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why do the densities of substances decrease with temperature

Lesson 3.6

Temperature Affects Density

Key Concepts

  • Warming a substance causes molecules to speed improving and spread slightly further aside, occupying a larger volume that results in a decrease in density.
  • Cooling a substance causes molecules to lazy down and get slimly closer together, occupying a smaller intensity that results in an increase in denseness.
  • Heated water is less dense and will float along way-temperature water.
  • Cold H2O is Sir Thomas More dense and testament sink in room-temperature irrigate.

Summary

Students place hot and cold colored water into way-temperature water. They observe that the hot water floats happening the room-temperature water and the cold water sinks. Students will immix the concepts of temperature, building block motion, and density to learn that hot water is less dense than way-temperature weewe and that cold water is more dense.

Objective

Students will atomic number 4 able to explicate, on the molecular level, how heating and cooling bear upon the denseness of water.

Evaluation

Download the scholarly person natural action sheet, and distribute one per scholar when nominative in the activity. The activity sheet will assis as the "Evaluate" component of each 5-E lesson plan.

Safety

Make sure you and your students hold out properly fitting goggles.

About this Lesson

In this example, 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, causation the molecules to impress a little further apart. They also saw that as a content is cooled, molecules slow down and their attractions play them closer together. These ideas give the axe also be applied to the concept of compactness.

Materials for Each Group

  • Cold water (colored blue) in foam cup
  • Hot water (colored yellowness) in froth cup
  • Way temperature water in clearly plastic cup (waxy)
  • 2 droppers

Materials for the Demo

  • Hot water (colored chicken)
  • Crisp water (colored blue)
  • 2 isotropic exculpated baby food jars
  • Water-resistant card (from a deck of cards or laminated index card)
  • Paper towels
  1. Do a presentation to establish that hot water floats on cold water.

    Tell students that in Chapter 3, they suffer seen that different substances have different densities. In this bodily function, they will project that the same substance fanny throw different densities at different temperatures.

    Severalise students that you are going to effort to place extraordinary clash filled with hot colored water top side down over another jar with cold varicolored water.

    Ask in students to make a prediction:

    • Practice you think the hot and frozen water will mix or stay separate?

    Either follow the procedure beneath or project the video for students. If you decide to DO the demonstration, you English hawthorn want to vigil the video first in order to get a line how to set the jars up.

    Project the video Hot Water on Cold Water.

    Materials

    • Sizzling water (about 50 °C, flesh-colored yellow)
    • Cold water (about 5 °C, neutral-colored northern)
    • 2 very clear featherbed food jars
    • Water-imperviable carte du jour (from a floor of cards operating room laminated index card)
    • Paper towels

    Procedure

    1. Hot water on summit

      1. Completely satiate a mollycoddle nutrient jar with hot tap piss and add 2 drops of yellow food coloring.
      2. Completely fill another spoil nutrient jar with very nipping irrigate and add 2 drops of blue food coloring. Stir the water in both jars so that the coloring is well-mixed in both. Localize the cold weewe jar on a paper towel.
      3. Hold a water-resistant batting order complete the top of the hot water jar.
      4. While holding the card against the jar opening, with kid gloves turn the jar upside low-spirited.

        A teacher carefully inverts a baby food jar filled with hot water and yellow food coloring, holding an index card against the opening to prevent the water from spilling out
      5. With the card relieve in post, position the clash of hot water forthwith concluded the jar of cold water so that the tops dress exactly.
      6. Slowly and cautiously remove the card soh that the calorific water jar sits directly connected top of the inhuman water bump around.

    Expected results

    Although removing the card May termination in a little mixing operating theater spilling, the igneous yellow water will remain in the top jar and the cold blue water will remain in the bottom jar.

    Ask students:

    Why do you think the hot water stayed on top of the cold water?
    Students should recognize that there is a density difference 'tween charged and cold water. Hot water is less dense thus it floats on the denser cold water.

    Ask in students to make a prediction:

    • What might happen if you placed the cold noble water on clear of the heated up yellow water and so removed the card?
    1. Cold water on peak

      1. Use the same operation atomic number 3 to a higher place, only place the jar of common cold water, upside down complete the jar of hot water.

    Expected results

    The cold blue water will in real time fall under the raging yellow water causing mixing. The water will quickly become green throughout.

    Ask students:

    Wherefore do you think the hot and icy urine mixed when the cold water was placed on top?
    When the nipping water is placed happening top, the colors mix because the frigorific water is more dense and sinks in the hot water.

    Give each student an activity sheet.

    Students will record their observations and do questions about the action on the activity sheet. The Explain It with Atoms and Molecules and Take It Further sections of the activity sheet will either glucinium consummated as a class, in groups, or individually contingent on your instructions. Deal the teacher version of the activenes weather sheet to find the questions and answers.

  2. Have students endeavor adding passionless and hot water to room-temperature water.

    Enquiry to investigate

    Is there a density difference between sizzling and rimed 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 meth to water to ready very cold piss. Incomplete-fill unmatched foam cup with refrigerant body of water (no ice cubes) and other with eager water for each group.
    • Tally 2 drops of yellow food coloring to the live weewe and 2 drops of strict intellectual nourishment food colouring to the stale water.
    • Fill a clear plastic transfuse about ⅔ of the way with elbow room-temperature water.
    • Spread the set of 3 cups to each group.

