Floating Boats

Essential Question
How does salinity affect density?

Objectives

 * Explain the correlation between salinity and density
 * Predict the effect of added weight to the boat.

Introduction & Student Background
What causes objects to sink or float in water? Is it size or weight? If so, how can a person or a ship float but a phone sink? Density is how heavy something is in comparison to its size, or how spread out that weight is. A phone is compact, so it sinks.

This lesson provides an awesome opportunity for you to mess with the students’ heads before you start the project. Getting students surprised and confused and then helping them to understand what is going on is a very good tool for student learning.

Maybe start with 2 tubs, one with fresh water and the other with very salty water. Then, before you meet with the students, figure out what number of pennies will allow the boat to float in freshwater yet sink in salt water.

Start your demonstration, by holding your boat above the tub of salt water. Ask the students if they think is is going to sink or float. have them try to explain why they think that. Then have one student place the boat in the tub and watch it float.

Now take the exact same boat and hold it above the tub of fresh water (of course at this point they will not know that there is any difference in the water). have the second kid place the same boat in the water and watch it sink. Now they are ready to learn about buoyancy.

Vocabulary

 * Salinity - Salt concentration
 * Freshwater - Water with less than 0.05% salt concentration. Typically found in lakes, rivers, and streams.
 * Brackish - Water with 0.05% to 3.0% salt. It often results from the mixing of freshwater and seawater, such as in estuaries.
 * Saline - Water with 3.0% to 5.0% salt concentration. Typically found in seawater.
 * Brine - Water with a concentration of greater than 5.0%.
 * Density - Mass over volume -- how light/heavy an object is with respect to its weight.
 * Buoyancy - the ability or tendency to float in water or air or some other fluid.

Materials

 * Table Salt
 * Graduated Cylinder
 * Beakers
 * Tub (X Liters?)
 * Aluminum
 * Pennies
 * Tap water
 * Stickers/thick markers
 * Butcher paper

PREP

 * 1) Fill two tubs with tap water.
 * 2) Put four tablespoons of salt into one of the tubs and label it 'salt water'. Label the other tub 'fresh water'.
 * 3) Create a model or extra supply of box-shaped aluminum boats ahead of time.
 * 4) Create a large grid for a graph. Have the horizontal axis be salt concentration (depending on how many tubs/different concentrations you used) and the vertical axis be number of pennies.

ACTIVITY

 * 1) Explain how buoyancy and density work to have objects float (see introduction.)
 * 2) Help students create their own aluminum boat by giving them a model to base it off of.
 * 3) Have the student put the boat in one of the tubs and place one penny in the boat at a time. Count how many pennies it takes for the boat to sink to the bottom of the tub.
 * 4) Repeat penny adding for the other tub.
 * 5) Have students reflect on the trend between salinity and density by using a sticker or marker to mark how many pennies their boat held in each tub.

How it fits to Standard

 * Carry out investigations by using instruments, observing, recording, and drawing evidence-based conclusions.

Follow Up
What other factors would affect the number of pennies held? Have students try to figure out the controlled variables and change the shape of the boat to test again.