Acids, Bases, and Salts

The governing concept is the identification of bases by indicators and the pH scale. A base changes red litmus to blue, while a neutral substance generally leaves it unchanged. Also, pH 10 is greater than 7, so the solution has alkaline character. Both observations lead to option C. Option A contradicts both tests, option B would fit pH 7, and the test gives no evidence that the sample is a gas.

Mouth bacteria break down sugars and produce acids. As the pH decreases, the acidic environment begins to dissolve the mineral component of tooth enamel. Tooth decay becomes rapid when the pH falls below about 5.5, making option A correct. A pH of 7 is neutral, while 8.5 and 14 are alkaline values and do not represent the acidic condition that attacks enamel. Regular cleaning helps limit this process.

The pH scale indicates acidity or basicity: 7 is approximately neutral, values below 7 are acidic, and values above 7 are basic. A pH close to 14 indicates a very high concentration of hydroxide ions and therefore a strongly alkaline solution. Such substances may be caustic and can damage skin or eyes. Option A is correct; the other choices conflict with the meaning of the pH value.

The governing concept is that acids commonly occur in several natural food substances. Lemon juice contains citric acid, vinegar contains ethanoic acid, and tamarind contains organic acids such as tartaric acid. Therefore, option A is the only group in which all listed substances are acidic. The other groups contain soaps, hydroxides, carbonates, or antacids, which are basic or alkaline rather than acidic.

The governing concept is the identification of bases by their alkaline nature. Soap solution is generally basic, lime water contains calcium hydroxide and is basic, and sodium hydroxide is a strong base. Thus option A contains only basic substances. The substances in options B and C are acidic foods or liquids, while option D contains strong mineral acids and lemon juice, so those groups cannot be correct.

The governing reaction is: acid + reactive metal → salt + hydrogen gas. For example, zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen. Hydrogen is combustible and produces the characteristic ‘pop’ when tested with a burning splint, so option A is correct. Carbon dioxide generally extinguishes a flame, while chlorine and ammonia do not give this standard pop test.

Plaster of Paris is calcium sulphate hemihydrate, written as CaSO₄·½H₂O. When water is added, it combines with the required water and changes into calcium sulphate dihydrate, CaSO₄·2H₂O, which is gypsum. The formation of the harder solid explains its use for casts and moulds. Therefore option A is correct. Washing soda, baking soda and bleaching powder are different salts and are not produced in this hydration reaction.

Marble consists mainly of calcium carbonate, CaCO₃. Acids present in acid rain react with carbonates to form a salt, water and carbon dioxide; in general, acid + carbonate → salt + H₂O + CO₂. Repeated exposure can dissolve or weaken the marble surface and damage carved details. Therefore option A correctly states the governing reaction. The other options either claim an impossible conversion or deny the chemical reaction involved.

Bacteria in the mouth can break down sugars and produce acids. These acids lower the local pH and may gradually demineralise tooth enamel, causing tooth decay. A mildly basic toothpaste can react with and neutralise some of the excess acid, helping protect the enamel. Thus option A expresses the correct acid-base principle. Toothpaste does not dissolve enamel immediately, convert sugar into acid, or make the mouth’s pH zero; those statements are chemically incorrect.

Turning blue litmus red identifies the first substance as an acid. The characteristic neutralisation reaction is acid + base → salt + water. Since the first substance is already identified as an acid and the products are salt and water, the other reacting substance must be a base, or alkaline substance. Therefore option A is correct. Another acid would not normally give this neutralisation pattern, while a neutral gas or an unreactive metal does not fit the stated evidence.

Sodium hydroxide is strongly hygroscopic and also deliquescent: it attracts water vapour from the atmosphere and can dissolve in the absorbed moisture. When its pellets are left exposed, they therefore become moist, sticky or apparently wet. This behaviour is a physical absorption process, not the creation of water or conversion into oxygen. Sodium hydroxide pellets do not normally release chlorine merely because they are exposed to air. Hence option A is correct.

The governing concept is the pH scale and the indicator behaviour of bases. A soapy feel suggests that the solution is alkaline, while turning red litmus blue is a direct test for a base. Basic solutions have a hydrogen-ion concentration lower than that of neutral water, so their pH is greater than 7. Option B describes an acidic solution, and C and D are fixed values that do not follow from the observations. Therefore, option A is correct.

The governing concept is the colour change of a natural acid-base indicator. Turmeric contains curcumin, which remains yellow in its ordinary form but changes to a reddish-brown colour in a basic medium. Therefore turmeric paper is especially useful for detecting alkaline solutions. Neutral water generally causes no characteristic change, and the test is not a universal test for gases or metals. The observation identifies basicity rather than a particular substance, so option A is correct.

