水的問題---何謂硬水,何謂軟水, 兩者有咩分別

硬水：水中溶解礦物質較多的水，如地下水
軟水：水中溶解礦物質較少的水，如雨水

這裡的礦物質多指鈣和鎂等離子
鹽化後易成為固體鹽（不溶於水）而名
可按鈣和鎂的含量分為不同硬度
香港的食用水屬軟水

硬水煮後能變成軟水
但經常煮硬水的水煲易長水垢（結在水煲壁的黃白色物質）
其實是水中的礦物移到水煲壁上
蒸餾後或者經離子交換樹脂過濾也可變成軟水

硬水洗衣時與簡單肥皂易長成浮渣
即不溶於水的長碳鏈酸鈣/鎂浮在水上和沾在衣物上
須浪費更多肥皂才能達到和同量軟水洗衣效果
不過目前一般清潔劑都能不受硬水影響發揮良好清潔效果

也有飲用硬水能保健
製成食品（如面食或糕點）更可口的說法

硬水是指礦物質成份高的水，軟水則是礦物質成份低的水

Hard water that has a high mineral content (water with a low mineral content is known as soft water). This content usually consists of high levels of metal ions, mainly calcium (Ca) and magnesium (Mg) in the form of carbonates, but may include several other metals as well as bicarbonates and sulfates. It is not generally dangerous. The simplest way to determine if water is hard or soft is the lather/froth test. If the water is very soft, soap will tend to lather up easily when agitated, whereas with hard water it will not. Toothpaste will also not froth well in hard water. More exact methods of 'hardness' detection use a wet titration method to determine hardness.

Total water 'hardness' (including both Ca2+ and Mg2+ ions) is reported as ppm w/v (or mg/L) of CaCO3. Water hardness usually measures the total concentration of Ca and Mg, the two most prevalent divalent metal ions, although in some geographical locations iron, aluminium, and manganese may also be present at elevated levels. Calcium usually enters the water from either CaCO3, as limestone or chalk or from mineral deposits of CaSO4. The predominant source of magnesium is dolomite, CaMg(CO3)2.