Castles in the Sand

Background

One of the more special properties of water is its 'stickiness'. Because of attractive intermolecular forces, water beads up on waxy surfaces, it travels upwards through the trunk of a tree, and insects are able to walk across its surface.

Two types of forces contribute to water's stickiness: cohesive and adhesive forces.

Cohesive forces create the attraction between individual water molecules. These forces contribute to surface tension. Surface tension allows water strider insects to move almost effortlessly across a creek's surface without sinking. Surface tension keeps water from quickly evaporating from lakes. Without surface tension, the morning dew could not bead up on spider webs.

Through adhesive forces, water is attracted between unlike molecules. Look closely at water rising up the sides of thin a glass tube. This adhesive attraction is even greater than water's cohesive force (which causes surface tension). Water's adhesion to other surfaces, such as sand grains, causes a flow called capillary action. Capillary action helps water rise to the top of the highest trees, helps water flow through some aquifers, helps transport nutrients throughout your body, and allows the billions of sand grains in a sand castle to stick together.

Procedure

Select two areas in the sand at Great Sand Dunes. One should be near the edge of Medano Creek where the water is flowing and the other should be in a dry, sandy area. If Medano Creek is not flowing, be sure to bring a few large buckets to fill with water.

Students can work in groups, small or large. Tell the students that they are going to have a sand castle building competition. Distribute the supplies and tell them they will have twenty minutes to build their castles. First have them build castles in the area of dry sand. The only rule is that they cannot leave their building site.

In a short while, they will begin complaining that the dry sand will not hold castle walls. Before they get too frustrated, ask them what will help them in their castle-building endeavor. Explain the concepts of water tension and capillary action. Then have the groups move to the location with wet sand alongside the creek.

Allow them twenty minutes to build their castles with wet sand. When the the twenty minutes are up, look at each castle and discuss the effects of water on their castles. Let the students share their problems and ideas they had while building.

Critical Thinking

Make a list of things in nature or things made by people that would not work without surface tension or capillary action.

Extension

Sand Filters is a follow-up activity that helps students understand how capillary action filters water naturally.

Adapted from Educator's Guide to Great Sand Dunes, by Lori Cooper, Friends of the Dunes.