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水的硬度由其中钙盐、镁盐(有时有两价的铁盐)含量的多少来决定。水的硬度通常以相对硬度来表示。各国硬度的表示法有所不同,多以某种体积水中CaO或CaCO_3的含量为基础。其他两价金属换算成等当量的钙化合物来计算。以前苏联用100升水中含1克CaO,即是在1升水中含10毫克CaO来表示水的硬度。在很早以前,当时在化学界中统治着盐的是由金属氧化物和非金属氧化物形成(例如由CaO和CO_2所形成的CaCO_3)的二元论的学说,就应用过
The hardness of water is determined by the amount of calcium and magnesium salts (sometimes ferrous salts). Water hardness is usually expressed in terms of relative hardness. The representation of the hardness of different countries are different, mostly based on the content of CaO or CaCO_3 in a certain volume of water. Other divalent metals are converted into equivalent calcium compounds. In the past, the Soviet Union used 1 gram of CaO in 100 liters of water, that is, 10 milligrams of CaO in 1 liter of water to indicate water hardness. A long time ago, the doctrine of dualism, which dominated the chemical industry at the time, was formed by the formation of metal oxides and non-metal oxides (such as CaCO 3 formed by CaO and CO 2)