Student Activity - Investigating sea water

Activity idea

This activity was adapted from SEREAD in collaboration with NIWA.

Density is the mass of unit volume of a substance, which is a measure of how tightly the particles of something are packed in a certain amount of space. The SI (international system) unit of density is expressed in kilograms per cubic metre. For instance, a kilogram of salt is denser than a kilogram of lead.

Have students discuss sea water, thinking about questions such as why is sea water salty, and where does the salt come from.

By the end of this activity, students should understand:

• Salt dissolves in water
• The dissolved salt does not change the volume of the water
• Since the volume of the water does not change, the density of the water has increased
• Seawater is denser than freshwater
• When it rains, the freshwater reduces the salinity of the saltwater on the surface.

Instructions

Density is the mass of unit volume of a substance, which is a measure of how tightly the particles of something are packed in a certain amount of space. For instance, a kilogram of salt is denser than a kilogram of lead.

1. Introduction to sea water

What you will need

• large measuring cylinder
• marker pen
• water
• salt

Instructions

• Fill the measuring cylinder ¾ with water.
• Mark the water level on the outside of the cylinder.
• Add one tablespoon of salt and mix until nearly all the salt is dissolved, leaving just a little bit of salt at the bottom.
• Check the water level.

Thought questions and discussion

• What happens to the water level before and after adding salt?
• If the water and salt are made up of particles, how can what has happened be explained?
• Would one litre of sea water be more/ the same/less/ dense than one litre of freshwater? Explain your answer.
• What do you think the density of sea water would be like compared to fresh water?

2. Will fresh water and sea water mix?

What you will need

• 2 x 250 ml beakers
• salt
• blue food colouring
• green food colouring
• 2 x 50 ml clear containers
• dropper

Instructions

• Half full both beakers.
• Add one tablespoon of salt to one beaker and mix until all the salt is dissolved.
• ¾ fill one 50 ml container with the salt water and add a few drops of green food colouring.
• Repeat with the other container, using the fresh water and blue colouring.
• Place a few drops of the green salt water into the fresh water beaker and observe.
• Repeat adding the blue fresh water to the beaker of salt water.

Thought questions and discussion

• Does salt water mix with the fresh water? Does it float or sink? Draw what happens to the green seawater and the blue fresh water when put into the opposite beaker. Use colours in your drawing to show what is happening.
• Can you think of a reason for what you see happening when freshwater and seawater come together?
• What do you think happens when it rains out at sea? Does the rainwater and sea water mix straight away?