Adhesion

Water rises in a capillary tube due to adhesion between water molecules and the tube's polar surface. Water molecules form hydrogen bonds with the glass or cellulose wall, creating an upward pull stronger than gravity. This adhesive force exceeds the cohesive forces between water molecules, causing capillary action. The narrower the tube, the higher the water rises as more molecules contact the surface.

Adhesion occurs when water molecules attract to different molecules, such as polar surfaces like glass or cellulose. Cohesion describes attraction between water molecules themselves. In capillary action, adhesion to the tube wall must be stronger than cohesion between water molecules for the liquid to rise. Both forces work together in plants to transport water upward through xylem vessels.

Plants exploit adhesion between water and cellulose in xylem vessels to move water against gravity. Water molecules cling to the polar cellulose cell walls through hydrogen bonding, resisting downward pull. Combined with cohesive forces between water molecules, this creates a continuous water column. The narrow diameter of xylem vessels enhances capillary action, allowing water to reach leaves where vaporization sweating and transpiration occur.

Hydrogen bonds form between the positively charged hydrogen atoms in water and negatively charged oxygen atoms in polar surfaces like silica or cellulose. These bonds create strong adhesive attractions that pull water molecules toward the surface. The strength of hydrogen bonding allows water to adhere more strongly to polar materials than water molecules adhere to each other, enabling capillary action.

Narrower tubes allow water to rise higher during capillary action because more water molecules contact the tube surface relative to the tube's volume. As tube diameter decreases, the adhesive forces acting on a larger proportion of water molecules overcome gravity more effectively. In plants, xylem vessels decrease in diameter toward tree tops, maintaining capillary action to deliver water to the highest leaves.

A concave meniscus is the curved surface of water that forms at the edges of a capillary tube, curving upward toward the center. It forms because water molecules adhere to the tube wall via hydrogen bonds, pulling the liquid upward at the edges. This curved shape reflects the stronger adhesive forces between water and the polar tube surface compared to cohesive forces between water molecules.

Water is polar because its hydrogen atoms carry a partial positive charge and oxygen atoms carry a partial negative charge. This polarity allows water to form hydrogen bonds with other polar molecules and surfaces, such as cellulose and silica. Polar water molecules are strongly attracted to polar surfaces, making adhesion possible and enabling capillary action in both laboratory tubes and plant xylem vessels.