The Scientific Method and Density: Thinking Practice

 

INTRODUCTION

 

Humans are naturally curious creatures.  One of our major evolutionary adaptations is our ability to learn from experience.  We ask questions about the world around us and within us and try to find the answers by trial and error methods.  Using this method, we make what we believe are reasonable guesses at the answers to our questions and then make observations that determine the accuracy of these guesses.  As we accumulate sets of related facts and concepts, we organize them into systems that we call theories.  Our picture of the world is made from these theories.  A major value of a good theory is that it has predictive value; it can tell us about things that we have not directly experienced.

 

The preceding paragraph describes the basic thought process in what we refer to as the “Scientific Method”.  The following outlines the steps of this method in a more formal fashion:

 

Steps of the Scientific method (with a simple example)

 

1. Observation

            Room looks completely dark

2. Questions

            Why is it dark?

3. Hypothesis

            1) Lights are off

            2) Light bulb is burnt out

            3) You are blind

            Null:   

Turning the light switch on will not increase the brightness of the room

            Alternative

                        Turning the light switch on will cause the room to become bright

4. Experiments or Collection of Data

            Independent variable – the one you will vary, the cause

            Dependent variable – the one that you measure, the effect

            Control – all other variables are kept constant

5. Conclusions

            accept or reject the null hypothesis

            not proven false if accepted but not proven as absolute truth – always open to new experimentation

Test all your hypotheses

6. Predictions

            make statements about how your results will apply to other situations

            Ex: If you conclude that you are blind, predict that the other rooms will be dark too       

 

                  


LAB OBJECTIVES:

 

1.         The main purpose of this lab is to give you the opportunity to use your critical thinking skills and what you now know about the scientific methods to answer the following research question:

 

Which has the higher density, salt water or fresh water?

 

In order to do this, you will have to first discover the density of fresh water (Question 1 below), then discover the density of salt water (Question 2 below) and compare the two.

 

2.         Once you have learned about the density of water, you can apply your knowledge and figure out how to find the density of solid objects (Question 3 below).

 

3.         Next, you can attempt to explain why boats float and how much cargo they can hold (Thought Questions below) using what you have learned about density.

 

LAB PROCEDURE:

 

An important idea in the marine world is that of density.  It has a major influence on weather, ocean currents, sediment distribution and floatation of boats and ships.  We can learn about density by making some simple observations.

 

Density is the mass of a unit of matter.  It is defined as:  

D = M/V

Where D = density; M= mass and V= volume

 

Density is typically expressed as grams per cubic centimeter (g/cm3) or grams per milliliter (g/ml)

 

Question 1

 

What is the density of fresh water?

Using the materials provided (pipettes, beakers, graduated cylinders, balances, etc.) discover the density of tap water.  First write a procedure.  Is one measurement enough or should you repeat the procedure to improve accuracy?  How many times?

 

Procedure (write out step by step):

 

 


Measurements (carefully label each measurement that you make, being sure to include units, and show all calculations) You may wish to construct a table to organize your measurements:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Question 2:

 

How does dissolved salt affect the density of water?

 

Mix a solution of salt water (10 g of salt added to each 100 ml of water) and measure its density following the procedure that you came up with for question #1.

 

Measurements:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Answer the following questions:

 

1. Was salt water more or less dense than plain water?

 

 

2. What caused the difference in density?

 

 

 


Question 3:

 

How can you discover the density of irregularly shaped solid objects?

 

Procedure:

Write out a step by step procedure to determine the density of one of the small solid objects on the instructor’s desk:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Measurements:

 

 

 

 

 

 

 

 

 

 

 

Thought Questions:

 

Apply what you have learned about density to attempt to answer the following questions.  Even if you do not know the “right” answer, make an attempt to think about the question and answer it.

 

  1. Why does a metal boat float?

 

 

 

 

 

  1. How can you find the cargo capacity of a ship?