The density of any substance is calculated by dividing the mass of the matter by the volume of the matter. In Fig. If the amount of matter is increased without changing the volume, then the density increases Fig. If volume increases without an increase in mass, then the density decreases Fig. Adding additional matter to the same volume also increases density, even if the matter added is a different type of matter Fig. When salt is dissolved in fresh water, the density of the water increases because the mass of the water increases.
This is represented by the addition of red spheres and blue cubes to the box from Fig. Salinity describes how much salt is dissolved in a sample of water. The more salt there is dissolved in the water, the greater its salinity. When comparing two samples of water with the same volume, the water sample with higher salinity will have greater mass, and it will therefore be more dense. The density of water can also be affected by temperature. When the same amount of water is heated or cooled, its density changes.
When the water is heated, it expands, increasing in volume. This is represented by the increase in the size of the box from Fig. The warmer the water, the more space it takes up, and the lower its density. When comparing two samples of water with the same salinity, or mass, the water sample with the higher temperature will have a greater volume, and it will therefore be less dense. The bag of liquid simulates a layer of water. The relative density of the liquid in the bag compared to the liquid in the beaker can be determined by observing whether the bag sinks or floats.
Activity Activity: Density Bags. Test the effects of salinity and temperature on the floating and sinking of liquid samples in bags. If water masses have salinity or temperature differences, they will form water layers because they have different densities.
Water layers can sometimes be felt when swimming. The relative density of one water mass in relation to another determines whether a layer of water floats or sinks. Density can be determined by measuring the mass and volume of an object.
In the Density Bags Activity , density was not calculated. Instead, relative density was determined by observing whether a bag of one liquid floated or sank in another liquid.
A bag of liquid that sank was determined to be more dense than the liquid in the beaker. A bag of liquid that floated was determined to be less dense than the liquid in the beaker. The motion of any object is due to forces , which are pushes or pulls.
Vertical—up-and-down—movement of water masses in the ocean can be explained in terms of two forces. 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 cold water is more dense.
Students will be able to explain, on the molecular level, how heating and cooling affect the density of water. Download the student activity sheet , and distribute one per student when specified in the activity. In this lesson, you can help students connect some of the concepts about density to ideas from Chapter 1.
In Chapter 1, students saw that heat increases molecular motion. This increased motion competes with the attractions between molecules, causing the molecules to move a little further apart. They also saw that as a substance is cooled, molecules slow down and their attractions bring them closer together.
These ideas can also be applied to the concept of density. Tell students that in Chapter 3, they have seen that different substances have different densities. In this activity, they will see that the same substance can have different densities at different temperatures. Tell students that you are going to try to place one jar filled with hot colored water upside down over another jar with cold colored water.
Either follow the procedure below or project the video for students. If you decide to do the demonstration, you may want to watch the video first in order to see how to set the jars up.
Project the video Hot Water on Cold Water. 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. The cold blue water will immediately fall into the hot yellow water causing mixing. The water will quickly become green throughout. 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.
Fill one dropper with blue cold water. Poke the end of the dropper about halfway into the colorless room-temperature water. The cold blue water will flow down and collect at the bottom of the room-temperature water. The hot yellow water will rise and collect at the surface. Project the animation Cold and Hot Water. Point out that the molecules of cold water move slower and are a little closer together than the hot or room-temperature water.
Also point out that when the water is cooled, the water level falls slightly in the graduated cylinder. Point out that the molecules in the hot water are moving faster and are a little farther apart than the molecules in room-temperature water.
Make sure students notice that when the water is heated, the water level rises slightly in the graduated cylinder. Tell students that in winter, ice can form on the tops of ponds and lakes. In the spring when the ice melts, the cold water sinks.
This causes mixing from the bottom which brings nutrients up to the surface. Tell students that you will model this process. The coloring in the cup with the ice will move up from the bottom and begin to mix throughout the water.
The coloring in this cup will mix faster than the coloring in the cup without ice. The American Chemical Society is dedicated to improving lives through Chemistry. Skip Navigation. Lesson 3. Engage Do a demonstration to show that hot water floats on cold water. Ask students to make a prediction: Do you think the hot and cold water will mix or stay separate? Completely fill another baby food jar with very cold water and add 2 drops of blue food coloring.
When temperatures increase, objects expand and become larger and therefore the density decreases. When temperatures decrease, objects condense and become smaller so density increases. Temperature can only alter the number of molecules in an area.
However, temperature cannot affect how many protons and neutrons are in each atom. Mark Kennan is a writer based in the Kansas City area, specializing in personal finance and business topics. Related Articles Volume Vs. Mass Density. Atomic Number Vs.
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