Temperature, salinity and water density
Cold water is denser than warm water, so it tends to sink. Seawater is denser than freshwater. Salinity, temperature and depth all affect the density of seawater.
Investigating water density
Temperature and salinity affect water density. Cold water is denser than warm water. Freshwater is less dense than 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. Learn more in the article Climate change, melting ice and sea level rise.
By the end of this activity, students should 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.
Download the Word file (see link below) for:
background information for teachers
equipment list
teacher instructions
extension ideas/prompting questions for teachers.
Nature of science
In science, models are a visual way of linking theory with experiment. In this activity, students are able to visualise differences in water density. This fits into the investigating in science aspect of the NZC.
Related content
Explore the big ideas that underpin floating and sinking in the article Building Science Concepts: Floating and sinking.
Read the articles Ocean salinity and Ocean density to learn about the vital role salinity and density play in ocean currents and heat circulation.
Activity ideas
Floating eggs – students investigate water density by floating an egg in freshwater and saltwater.
Buoyancy in water – students make a Cartesian diver to demonstrate the relationship between volume, mass and density.
Investigating seawater – students investigate some of the properties of seawater.
