Foating: A Chemical Phenomenon
Insects and boats can be seen skimming over the surface of a pond. Different chemical phenomena, including density, buoyancy, and surface tension, explain the floating and sinking of objects.
In this project, you will determine why objects float on water. You will also look at the effect of solutes on the buoyancy and surface tension of water.
Purpose: To determine why objects float on water.
- 2-quart (2-liter) bowl
- modeling clay
- Fill the bowl three-fourths full with cold tap water.
- Separate two walnut-size pieces of clay. Note: The pieces should be of equal size.
- Roll one of the clay pieces into a ball.
- Carefully place the clay ball on the surface of the water in the bowl (see Figure 20.1).
- Observe and record the results.
- Remove the clay ball from the water and set it aside.
- Take the second piece of clay and press it into a thin, 3-inch (7.6-cm) square.
- Turn up about 1/2 inch (1.3 cm) of each edge to form an open box.
- Gently place the clay box, open side up, on the surface of the water in the bowl (see Figure 20.2).
- Observe and record the results.
The clay ball sinks in the water. The same amount of clay, when formed into the shape of a box, floats on the surface of the water.
The mass and thus the weight of the equal-size pieces of clay are the same. Therefore, the sinking of one clay piece and the floating of the other are not dependent on weight. Clay, like all matter, occupies space and therefore has volume. The clay pieces have the same weight but not the same volume. The explanation of the seemingly paradoxical behavior of the clay can be explained by comparing the density and buoyancy of the two pieces.
Density is a comparison of the mass and volume of substances and, in the scientific community, is commonly measured in units of grams per milliliter (cubic centimeter). The density of a substance is calculated by the following formula:
The density of water is 1 g/ml and is used as a standard for density measurements. Substances with a density greater than 1 g/ml sink in water; objects with a density less than 1 g/ml float in water. When placed in water, an object displaces (pushes aside) the water. Floating objects weigh less than the volume of water that they displace, and sinking objects weigh more than the volume of water that they displace. The weight of the water displaced by an object is equal to the buoyant force (amount of upward force) exerted by the water on the object
The open clay box floats because it takes up more space than the clay ball of equal weight; thus, it has a lower density. The larger-size clay box displaces more water, making it more buoyant than the smaller clay ball of equal weight.
Try New Approaches
How much difference is there between the density of the clay ball and that of the clay box? Repeat the experiment using a scale to determine the mass of the clay pieces.
With a ruler, determine the height, length, and width of the clay box.
Use the following equation to calculate the volume of the clay box:
- volume of a box = length × width × height
Determine the density of the clay box by using its mass and dimensions. See Appendix 11 for example calculations. Record your data in a table such as the one shown here.
Use the following equation to determine the volume of the clay ball:
- volume = 4/3 × pi (3.14) × radius cubed
The radius can be calculated by determining the circumference (distance around a sphere) of the clay ball. Measure the circumference of the clay ball by wrapping a string around it and measuring the length of the string with a ruler. Use the following equation to determine the radius:
- circumference = 2 × 3.14 × radius
Design a data table and record your measurements of the clay ball in it. See Appendix 11 for example calculations.