A boat with the same mass will have greater buoyancy in salt water than in sugar water and sugar water will be more buoyant than pure water. Adding salt to the water forces more molecules into the water. This makes the water more dense. When there was no salt in the water, the boat is comparatively more dense and would ride lower (sink more) in the water. Adding salt to the water makes the water more dense which makes the boat float higher. If you weigh a cup of saltwater and a cup of fresh water, the saltwater will weigh more than the fresh water even though it is taking up the same amount of space. This is because the saltwater is more dense than the fresh water.
The property that is important is the *density* which is the mass (or you
can think of weight) per volume. Things that have a higher density than water (rocks, coins) will sink in water, while things with lower density than water (ice, most wood) will
float. That is true for anything floating in any liquid (the liquid doesn't have to be water) -- it all depends on whether it is more or less dense than the liquid. Ading sugar will increase the density and hence the buoyancy of the boat. However, sugar is less soluble than salt, which means you can put less sugar in a solution of water before it precipitates out. Because more salt can be added to water it will provide more density than sugar and therefore will allow the boat to float better than sugar in water at the outer range of concentrations.
The property that is important is the *density* which is the mass (or you
can think of weight) per volume. Things that have a higher density than water (rocks, coins) will sink in water, while things with lower density than water (ice, most wood) will
float. That is true for anything floating in any liquid (the liquid doesn't have to be water) -- it all depends on whether it is more or less dense than the liquid. Ading sugar will increase the density and hence the buoyancy of the boat. However, sugar is less soluble than salt, which means you can put less sugar in a solution of water before it precipitates out. Because more salt can be added to water it will provide more density than sugar and therefore will allow the boat to float better than sugar in water at the outer range of concentrations.