What we perceive as temperature is simply a measure of the average kinetic energy of the substance (the energy of the molecules in the bulk is what’s called Normally Distributed). When we mix substances at different temperatures they quickly exchange energy to get rid of the temperature difference between them so the average energies combine rapidly. Once this happens, the molecules arrange themselves according to the energy they have (the density at that specific temperature).
Mass is conserved so the mass of the mixture is simply the sum of the masses of the two liquids being mixed; volume can be found by dividing the total mass by the density (or by measuring it).
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