In novels, TV shows, and legends, there are often scenes where ancient people used silver chopsticks to test whether food was poisoned. If the surface of the silver chopsticks turned gray or black after touching the food, it meant the food was poisoned. But does this method really work? Silverware will turn gray or black over time. Originally, in ancient times, the most common poison was arsenic trioxide. Due to the low level of production technology in ancient times, arsenic trioxide often contained sulfur. Silver is a very stable metal and usually does not react with other substances, but it is particularly sensitive to sulfur. As long as it comes into contact with sulfur-containing substances, its surface will turn gray or black. Therefore, ancient people often used this method to test for poison and it was widely spread. The reason why silver turns gray or black when it comes into contact with sulfur is that silver reacts with sulfur to form black silver sulfide, which is very stable. Therefore, in ancient cases involving poison, the silver needle test for poison often became the key to solving the case.
As for why silver sulfide is stable, it can be qualitatively explained according to the soft-hard acid-base theory of modern chemistry. Generally, hard acids and hard bases, soft acids and soft bases can form stable compounds, while the products formed by hard (soft) acids and soft (hard) bases are unstable. Hard acids generally refer to substances with a small atomic (ionic) radius, low polarizability, and a high positive charge, while soft acids refer to substances with a large atomic (ionic) radius, low polarizability, and a high negative charge. For example, cobalt(III) ion (Co3+) is a hard acid, while cobalt(II) ion (Co2+) is between hard and soft, and cobalt (Co) is a soft acid; silver ion also belongs to a soft acid. Hard bases refer to substances with a small atomic (ionic) radius, low polarizability, and a high negative charge, such as oxygen anion (O2-), fluoride ion (F-), water (H2O), ammonia (NH3), etc. Conversely, soft bases refer to substances with a large atomic (ionic) radius, low polarizability, and a high positive charge, such as sulfide anion (S2-), iodide anion (I-), and ligands such as sulfur and phosphorus. This explains why the soft acid silver ion and the soft base sulfur ion "fall in love at first sight" and firmly bond together upon encounter. In fact, if silverware is stored for a long time, it will turn gray or black on its own, which is due to its surface contact with airborne sulfur compounds.
However, today, using silverware to test food for poison is clearly unreliable. This is because many toxic substances do not necessarily contain sulfur, and some sulfur-containing substances are not necessarily toxic. As for some toxic substances in food, we can now completely rely on more advanced and scientific methods and instruments to test for them, rather than using silverware to test for poison anymore.