Which of the following statements about a radioactive substance is/are correct? l. lt emits radiation…

Analyze the question: We need to decide which of the three statements about radioactive substances are true.

Concept: Radioactivity is the spontaneous disintegration of unstable nuclei, giving off alpha, beta and gamma radiation. It is a nuclear process, not a chemical one.

Checking each statement:

• I. It emits radiation continuously and spontaneously. TRUE. Unstable nuclei decay on their own, without any outside trigger, and keep doing so until they become stable.

• II. The emitted radiations are affected by temperature and pressure. FALSE. Because the process happens in the nucleus, it is not affected by temperature, pressure, or chemical combination. Chemical reactions depend on outer electrons, but radioactive decay does not. (This is why half-life is constant.)

• III. The radiation can penetrate opaque matter. TRUE. Radiation, especially gamma rays and, to a lesser extent, beta particles, can pass through opaque materials such as paper, wood, and thin metal sheets. That is why photographic plates wrapped in black paper still get fogged by radioactive substances.

Conclusion: Statements I and III are correct, and II is wrong.

Answer: C. I and III only

Tip: Remember "nuclear, not chemical." Anything that affects chemical reactions (temperature, pressure, catalysts) has no effect on radioactive decay. If a statement says otherwise, it is almost always false.