The question of whether salt sinks or floats in water is a common one that bridges the observable world with the principles of chemistry and physics. Table salt, or sodium chloride (\(\text{NaCl}\)), interacts with water in a complex way, ultimately altering the water itself. Exploring this interaction reveals the power of molecular forces and the concept of density, providing a clear answer to the initial query and explaining the significant changes that occur afterward.
The Initial Action: Sinking Before Dissolving
When a grain of solid table salt is dropped into a glass of water, the immediate and visible action is that it sinks. Solid sodium chloride has a density of approximately \(2.16\) grams per cubic centimeter (\(\text{g}/\text{cm}^3\)) at room temperature. This is significantly greater than the density of pure water, which is about \(1.0\) \(\text{g}/\text{cm}^3\). Since any object that is denser than the fluid it is placed in will displace less weight of fluid than its own weight, gravity pulls the salt downward.
The dense crystalline structure of the salt dictates this sinking behavior. The relatively high mass packed into a small volume means that the salt rapidly falls to the bottom of the container. This initial sinking is a straightforward physical response governed by the difference in density between the solid salt and the liquid water.
The Chemistry of Dissolution: Why Salt Disappears
The immediate sinking is quickly followed by the process of dissolution, where the salt seems to “disappear” into the water. Salt is an ionic compound, composed of positively charged sodium ions (\(\text{Na}^+\)) and negatively charged chloride ions (\(\text{Cl}^-\)) held together by a strong electrostatic attraction in a crystal lattice. Water molecules (\(\text{H}_2\text{O}\)) are polar, possessing a negative charge near the oxygen atom and a positive charge near the two hydrogen atoms.
The polarity of the water molecule allows it to act as an effective solvent, especially for ionic compounds like salt. The negatively charged oxygen end of the water molecules is attracted to the positive sodium ions, while the positively charged hydrogen ends are attracted to the negative chloride ions. This attraction is strong enough to overcome the internal attractive forces of the salt’s crystal lattice.
Water molecules surround the individual ions, effectively pulling the sodium and chloride ions out of the solid structure one by one. This process is known as hydration, and it results in the formation of hydration shells around each separated ion. Once surrounded by water molecules, the individual ions are dispersed evenly throughout the water, turning the mixture into a homogeneous solution. The salt has not vanished; its constituent ions are now fully integrated into the liquid at a molecular level.
How Dissolved Salt Changes Water’s Density
Once the salt dissolves, its presence as dissolved ions fundamentally changes the physical properties of the water. The most significant change is an increase in the liquid’s density. When salt is added and dissolves, the ions fit into the existing spaces between the water molecules.
This molecular arrangement means the total mass of the solution increases by the mass of the added salt, but the overall volume of the liquid increases only slightly. Since the mass has increased without a proportional increase in volume, the resulting saltwater solution is denser than the original pure water. A saturated salt solution, for example, can have a density of around \(1.202\) \(\text{g}/\text{cm}^3\).
This increase in density directly impacts buoyancy, the upward force a fluid exerts on an object. Because the saltwater is heavier per unit of volume, it leads to a greater buoyant force. This explains why it is easier for an object, such as a human body, to float in saltwater compared to freshwater, a phenomenon demonstrated in highly saline bodies like the Dead Sea, where the salt concentration can be over eight times that of the ocean.