Softening is recommended when hardness of water is more than:
**Core Concept**
Water hardness is typically measured in terms of the concentration of calcium and magnesium ions. Hard water can cause scaling and deposits in pipes, appliances, and skin, leading to skin conditions like eczema and dermatitis. Softening water involves removing these ions to prevent such issues.
**Why the Correct Answer is Right**
The recommended threshold for water softening is when the water hardness exceeds 7-10 grains per gallon (gpg), which is equivalent to 120-200 milligrams per liter (mg/L) of calcium carbonate (CaCO3). At this level, the risk of scaling and skin irritation becomes significant. Water softeners use ion exchange resins to remove calcium and magnesium ions, replacing them with sodium or potassium ions.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because water hardness below 1 gpg (17 mg/L CaCO3) is generally considered soft and does not require softening.
**Option B:** This option is incorrect because water hardness around 5 gpg (85 mg/L CaCO3) may not cause significant scaling or skin irritation, and softening may not be necessary.
**Option C:** This option is incorrect because water hardness above 20 gpg (340 mg/L CaCO3) is extremely hard and requires more aggressive softening measures.
**Clinical Pearl / High-Yield Fact**
A common mnemonic to remember the order of water hardness levels is "H2O" (Hard, 7-10 gpg; Moderately Hard, 5-6 gpg; Moderately Soft, 1-4 gpg; Soft, <1 gpg).
**Correct Answer:** C. 20