For many everyday substances, simply adding water causes them to disappear, a process known as dissolving. This common phenomenon involves a substance breaking apart and evenly dispersing within water, forming a uniform mixture called a solution. Water’s unique molecular structure allows it to play a significant role as a solvent in countless natural and industrial processes.
How Dissolving Works
Dissolving involves a solvent, the substance doing the dissolving, and a solute, the substance being dissolved, combining to form a solution. Water is frequently called the “universal solvent” because it can dissolve more substances than almost any other liquid. This ability stems from its molecular structure: each water (H₂O) molecule has a bent shape with a slight positive charge on the hydrogen atoms and a slight negative charge on the oxygen atom. This uneven charge distribution makes water a polar molecule, acting like a tiny magnet with distinct positive and negative ends.
When a polar substance or an ionic compound (like salt) is introduced to water, the charged ends of the water molecules are attracted to the oppositely charged parts of the solute. For instance, the slightly negative oxygen atom in water attracts positive ions, while the slightly positive hydrogen atoms attract negative ions. These attractions are strong enough to pull the solute’s particles away, surrounding them with water molecules in a process called solvation. This “like dissolves like” principle explains why polar solvents, such as water, effectively dissolve polar solutes, while nonpolar substances generally do not.
Everyday Items That Dissolve
Many common items readily dissolve in water. Table salt, or sodium chloride, is a prime example; its ionic bonds are easily broken by the attractive forces of water molecules, separating into individual sodium and chloride ions that become surrounded by water. These ions then disperse evenly throughout the water, creating a homogeneous solution.
Sugar, specifically sucrose, also dissolves well in water because it is a polar molecular compound. Sugar molecules contain multiple hydroxyl (-OH) groups, which can form hydrogen bonds with water molecules. These strong attractions overcome the forces holding the sugar crystal together, allowing individual sugar molecules to disperse without breaking down into atoms.
Ethanol, a common alcohol, mixes completely with water due to its polar nature and ability to form hydrogen bonds with water. While water is more polar than ethanol, ethanol’s hydroxyl group allows it to interact favorably with water. Additionally, certain gases, such as oxygen and carbon dioxide, can dissolve in water. Oxygen dissolves in water through diffusion, and carbon dioxide’s slightly polar bonds allow water molecules to attract and surround it.
Everyday Items That Do Not Dissolve
Conversely, many familiar substances do not dissolve in water due to their chemical properties. Oils and fats are examples; they are nonpolar molecules composed primarily of long hydrocarbon chains. Water molecules, being polar, are more attracted to each other than to the nonpolar oil molecules, causing oil to separate and float on top of water.
Sand, predominantly silicon dioxide (SiO₂), does not dissolve in water because its silicon and oxygen atoms are held together by very strong covalent bonds. Water molecules lack the necessary strength to break these robust bonds, so sand particles remain intact. Similarly, most plastics are large, nonpolar polymers that resist interaction with water. Their tightly bonded chains do not allow water molecules to effectively pull them apart or surround them, making them largely insoluble.
Wood, primarily composed of cellulose, hemicellulose, and lignin, largely resists dissolution in water. Cellulose and hemicellulose are complex carbohydrates with strong internal bonds and ordered structures that water molecules cannot easily overcome. While wood can absorb some water and swell, its fundamental structure remains intact because the forces holding its components together are stronger than the attraction to water molecules.
Influences on Dissolving Speed
Several factors can influence how quickly a soluble substance dissolves in water. Increasing the temperature of the water speeds up dissolution. Higher temperatures provide more kinetic energy to both water and solute particles, causing them to move faster and collide more frequently and forcefully. These more energetic collisions help break the bonds within the solute more rapidly and distribute its particles throughout the solvent.
Stirring or agitation accelerates the dissolving process. When a solute begins to dissolve, a layer of concentrated solution forms around the undissolved particles. Stirring disperses this concentrated layer, bringing fresh, unsaturated solvent into contact with the solid solute. This continuous exposure allows more solute to dissolve more quickly.
The surface area of the solute impacts dissolving speed. Breaking a solid solute into smaller pieces, such as using granulated sugar instead of a sugar cube, increases its surface area exposed to the solvent. With more surface available for interaction, water molecules can attack and pull apart the solute particles simultaneously from many points, leading to a faster dissolution rate.