This is because water has high specific heat capacity that is 4200 J/kg degrees celsius due to which it remains hot for long time but other liquid does not remain hot like water . Hence, water is kept in water bag.
Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to a given mass of a material to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K). ... The volumetric heat capacity measures the heat capacity per volume.
The symbol for specific heat is cp, with the p subscript referring to the fact that specific heats are measured at constant pressure. The units for specific heat can either be joules per gram per degree (J/goC) or calories per gram per degree (cal/goC).The SI unit of specific heat is joule per kelvin and kilogram, J/(K kg). For example, at a temperature of 25 °C the heat required to raise the temperature of 1 kg of water by 1 K is 4179.6 joules so the specific heat capacity of water is 4179.6 J. kg. K−1.
Molar heat capacity is a measure of the amount of heat necessary to raise the temperature of one mole of a pure substance by one degree K. Specific heat capacity is a measure of the amount of heat necessary to raise the temperature of one gram of a pure substance by one degree K.In a system, Cp is the amount of heat energy released or absorbed by a unit mass of the substance with the change in temperature at a constant pressure. ... So, Cp represents the molar heat capacity, C when pressure is constant. The change in temperature will always cause a change in the enthalpy of the system.
The specific heat capacity of water is 4.18 J/g/°C. We wish to determine the value of Q - the quantity of heat. To do so, we would use the equation Q = m•C•ΔT. The m and the C are known; the ΔT can be determined from the initial and final temperature.
If oil is used instead of water in hot water bags the warmth will not remain as long as it would with the water. The product of density and specific heat for oil is much smaller than that of water.