Acids, Bases, and Salts

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, so option A gives the correct name and formula description. Bleaching powder is not sodium chloride; it is commonly represented as calcium oxychloride. Plaster of Paris is calcium sulphate hemihydrate, not calcium carbonate. Baking soda is sodium hydrogen carbonate, not calcium sulphate. Therefore only option A is correct.

An active metal reacting with a dilute acid generally produces hydrogen gas and a salt: metal + acid → salt + H₂. Hydrogen is combustible and, when a small sample is brought carefully near a burning splint, it burns with a characteristic ‘pop’ sound. Limewater turning milky identifies carbon dioxide, while turmeric and phenolphthalein are indicators rather than confirmation tests for hydrogen. Hence option B is correct.

When hydrogen ions and hydroxide ions are present in nearly equal effective amounts, their acid and base effects balance, giving an approximately neutral solution. Neutral water has pH close to 7. Universal indicator shows green around pH 7. Red and orange indicate acidic conditions, whereas violet indicates a strongly alkaline condition. Therefore the expected colour is green, making option B correct.

Limewater is a dilute aqueous solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is bubbled through it, the reaction is Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is insoluble in water and forms a suspended white precipitate, producing the characteristic milkiness. Calcium hydrogen carbonate is soluble, while the sodium salts listed do not result from this test. Thus option B is correct.

The governing concept is laboratory safety: an unknown acid must never be tasted because even a small amount may burn tissues, poison the body, or cause other harm. Taste also cannot reliably identify an acid or measure its pH. Instead, a teacher-approved indicator such as litmus, universal indicator, or pH paper should be used. Therefore option A is correct; B, C, and D make scientifically unsafe or false claims.

The governing concept is the relation between pH and corrosive hazard. A pH of 13 indicates a strongly basic solution, with a very low hydrogen-ion concentration and substantial hydroxide activity. Such a solution may irritate or burn skin and seriously damage eyes. Gloves, goggles, careful labelling, and approved handling procedures are therefore required. Option A is correct; tasting, direct contact, and beverage storage are unsafe.

The governing concept is the pH colour scale of a universal indicator. Blue generally represents a basic solution, while green is near neutral, around pH 7. When acid is added, hydrogen-ion concentration increases and the basic character is reduced. Thus the solution moves from basic toward neutral, making option A correct. Option B predicts the opposite change, and C would require a strongly acidic colour rather than green.

The governing reaction pattern is acid plus carbonate producing salt, water and carbon dioxide: carbonate + acid → salt + H₂O + CO₂. For example, sodium carbonate reacting with hydrochloric acid releases CO₂, not hydrogen. The gas can turn limewater milky, providing a useful confirmation. Hydrogen is associated with many acid–active-metal reactions, so option A correctly fixes the student’s misconception; B, C and D do not fit the reaction.

Colour alone is not a reliable chemical test because many common acids and bases, such as dilute hydrochloric acid and sodium hydroxide solution, are colourless. Their nature must be tested safely with an indicator, pH paper, or a calibrated pH meter. Litmus gives an acid–base indication, while a universal indicator gives an approximate pH range. Therefore A is correct; B and C make false universal claims, and D overstates what appearance can show.

Dilution of a concentrated acid is highly exothermic because hydration of its ions releases considerable heat. Adding acid slowly to a larger quantity of water allows the heat to disperse gradually and reduces the chance of local boiling, splashing, and burns. Therefore option A is correct. The acid does dissolve in water, is not made solid, and dilution does not itself neutralise it.

Tooth enamel is mainly a calcium-containing mineral that is vulnerable to acidic conditions. Oral bacteria can convert sugars into acids, lowering the pH at the tooth surface. When the pH falls below approximately 5.5, demineralisation of enamel becomes more likely and tooth decay risk increases. Therefore option A is correct. Values above neutral or strongly alkaline values do not represent this usual acid-dissolution threshold.

Indicators change colour in characteristic media. Turmeric turns reddish-brown in the presence of a base, and phenolphthalein becomes pink in a basic solution, generally when the pH is above its transition range. Since both independent observations indicate alkalinity, the solution is basic. Option B is correct. An acidic solution would not produce this combination of colours, and neutral or metallic are not suitable descriptions of the solution’s acid–base nature.

