$1.$ What is the primary source of hard water?a$)$ Rainwaterb$)$ Underground springsc$)$ Surface waterd$)$ River waterWhich of the following ions is primarily responsible for hardness in water?a$)$ Sodium ionsb$)$ Chloride ionsc$)$ Calcium ionsd$)$ Potassium ionsWhat is the acceptable limit for hardness in drinking water, as per WHO standards?a$)100$ ppmb$)200$ ppmc$)500$ ppmd$)1000$ ppmWhich method is commonly used to estimate the hardness of water?a$)$ Gravimetric methodb$)$ Titration methodc$)$ Spectrophotometric methodd$)$ Chromatographic method