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水的硬度常用一种规定的标准来衡量。通常把1 L的水中含10 mg CaO(或相当于10 mg CaO的物质)称为1。Ca2、Mg2的硬度之和称为总硬度。总硬度在8以上的水称为硬水。例如某水样分析含Ca244 mg/L,含Mg218 mg/L,则通过Ca2~CaO和Mg2~CaO的关系式,计算出1 L这种水相当于含103.6 mg CaO,即总硬度为10.4。

现在我区某地用水所含Ca2、Mg2、HCO3物质的量浓度如下:

c(Ca2)=1×103mol/L、c(Mg2)=5×104mol/L、c(HCO3)=8×104mol/L。

通过计算回答:

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    Getting rid of dirt(灰尘), in the opinion of most people, is a good thing. However, there is nothing fixed about attitudes to dirt.

    In the early 16th century, people thought that dirt on the skin was a way to block out disease, as medical opinion had it that washing off dirt with hot water could open up the skin and let illnesses in. A particular danger was thought to lie in public baths. By 1538, the French king bad closed the bath houses in his kingdom. So did the king of England in 1546. Thus it began a long time when the rich and the poor in Europe lived with dirt in a friendly way. Henry IV, King of France, was famously dirty. Upon learning that a nobleman had taken bath, the king ordered that, to avoid the attack of disease, the nobleman should not go out.

    Though the belief in the merit of dirt was long-lived, dirt has no longer been regarded as a nice neighbor ever since the 18th century. Scientically speaking, cleaning away dirt is good to health. Clean water supply and hand washing are practical means of preventing disease. Yet, it seems that standards of cleanliness have moved beyond science since World War ll. Advertisements repeatedly sell the idea: clothes need to be whiter than white, cloths ever softer, surfaces to shine. Has the hate for dirt, however, gone too far?

    Attitudes to dirt still differ hugely nowadays. Many first-time parents nervously try to warm(警告)their children of touching dirt, which might be responsible for the spread of disease. On the contrary, Mary Ruebush, an American immunologist(免疫学家), encourages children to play in the dirt to build up a strong immune system. And the latter position is gaining some ground.