When hydrogen-ion and hydroxide-ion effects are approximately balanced, the solution is nearly neutral. Its pH is close to 7 because neither acidic nor basic character predominates. Universal indicator displays green around neutral pH. Red and orange indicate acidic conditions, whereas violet indicates a strongly basic condition. Therefore option A, green, is the correct answer.

The governing concept is laboratory safety and the use of chemical indicators. Many acids are corrosive or toxic, so tasting even a small amount may burn tissues or cause poisoning. Litmus paper, universal indicator, or a pH strip can test acidic character without direct contact. Therefore option A is correct; B is false because acids may have tastes, while C and D incorrectly treat taste or indicators as unsuitable tests.

The governing concept is the relationship between pH and alkalinity, together with laboratory safety. A pH of 13 indicates a strongly basic solution with a high hydroxide-ion concentration; it may irritate or damage skin and eyes. Gloves and goggles reduce exposure and protect these tissues, so option A is correct. Tasting, direct touching, and storing it in a drink container are unsafe practices, not precautions.

The governing concept is acid-base identification followed by neutralisation. A solution that turns red litmus blue is basic, while one that turns blue litmus red is acidic. When suitable amounts react completely, hydrogen ions combine with hydroxide ions to form water, and the remaining ions form a salt. Thus option A is correct; the other choices are not general products of neutralisation.

Litmus is an acid–base indicator with a characteristic colour change. Blue litmus turns red in an acidic medium because the indicator responds to the presence of excess hydrogen ions. A basic solution generally turns red litmus blue, while a neutral solution does not produce this acidic change. Therefore the solution described is acidic, making option A correct; “salty” describes neither a definite indicator response nor a guaranteed pH.

The governing indicator rule is that a basic medium changes red litmus to blue. Bases produce hydroxide ions in aqueous solution, and the litmus dye responds with this characteristic colour change. Acids show the opposite effect on blue litmus, while a neutral solution normally does not turn red litmus blue. Hence option B is correct. The terms acidic, neutral, and oily do not explain the observed indicator response.

The pH scale expresses the acidic or basic nature of an aqueous solution. At approximately 25°C, pH 7 represents neutrality, where hydrogen-ion and hydroxide-ion concentrations are equal. Values below 7 are acidic and values above 7 are basic. Therefore option C, neutral, is correct. The answer assumes the usual school-level aqueous pH scale and conditions; pH 7 is not highly acidic or basic.

The pH scale indicates the acidic or basic nature of an aqueous solution. A pH of 7 is neutral, values below 7 are acidic, and values above 7 are basic. Since pH 3 is below 7, the solution contains a relatively higher concentration of hydrogen ions and is therefore acidic. It is not neutral or basic; sweetness is unrelated to pH.

The pH scale classifies aqueous solutions: pH 7 is neutral, a pH below 7 indicates acidity, and a pH above 7 indicates basicity. Because the given pH is 11, it is clearly greater than 7, so the solution is basic and has a comparatively greater hydroxide-ion concentration. It cannot be acidic or neutral, while metallic describes a material rather than its pH nature.

An active metal can displace hydrogen from a dilute acid. The general reaction is: metal + acid → salt + hydrogen gas; for example, Zn + 2HCl → ZnCl₂ + H₂. Therefore hydrogen is released, making option A correct. Oxygen and nitrogen are not produced in this ordinary acid-metal reaction, while carbon dioxide is characteristic of an acid reacting with a carbonate or bicarbonate.

Hydrogen is a combustible gas. When a sample of hydrogen is brought near a burning splint, it combines rapidly with oxygen and burns, producing the characteristic ‘pop’ sound. Thus option B is the identification test. Limewater turning milky indicates carbon dioxide, turmeric changing colour suggests a basic medium, and blue litmus remaining blue is not a specific hydrogen test.

Acids react with carbonates in a characteristic reaction that produces a salt, water, and carbon dioxide. In general, acid + carbonate → salt + water + CO₂; for example, 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂. Therefore option B is correct. Hydrogen is associated with many acid-metal reactions, whereas oxygen and chlorine are not products of this reaction.

Carbon dioxide is identified by passing it through limewater, an aqueous solution of calcium hydroxide. The gas reacts to form insoluble calcium carbonate, making the limewater appear milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Hence option B is correct. A pop sound identifies hydrogen, while litmus or unchanged turmeric does not specifically confirm carbon dioxide.