The governing concept is indicator colour change with pH. Methyl orange is a synthetic acid-base indicator that appears red in an acidic medium, changes through orange in its transition range, and appears yellow in a sufficiently basic medium. Because the question specifies an acidic solution, the correct observation is red. Yellow corresponds to the basic side, while blue and pink are not the standard methyl-orange colours for this test. Therefore option A is correct.

The governing concept is neutralisation, in which an acid reacts with a base to form salt and water, often with heat release. Excess hydrochloric acid in the stomach causes acidity, while a mild basic antacid reacts with and reduces that acid. Dissolving sugar is not an acid-base reaction, and drying cloth or melting ice are physical changes. Therefore using an antacid for acidity is the valid daily-life example, so option A is correct.

The governing principle is laboratory safety and objective testing. Some acids are corrosive, poisonous, or harmful to body tissues, so tasting an unknown substance can cause injury and is never an acceptable identification method. Indicators, pH paper, and controlled chemical tests provide safer evidence of acidic behaviour. Acids can have tastes, so B is false; taste cannot measure pH reliably, and indicators are not limited to metals. Hence A is correct.

The governing pattern is neutralisation: an acid reacting with a base produces salt and water. Metal oxides such as copper(II) oxide or magnesium oxide consume acid and form corresponding salts, for example CuO + 2HCl → CuCl₂ + H₂O. Since the oxide neutralises H⁺ ions, it behaves as a basic oxide. An acidic oxide would generally react with a base instead, and the other choices do not describe this chemical behaviour. Therefore option A is correct.

Blue litmus turning red is the characteristic test for an acidic medium. If a non-metal oxide dissolves in water and produces such a solution, the oxide has formed an acid, for example SO₂ + H₂O → H₂SO₃ or CO₂ + H₂O ⇌ H₂CO₃. This is why many non-metal oxides are classified as acidic oxides. Basic oxides would turn red litmus blue, while a neutral salt or metallic base does not fit the observation. Thus A is correct.

Indicator colour ranges provide the governing evidence. Methyl orange is yellow in a neutral or alkaline medium and red in a sufficiently acidic medium. Phenolphthalein is pink in an alkaline medium and colourless in acidic or near-neutral conditions. Seeing both yellow methyl orange and pink phenolphthalein therefore indicates that the solution is basic, sufficiently above the phenolphthalein transition range. An acidic solution would make methyl orange red and phenolphthalein colourless. Hence option A is correct.

The governing concept is chemical laboratory safety. Acids and bases can cause burns, damage materials, and react dangerously if they are mixed or mistaken for another substance. Separate, compatible containers with tight closures and clear labels identify the contents and reduce accidental contact or mixing. Therefore option A is correct. Mixing them may cause heat or splashing, a drinking bottle invites accidental ingestion, and removing labels removes essential safety information.

The governing concept is neutralisation and soil pH management. Acidic soil has a pH below the preferred range for some plants. Slaked lime, Ca(OH)2, is a basic substance; when applied in a suitable measured quantity, it neutralises some soil acidity and raises the pH. Hence option A is correct. Vinegar, lemon juice, and dilute acids are acidic, so adding them would generally increase rather than reduce soil acidity.

The governing concept is gas evolution and expansion in a food mixture. Sodium hydrogen carbonate reacts with an acid to release carbon dioxide, for example NaHCO3 + H+ → Na+ + H2O + CO2. During heating, the carbon dioxide forms and expands as many small bubbles trapped in the batter or dough. These bubbles increase the volume and create a lighter, softer texture, so option A is correct.

The governing concept is neutrality in an aqueous solution. Hydrogen ions are associated with acidity, while hydroxide ions are associated with basicity. When their effects are nearly balanced, neither acidic nor basic character dominates, so the solution is approximately neutral. At ordinary conditions, a neutral aqueous solution has pH about 7; therefore option A is correct. pH 0 and pH 2 indicate acidity, while pH 14 indicates strong basicity rather than neutrality.

The governing concept is thermal decomposition of sodium hydrogen carbonate, commonly called baking soda. On heating, the balanced change is 2NaHCO3 → Na2CO3 + CO2 + H2O. Carbon dioxide and water are released, while sodium carbonate remains as the solid product. Thus option C is correct; the calcium compounds and sodium chloride are not products of this decomposition.

The governing concept is neutralisation: an acid reacts with a suitable base to reduce acidity and form salt and water. Ant venom is commonly described at this school level as acidic, so a mild baking soda solution, which is weakly basic, can neutralise part of it and provide relief. Option C is therefore suitable. Vinegar, lemon juice, and dilute acid are acidic and would not neutralise the sting.