What color does phenolphthalein turn in the presence of an acid?

Correct Answer: C

Rationale: In the presence of an acid, phenolphthalein turns pink. Phenolphthalein is a pH indicator that is colorless in acidic solutions but turns pink in basic solutions. Therefore, when added to an acidic solution, phenolphthalein will exhibit a pink coloration. Choice A, 'Clear,' is incorrect because phenolphthalein does not remain colorless in the presence of an acid. Choice B, 'Blue,' is incorrect as phenolphthalein does not turn blue in the presence of an acid. Choice D, 'Red,' is incorrect as phenolphthalein does not exhibit a red color in acidic solutions.

Correct Answer: B

Rationale: The compound CH₃-CH₂-CH₂-CH₃ is named butane. Butane is a straight-chain alkane comprising four carbon atoms connected by single bonds. The prefix 'but-' denotes the presence of four carbon atoms, while the suffix '-ane' indicates it is an alkane with single bonds between the carbon atoms. Choice A, Cyclobutane, is incorrect as it refers to a cyclic hydrocarbon with four carbon atoms in a ring structure. Choice C, Butene, is incorrect because it is an alkene with a double bond between two carbon atoms, not a saturated hydrocarbon like butane. Choice D, Butyne, is also incorrect as it is an alkyne with a triple bond between two carbon atoms, unlike the single bonds in butane.

Correct Answer: C

Rationale: Solubility generally tends to increase with temperature for most solid solutes in liquid solvents due to higher kinetic energy leading to better solute-solvent interactions. As the temperature increases from 0°C to 20°C, all four solubility figures are expected to increase. Choice A is incorrect because solubility tends to increase with temperature. Choice B is incorrect as well for the same reason. Choice D is incorrect because the solubility of solid solutes typically increases with temperature.

Correct Answer: C

Rationale: The molar mass of sodium chloride (NaCl) is calculated by adding the atomic masses of sodium (Na) and chlorine (Cl). The atomic mass of sodium is approximately 23 g/mol and chlorine is approximately 35.5 g/mol. Adding these two atomic masses gives us a molar mass of approximately 58 g/mol for sodium chloride (NaCl). Therefore, the correct answer is C, 58 g/mol. Choice A (36 g/mol) is incorrect as it does not account for the individual atomic masses of sodium and chlorine. Choice B (43 g/mol) and choice D (72 g/mol) are also incorrect as they do not reflect the accurate molar mass of sodium chloride.

Correct Answer: B

Rationale: 0 Kelvin, also known as absolute zero, is equal to -273°C. This is the point at which all molecular motion stops, making it the lowest possible temperature on the Kelvin scale. Choice A (-300°C) is incorrect as it is not the correct conversion of 0 K to °C. Choice C (-250°C) and Choice D (-200°C) are also incorrect as they do not correspond to the accurate conversion of 0 K to °C.

Correct Answer: C

Rationale: To balance the chemical equation Fe + O₂ → FeO, the coefficients needed are 2 for Fe and 1 for O. Therefore, the balanced equation becomes 2Fe + O₂ → 2FeO, which translates into 2Fe + 3O₂ → 2FeO. This corresponds to option C. Choice A has the incorrect number of oxygen molecules. Choice B has an incorrect number of Fe atoms on the product side. Choice D also has an incorrect number of Fe atoms on the product side.

Correct Answer: B

Rationale: The correct answer is 100°C. The boiling point of water in Celsius is 100°C, which is standard at sea level. This is the temperature at which water changes from a liquid to a gas phase under standard atmospheric pressure. Choice A (90°C), Choice C (95°C), and Choice D (80°C) are incorrect because they do not represent the standard boiling point of water at sea level.

Correct Answer: C

Rationale: To decay from 56 kg to 7 kg, the substance needs to go through 3 half-lives (56 kg · 2 · 2 · 2 = 7 kg). Since each half-life is 12 days, the total time required is 12 days per half-life x 3 half-lives = 36 days. Choice A is incorrect because it does not consider the concept of half-lives. Choice B is incorrect because it represents the duration of a single half-life, not the total time required for the decay. Choice D is incorrect as it does not account for the multiple half-lives needed for the substance to decay from 56 kg to 7 kg.

Correct Answer: B

Rationale: The correct answer is 'Acid.' An acid is a compound that donates protons (H+), is corrosive to metals, and turns blue litmus paper red. When an acid is mixed with a base, they react to form salts and water, resulting in a decrease in acidity. Choices A, C, and D are incorrect because bases accept protons rather than donate them, salts are the products of acid-base reactions, and hydroxides are typically bases, not acids.