Water, density and temperature
Since PV = nRT the fractional increase in density, at constant pressure, is equal to minus the fractional change in temperature, for small. An important property of water is the relationship between temperature and density. Those who live in colder regions of the world are aware of this property. Students will combine the concepts of temperature, molecular motion, and density to learn that hot water is less dense than room-temperature water and that.
When the cold water is placed on top, the colors mix because the cold water is more dense and sinks in the hot water. Give each student an activity sheet. Students will record their observations and answer questions about the activity on the activity sheet. The Explain It with Atoms and Molecules and Take It Further sections of the activity sheet will either be completed as a class, in groups, or individually depending on your instructions.
Look at the teacher version of the activity sheet to find the questions and answers. Explore Have students try adding cold and hot water to room-temperature water.
Question to investigate Is there a density difference between hot and cold water? Materials for each group Cold water colored blue in foam cup Hot water colored yellow in foam cup Room-temperature water in clear plastic cup colorless 2 droppers Teacher preparation Add ice to water to make very cold water.
Half-fill one foam cup with cold water no ice cubes and another with hot water for each group. Add 2 drops of yellow food coloring to the hot water and 2 drops of blue food coloring to the cold water.
Temperature and Density | Chapter 3: Density | Middle School Chemistry
Distribute the set of 3 cups to each group. Procedure Fill one dropper with blue cold water. Poke the end of the dropper about halfway into the colorless room-temperature water. While observing from the side, very gently squeeze the dropper so that the cold water slowly flows into the room-temperature water.
Fill another dropper with yellow hot water. Poke the end of the dropper about halfway into the room-temperature water.
While observing from the side, very gently squeeze the dropper so that the hot water slowly flows into the room-temperature water. Record your observations on the activity sheet. Remember that liquid water and solid water have the same network of bonds.
Temperature Effects on Density
Liquid water at 25 deg is so rapidly breaking bonds between H2O units and reforming them that extra water molecules get trapped inside the water lattice. This is the reason why liquid water is more dense than solid water. The bonds in water break more slowly as temperature decreases and the structure tend to trap fewer extra water molecules. At low temperature, more of the water has the same lattice as ice. Wikipedia, Water Density It is possible to have liquid water at temperatures well below 0 deg C.
Molecules in this supercooled water are free to move.
Bonds are made and broken. The long range structure is not perfect but the short range structure of supercooled water is very much like ice.
Temperature Affects Density
Adding a crystal to supercooled water causes instantaneous ice formation. It is similar to water in that it is polar, with a permanent dipole moment, and forms hydrogen bonds with itself. It doesn't have the same type of 3-dimensional lattice that water has though. We can see that the density decreases with temperature through this range. Unlike the situation with water, there is no maximum density point. Most other pure liquids are like ethanol in this respect.