ion and hydroxide-ion concentrations are approximately equal. Universal indicator normally appears green near neutral pH, so both pieces of evidence support option B. Highly acidic solutions have low pH and red or orange colours, whereas highly basic solutions have high pH and blue or violet colours; metallic is not an acid-base classification.

The governing principle is laboratory safety: no chemical should be tasted because even a small quantity of an acid may burn tissues or be poisonous. Acids also differ in concentration, so taste cannot reliably identify their strength or exact pH. Instead, litmus, universal indicator, or pH paper can test acidity without direct contact. Therefore option A is correct; B is false because acids may have tastes, C confuses taste with measurement, and D wrongly limits indicators to metals.

On the usual aqueous pH scale, values near 14 indicate a very high concentration of hydroxide ions and strong basic character. Such a solution may be caustic, meaning it can damage skin, eyes, or other tissues. The pH value alone does not make a substance pure, edible, or acidic, so careful handling with goggles, gloves, and proper laboratory practice is required. Therefore A is correct; B, C, and D contradict the meaning of high pH.

The governing reaction pattern is acid plus carbonate, which produces a salt, water, and carbon dioxide: carbonate + acid → salt + water + CO₂. Hydrogen is commonly released when a suitable active metal reacts with a dilute acid, not when a carbonate reacts. Carbon dioxide can be confirmed because it turns limewater milky. Thus option A correctly replaces the student’s statement; chlorine and oxygen are not products of this reaction, and option D wrongly denies gas evolution.

Both observations identify the test solution as acidic: blue litmus turns red in acid, and methyl orange is red in a sufficiently acidic medium. Neutralisation requires an acid to react with a base, forming salt and water. Sodium hydroxide solution is a base and supplies hydroxide ions that combine with hydrogen ions. Lemon juice, vinegar, and hydrochloric acid are acidic, so they would not appropriately neutralise this solution. Therefore option A is correct.

The governing idea is that the colour of a substance alone is not a reliable chemical test. Many acids and bases are colourless, and the colours of solutions can vary for reasons unrelated to acidity or basicity. Litmus, universal indicator, or pH paper gives an observable response linked to hydrogen-ion concentration and pH. Therefore option A is the scientifically safe and exam-correct answer. Options B and C make unjustified generalisations, while D rejects a useful measurement.

Indicator colour ranges provide the governing evidence. Phenolphthalein is colourless in acidic or neutral conditions but becomes pink in a basic solution. Methyl orange is red in acidic medium and yellow in neutral to alkaline medium. Since both observed colours are consistent with a basic medium, the solution is classified as basic. A neutral solution would not make phenolphthalein pink, so option B is the unambiguous answer.

Both observations indicate a basic medium. Turmeric changes to a reddish-brown colour in the presence of a base, and red litmus turns blue when the solution is basic. In the Arrhenius description, basicity in aqueous solution is associated with an excess of hydroxide ions, OH⁻, relative to hydrogen ions. Chloride and sulphate ions may be present in salts but do not alone explain these indicator changes. Therefore option B is correct.

A pH of 4.5 indicates that the soil is distinctly acidic. Slaked lime, calcium hydroxide, is a basic substance that can neutralise some of the excess acidity and raise the soil pH when applied in a suitable, tested amount. Therefore option A is correct. Vinegar, lemon juice, and dilute acid would add acidity and make the soil condition worse rather than correct it.

The governing concept is the indicator test for an acidic solution. Excess hydrogen ions in water indicate that the solution is acidic, meaning its pH is below 7. Blue litmus changes to red in an acidic medium, so option A follows directly. Blue litmus does not normally become pink or white in this test, while remaining blue would indicate a neutral or basic condition rather than excess hydrogen ions.

The governing concept is safe laboratory identification of acids and bases. Unknown laboratory chemicals must never be tasted because even a small amount may be corrosive, poisonous, or otherwise harmful. Litmus, universal indicator, or pH paper can provide evidence without direct contact; a pH value also requires suitable measurement conditions. Therefore option A is the correct exam response. The remaining choices promote unsafe practice or make false claims about indicators and taste.