An appliance using water including an incoming water valve is provided. The appliance includes a sensor disposed in-line to an incoming flow of water received from the incoming water valve and is configured to sense a degree of hardness in the incoming flow of water. The sensor includes a sensing element disposed on a substrate. The sensing element includes a sensing matrix, an indicator for one or more chemical species in flow of water, and a selectivity component that reacts reversibly with one or more chemical species in the water. The sensor also includes a light source configured to direct light through the substrate and the sensing matrix. The sensor further includes a light detector configured to receive transmitted light from the substrate and the sensing matrix and to generate a signal representative of selective wavelengths of the light indicative of one or more chemical species in flow of water.