If you live in a region where winter weather is a regular risk, you are likely used to pouring salt on your sidewalks. But how does it work? And how much salt do humans pour onto our planet’s surface? The second question is easier to answer: a lot.
Salt doesn’t directly melt ice, nor does it make snow simply disappear. Instead it makes water less likely to freeze in a phenomenon called freezing point depression. In the case of simple rock salt, which is a rawer, less pure version of table salt, each molecule (分子) splits into smaller elements. Normally, when water freezes into ice, its molecules line up to form a stable, orderly structure. Salt interrupts the process, however, and temperatures must drop lower to overcome that interruption and for freezing to occur.
But if salt needs to interact with liquid water, how does it do anything when temperatures are stubbornly below freezing, and water should already be in the form of ice? That’s where cars help clear their own way by creating friction and, in turn, heat. The friction allows for the ice that has already frozen to melt a little bit, making it unlikely to freeze.
Beyond rock salt’s ability to clear icy streets, it can also be destructive. Chloride ions (氯离子) can cause wear and tear on vehicles and facilities. Increasing chloride densities in North American lakes could begin to upset local ecology and degrade sources of drinking water.
Many local governments are looking for alternatives to rock salt. Other salts such as magnesium chloride and calcium chloride work in the same way as rock salt, and they’re perhaps even more efficient. Some experiments are testing other ice-preventing solutions, including those that contain sugars instead of or in addition to salts. Another approach relies more heavily on sand, which can mechanically make roads safer. All of this experimentation is aimed at ensuring people can travel in any weather condition — a Herculean task when winter brings its worst.
【小题1】What’s the purpose of the first sentence in Paragraph 2?| A．To introduce the topic. | B．To give some evidence. |
| C．To define the phenomenon. | D．To reveal the misconception. |
| A．By lowering the temperature. | B．By melting and splitting itself. |
| C．By lining up to form a solid structure. | D．By stopping the formation of the structure. |
| A．Concentrations. | B．Weights. | C．Qualities. | D．Salts. |
| A．Other salts won’t disturb local ecology. |
| B．Efforts are being made to guarantee safe travels. |
| C．Many alternatives are available to solve the problem. |
| D．Bitter winter make it impossible to secure road safety. |
