In the first lab of the year, students were asked to investigate the properties of water by completing a myriad of different activities. Each station's activity exemplified a different phenomenon made possible by water due to its unique characteristics. During my time in lab, I experimented with the following:
Specific Heat of Water
Cohesion and Adhesion of Water
Capillary Action
Specific Heat of Water
Specific heat is a property of matter that describes a substance's ability to resist change in temperature. To see conceptually just how stubborn water can be, I heated a penny over a Bunsen burner to an incredible temperature, and dropped it into a cup filled about half-way with room temperature water. Surprisingly, the water temperature failed to rise even a single degree, as shown below.
Cohesion of Water
Water is referred to as a polar compound. Polar compounds are clusters of atoms with unequal distributions of charge. Water is polar because oxygen (one part of the water formula) is extremely electronegative, meaning it is very good at pulling electrons away from other atoms. Hydrogen, on the other hand, sits at the far other end of the spectrum, where it easily loses electrons and bonds with various elements to make different compounds. When an oxygen atom forms two covalent bonds with two different hydrogen atoms, the result is a slightly negative charge on the oxygen atom and positive charges on the hydrogen atoms, effectively resulting in a polar compound.
Cohesion is Water's tendency to bond with other water molecules. This occurs when a hydrogen atom already attached to a water molecule forms a hydrogen bond with an oxygen atom, because they are positive and negative (respectively). Because water likes to bond to itself, we see water bead up on wax paper instead of spread out or be absorbed, as shown here.
In addition to this, water bonds to itself even on something as simple as a penny, which is why plenty of drops can fit on something so small before the water runs off. However, touching the water droplets with something nonpolar like a soap-covered toothpick alleviates the cohesion phenomenon because polar and nonpolar molecules do not mix. This is commonly seen with oil and water. Once the toothpick ruptured the water bubble, water drained from the penny since the attracting force had been reduced severely.
Capillary Action
Capillary Action is the ability for water to flow through tight spaces without need of assistance from external forces such as gravity. This is only seen with polar substances like paintbrushes or glassware, becuase capillary action cannot occur without adhesion.
Adhesion is water's tendency to bond with polar surfaces. Adhesion is seen when water runs down the side of your cup before it dumps into the sink. Adhesion is also at play when looking at a meniscus in a graduated cylinder. The reason water does not form a flat surface in a glass container is because water molecules are bonding to the glass, thus allowing it to rise on the sides yet not in the middle.
At the station involving capillary action, a small glass tube was placed in a cup of water, which (to my shock) raised the level of water within the tube half an inch above the water level in the basin of the cup. Again, this would not be seen if the tube was not a polar surface, but it was still really cool. Capillary action was also seen when a special paper was placed in water so that only the very bottom dipped beneath the surface. I observed the water move through the paper by means of adhesion, because the colors of the rainbow began to subtly appear up the length of the paper.
No comments:
Post a Comment