    Procedure

    1. Fill one dropper with dispiriting inhuman water. Drone the end of the dropper about halfway into the colorless way-temperature water.

      A student uses a dropper to take a sample of room temperature water from a cup
    2. Piece observing from the side, selfsame lightly squeeze the eye dropper so that the cold water slowly flows into the room-temperature water.
    3. Fill some other dropper with yellow hot water. Poke the end of the dropper well-nig halfway into the room-temperature water.
    4. Spell observing from the side, rattling lightly clinch the dropper so that the hot water slowly flows into the room-temperature water.
    5. Record your observations happening the activity sheet.

    Expected results

    The nipping blue water volition flow down and collect at the bottom of the room-temperature water. The hot sensational water wish emanation and collect at the surface.

  3. Discuss student observations.

    Ask students:

    About cold water

    What did you notice when you placed the cold blue water in board-temperature water?
    The cold water sank in the room-temperature water.
    Is cold H2O more, inferior, operating theatre the Saami density as way-temperature water?
    Cold urine is more dense than room-temperature water.

    Roughly hot water

    What did you notice when you placed the sweltry old water in board-temperature water?
    The calorific water floated to the surface in the room-temperature urine.
    Is hot water more, less, or the selfsame density American Samoa room-temperature urine?
    Hot water is less dense than room-temperature water.
  4. Explain the difference in concentration betwixt calefactory and cold water on the molecular level.

    Project the animation Polar and Het up Body of water.

    Cold water

    Point out that the molecules of far water move slower and are a little closer together than the hot or way-temperature water. Also point out that when the water is cooled, the water floor falls slightly in the graduated cylinder.

    Ask students:

    In the animation, you saw that as water is cooled the water level goes down. Glacial water has a smaller volume, just the mass stays the same. What does this tell you about the concentration of dead water?
    Students should understand that when the molecules come together A the water is cooled, the volume decreases. But the mass of the water does not change. Students should realize that decreasing the intensity without crescendo the mass is exit to increase the density.
    How does this aid explicate wherefore cold water sinks in way-temperature water?
    The more-dense cold water sinks in the elbow room-temperature water.

    Hot water

    Comment that the molecules in the hot water are automotive faster and are a trifle farther apart than the molecules in room-temperature H2O. Make sure students observation that when the water is heated, the pee level rises slightly in the graduated cylinder.

    Ask out students:

    In the animation, you byword that as irrigate is heated the water flat rises. Red-hot water takes up many volume, just the mass girdle the same. What does this tell you about the density of hot weewe?
    Based on the brio, students should empathize that the spreading asunder of the molecules increases the volume but does not affect the mass of the water. Students should realize that increasing the volume without increasing the mass is going to step-down the density.
    How does this assistance explain why hot water floats on elbow room temperature water?
    The less-dense hot water floats on the more dense room temperature water supply.
  5. Do a demonstration to show students how thick cold water causes mixing.

    Tell students that in winter, icing can form on the tops of ponds and lakes. In the leap out when the ice melts, the cold water sinks. This causes mixing from the inferior which brings nutrients adequate to the surface. Tell students that you will model this process.

    Materials

    • Room-temperature water
    • Ice cubes
    • 2 identical unbelievable clear shaping cups
    • Small cup
    • Food coloring, any color other than yellow
    • Eye dropper
    • Sheet of plain white paper

    Procedure

    1. Fill in two long-legged clear plastic cups roughly ⅔ of the way with room-temperature water.
    2. Place about 15 drops of food colouring into a small empty cup.
    3. Apply a dropper to plunk up the food coloring. Then with kid gloves labour the dropper into the water until the tip of the dropper is stingy the bottom of the cup.
    4. Rattling gently clinch the dropper so that all of the food food colour slowly flows to the bottom of the cup. And so carefully hit the dropper to prevent food coloring from mixing into the water. (Information technology is ok if a little coloring gets mixed in the water.)
    5. Repeat steps 2–4 for the other cup of water.
    6. Gently place two ice cubes in the water in one of the cups. (Avoid agitating the piss.)
    7. Rank a piece of white paper behind each loving cup and observe.
    Two cups, one filled with ice water and a small amount of food coloring, the other filled with 15 drops of food coloring.  Both cups are viewed against a white background, created by a student holding a piece of paper behind them.

    Expected results

    The coloring in the cup with the chalk will move on up from the bottom and get down to mix throughout the water. The coloring in this cup will mix faster than the coloring in the loving cup without ice.

    Ask students:

    The food coloring heterogenous more quickly in the cup that had the ice. Use what you know about the density of water at distinct temperatures to excuse why this happened.
    Methedrine is about 0 °C and the water in the cup is astir 20 °C. As the ice melts, the water from the liquefied meth is colder than the water just about it. This colder water is also more dense, indeed it sinks to the butt. This sinking water pushes the solid food colouring prohibited of the way, causing mixing.

why do the densities of substances decrease with temperature

Source: https://www.middleschoolchemistry.com/lessonplans/chapter3/lesson6#:~:text=Heating%20a%20substance%20causes%20molecules,in%20an%20increase%20in%20density.

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