Social & Preventive Medicine Hardness of water is expressed per litre (m Eq/L), one unit which is equivalent to which one of the following. A 20 mg CaCO3 (20 ppm) B 30 mg CaCO3 (30 ppm) C 40 mg CaCO3 (40 ppm) D 50 mg CaCO3 (50 ppm) 💡 Explanation Hardness of water is expressed per litre (m Eq/L), one unit which is equivalent to which one of the following. ✓ Correct Answer: D. 50 mg CaCO3 (50 ppm)