Final answer:
Water forms a dome on a penny due to adhesion and capillary action, caused by the polarity of water molecules, which leads to attraction between water and polar surfaces like glass and the movement of water along these surfaces against gravity.
Step-by-step explanation:
Water on a penny forms a dome mainly due to adhesion and capillary action. Water molecules exhibit polarity, which allows them to be attracted to other charged or polar substances, such as the glass in a capillary tube. This attraction between water molecules and a surface is the adhesion component, while their ability to travel along the surface, against gravity, is known as capillary action.
When a glass capillary is inserted into water, the strong adhesion between water and the polar surface of the glass combined with water's cohesive forces creates a concave meniscus, illustrating capillary action as water rises within the tube. The water molecules adhere to the glass walls and pull the rest of the water up along, and due to the water's surface tension and polar character, a concave upward curve forms at the liquid's surface. This delicate balance is maintained until gravitational forces exactly balance the upward adhesive force. In contrast, liquids like mercury, which have stronger cohesive forces than adhesive forces with glass, display a convex meniscus in a capillary tube.