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Science Activity: How Do Temperature and Salinity Affect Density?

written by: Marlene Gundlach • edited by: Benjamin Sell • updated: 1/5/2012

This science activity allows students to study the effect that temperature and salinity have on the density of water.

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    Prelab Discussion

    Have students read the entire activity and discuss the following points before beginning:

    • Emphasize the importance of pouring carefully. Point out that some liquid is likely to spill during this activity.
    • Advise students to carefully observe the initial movements of the different types of water as the food coloring will eventually spread throughout the water.
    • Ask students to predict the results of this activity.

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    Materials and Preparation

    Materials needed:

    • two large drinking glasses
    • two small drinking glasses
    • labels
    • pen
    • warm and cold tap water
    • blue food coloring
    • dropper
    • salt
    • plastic spoon
    • stir stick
    • data table

    Number the labels 1, 2, 3, and 4. Label the large drinking glasses 1 and 2. Label the small drinking glasses 3 and 4.

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    Procedure

    1. Make copies of the data table to record observations.
    2. Add warm tap water to glass 1 until half full.
    3. Add cold tap water to glass 3 until it is nearly full. Add three drops of food coloring and stir.
    4. Slowly pour the water in glass 3 down along the inside of glass 1. Record your observations.
    5. Add warm tap water to glass 4 until it is nearly full. Add three drops of food coloring and 5 spoonfuls of salt. Stir well.
    6. Add warm tap water to glass 2 until it is half full. Add 2 spoonfuls of salt. Stir well.
    7. Slowly pour the water in glass 4 down the inside of glass 2. Record your observations in the data table.

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    Analysis

    1. Which was more dense, the water in glass 1 or the water in glass 3? Explain. (The water in glass 3 was more dense because it sank below the water in glass 1.)
    2. Why did the water in glass 1 have a different density than the water in glass 3? (The water in glass 1 was warm, so it was less dense than the water in glass 3.)
    3. Which was denser, the water in glass 2 or the water in glass 4? Explain how you know. (The water in glass 4 was more dense because it sank below the water in glass 2.)
    4. Why did the water in glass 2 have a different density than the water in glass 4? (Glass 2 had less salt.)

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    Ideas to Extend the Activity

    Write a short paragraph explaining the relationship between temperature and density. Then write another paragraph explaining the relationship between salinity and density. Relate your conclusion to the movements of deep currents in the ocean.

    For an extension activity, imagine you had another small glass with 8 spoonfuls of salt dissolved in it. Suppose you poured this solution into glass 2 after completing step 6 of the procedure. Make a hypothesis about what would happen. Test your hypothesis.