Junior Science Density Ms. Hennessey

Temperature, salinity and water densityActivity

In this activity, you will investigate the impact of temperature and salinity on water density.By the end of this activity, you will be able to:● describe how temperature affects water density● describe how salinity affects water density● offer simple explanations of how climate change may impact the oceans’

chemical and physical properties.

This is to be completed by Monday 23rd March

Background information

Cold water is denser than warm water, so it tends to sink. This is because water expands when it warms up – heat energy makes its molecules move around more and take up more space. When water cools, it contracts, becomes denser and sinks.

Seawater is denser than freshwater. This is because seawater has additional chemicals like sodium chloride (NaCl – salt) dissolved in it. Salinity (amount of Salt), temperature and depth all affect the density of seawater.

The ocean has a complex circulation system called the Global Ocean Conveyor. It moves water, heat, salt and nutrients around the world. Surface currents in the top 400 m are driven mainly by wind. Deeper currents are driven by changes in water density. Both types of currents work with the atmosphere to help shape the Earth’s climate.

Melting land ice and increased rainfall – as consequences of climate change – have the potential to disrupt the oceans’ chemical and physical properties, which will impact this complex circulation system.

Equipment required

● 2 glasses – one labelled freshwater and the other saltwater

● 2 large ice cubes, made with fresh (tap) water and a few drops of food colouring

● Fresh (tap) water at room temperature● Salt water at room temperature (approximately

7 teaspoons of salt added to 1 glass of tap water)

● Camera or other recording device

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Junior Science Density Ms. Hennessey

Instructions

1. Make up Ice cubes with food colouring (red or blue) and allow to set.2. Get two glasses label one glass freshwater and the other saltwater, and add

equal amounts of water to the appropriate glasses.3. Place an ice cube in each glass.4. Observe, photograph and discuss what is happening in each glass at regular

intervals.Questions:

Either answer in your copy or in this document.If you use your copy take a picture and sent it to me.

If you use this document – either upload to your folder or attach and return via firefly.

1. What are you trying to show with this activity?

2. What do you predict will happen?

3. As you set up the activity, what parts are set up the same? (Controlled variables)

4. What part of the activity set-up is different? (Independent Variables)

5. Why are you observing / measuring? (Dependent variable)

6. We are using the equipment to model the impacts of temperature and salinity on water density. What do the different parts of the model represent?glasses, ice, tap water, saltwater

7. Explain what you observed in each model?

8. Were the predictions correct?

Learning Review

Why did the cold water sink to the bottom of the freshwater glass?

How could you tell the cold water was sinking?

Why did the freshwater float on top of the saltwater although the freshwater was colder?© Copyright. Science Learning Hub http://sciencelearn.org.nz

Junior Science Density Ms. Hennessey

What will the addition of freshwater do to the chemistry of the oceans?

What difference might this make to the Global Ocean Conveyor?

Why is climate change a factor in these changes?

© Copyright. Science Learning Hub http://sciencelearn.org.nz