Water, density and temperature
Mass, Specific Gravity or density, of water at various temperatures degree C. 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. 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.
Place the cold water jar on a paper towel. Hold a water-resistant card over the top of the hot water jar. While holding the card against the jar opening, carefully turn the jar upside down.
With the card still in place, position the jar of hot water directly over the jar of cold water so that the tops line up exactly. Slowly and carefully remove the card so that the hot water jar sits directly on top of the cold water jar. Expected results Although removing the card may result in a little mixing or spilling, the hot yellow water will remain in the top jar and the cold blue water will remain in the bottom jar. Why do you think the hot water stayed on top of the cold water?
Students should realize that there is a density difference between hot and cold water. Hot water is less dense so it floats on the denser cold water. Ask students to make a prediction: What might happen if you placed the cold blue water on top of the hot yellow water and then removed the card?
Cold water on top Use the same procedure as above, but place the jar of cold water, upside down over the jar of hot water.
Expected results The cold blue water will immediately fall into the hot yellow water causing mixing. The water will quickly become green throughout. Why do you think the hot and cold water mixed when the cold water was placed on top? 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.
This makes sense because, as heat is added to the liquid water, there is greater kinetic energy of the molecules and there are also more vibrations of the water molecules.
Together these mean that each H2O unit in liquid water takes up more space as the temperature increases. We see the same trend in going from liquid water at 25 deg C 0.
Temperature and Density | Chapter 3: Density | Middle School Chemistry
Density increase as the temperature decreases. Below 4 deg C, however, the density decreases again. How can we explain this? Remember that liquid water and solid water have the same network of bonds.
Density of Water
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.