How to Increase the Density of Water
The simplest way to increase the density of water is by dissolving denser substances in it. This increases the mass of the water without significantly changing its volume, thus increasing the density.
Several methods can effectively increase water's density:
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Adding Inorganic Salts: Dissolving salts like sodium chloride (table salt) significantly increases water's density. A saturated solution of sodium chloride in water can reach a density of around 2.16 g/cm³. This is supported by multiple sources including Reddit's r/chemistry community (https://www.reddit.com/r/chemistry/comments/m6890d/can_i_increase_the_density_of_water_to_above_15/) and numerous other online resources confirming this method.
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Cooling the Water (to a point): Water's density increases as it cools, reaching its maximum density at approximately 4°C. Further cooling causes the density to decrease as ice forms. This is mentioned on the American Chemical Society's website (https://www.acs.org/middleschoolchemistry/lessonplans/chapter3/lesson6.html).
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Increasing dissolved substances: While adding salt is the most common method, other dissolved substances, depending on their density and solubility in water, will increase the density of the water. For example, adding sugar would increase density but to a lesser extent than salt.
Considerations
While you can increase the density of water, achieving a 1.5 times increase under normal atmospheric conditions is not possible simply by adding substances or cooling it. This limitation is highlighted in a Quora answer (https://www.quora.com/How-can-I-increase-the-density-of-water-1-5-times). The degree of density increase depends on the type and amount of substance added.
Increasing dissolved oxygen increases density, but the effect is minimal (approximately 0.0022% increase per 1 mg/L of dissolved oxygen) as noted by a Reddit user in r/askscience (https://www.reddit.com/r/askscience/comments/16vldcn/dissolved_oxygen_vs_density_of_water/).
