这是我在2018年上海科普英语复赛的讲演稿，那次获得三等奖。
刚上初中时候对化学很着迷，学校9年级才开始学化学，而我6年级就把初中化学学了一遍。这次比赛介绍的就是氢和氧的同位素和重水。
Are you certain that you fully understand your drinking water? For example, is it correct that all purified water is drinkable?
To understand H2O fully, one should have an elementary knowledge of isotopes. Isotopes are elements with the same number of protons but different numbers of neutrons in each atom.
As is well known, a molecule of water consists of two atoms of hydrogen and one atom of oxygen.So there are nine categories of isotopic water molecules in total, all of which are found in natural water sources in varying proportions. In fact, one sip of water contains all nine kinds of water.
The chemical properties of an element are determined by the number and arrangement of electrons in its atoms. The same number of electrons in water isotopes leads to a similarity in their chemical features. Nevertheless, they have differences.
Actually, except for the first one mentioned above, which is the ordinary water found in the tap, the other eight are called“heavy water” and are certainly undrinkable in large quantities.
Why is“heavy water”poisonous to humans? The deuterium-oxygen bond is sturdier than the hydrogen-oxygen bond. As a result,the chemical properties of heavy water are stabler than those of drinking water.The chemical reactions of heavy water occurring in organisms are much slower than those of ordinary water ,which have a negative influence on metabolism.
According to experiments,mice will die if heavy water occupies one third of their body water content. It is not clear how much heavy water will cause death of an individual subject. Scientists estimate that people will become sick if heavy water exceeds ten percent of their body water content.
Although heavy water is of no great help to our health, it is widely used in the military and civilian fields. Heavy water acts as a cooling and moderating material in nuclear reactors. It is also used as an isotopic tracer in chemical and biochemical mechanisms. By tracking the metabolism of heavy water, scientists have found that the average residence time of common water in the human body is fourteen days.
The density of deuterium oxide is ten percent higher than that of that of ordinary water. Unlike common ice, deuterium oxide ice sinks to the bottom of ordinary water.
What we take for granted conceals many unknown details, which are worth exploring and pondering.
