Correct Answer: B

Rationale: Viscosity is the term used to describe the resistance of a substance to flow. It quantifies how thick or thin a fluid is and how easily it flows. Density (A) refers to the mass of a substance per unit volume and does not directly relate to resistance to flow. Conductivity (C) is the property of a material to conduct electricity or heat, not related to resistance to flow. Malleability (D) is the property of a material to be hammered or rolled into thin sheets, which is not related to resistance to flow.

Correct Answer: C

Rationale: The correct answer is Acetic acid (CH3COOH) because it is a weak acid that only partially dissociates in water, resulting in a lower concentration of H+ ions compared to strong acids like hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3) which completely dissociate in water. Weak acids do not fully ionize in water, leading to a lower concentration of H+ ions in solution. Hydrochloric acid, sulfuric acid, and nitric acid are strong acids that completely dissociate in water, producing a higher concentration of H+ ions. Therefore, they are not examples of weak acids.

Correct Answer: C

Rationale: Reactivity is the property of a substance that describes its ability to undergo a chemical change and form new substances. Density (A), mass (B), and volume (D) are physical properties of a substance and do not directly relate to its ability to undergo chemical changes. Density is the measure of how much mass is contained in a given volume, mass is the amount of matter in an object, and volume is the amount of space an object occupies. Therefore, reactivity (C) is the correct answer as it specifically addresses the substance's ability to undergo chemical changes.

Correct Answer: B

Rationale: The formation of a gas is evidence of a chemical reaction occurring. In this case, when magnesium metal reacts with hydrochloric acid, hydrogen gas is produced, indicating a chemical change is taking place. The production of gas is a clear indication of a chemical reaction, as new substances are being formed. A change in color might suggest a physical change, dissolving in a liquid could be a physical or chemical change depending on the context, and no change in temperature does not necessarily indicate a chemical reaction, as some reactions are endothermic or release small amounts of heat that may not be easily noticeable.

Correct Answer: D

Rationale: An atom is the smallest unit of matter that retains the chemical properties of an element. It consists of a nucleus containing protons and neutrons, surrounded by electrons that orbit the nucleus. The number of protons in the nucleus defines the atomic number of the element, and the number of neutrons determines the isotope of the element. While electrons, protons, and neutrons are fundamental particles, an atom as a whole is the smallest part of an element that maintains its unique identity. Electrons are negatively charged particles orbiting the nucleus, protons are positively charged particles in the nucleus, and neutrons are neutral particles in the nucleus. Therefore, the correct answer is 'D: Atom.'