Solubility Races
Introduction
The natural world is filled with problems which we are compelled to solve and phenomena which we find puzzling. We have questions about these phenomena, and scientists attempt to investigate the natural world in search of explanations. Science is the field of study that aims to provide explanations and answers to the many questions we have about our world.
Concepts
- Scientific method
- Independent/dependent variables
- Solubility rate
- Factors affecting solubility rate
Background
The scientific method is a way of solving problems using a systematic approach. A hypothesis, a possible explanation of a problem or question, is formulated based on a series of observations, and then the hypothesis is tested by means of experiments designed to either support or invalidate the hypothesis. Based on experimental evidence, a theory is proposed to account for initial observations. If the theory fails to explain further observations, a new hypothesis may be made and tested, leading to a refined theory. In a sense, scientific knowledge is continually changing and becoming more reliable as we gather more information and test new hypotheses.
An organized strategy, such as the scientific method, is an effective way of approaching a problem. A wide variety of strategies are described in the literature and the following is a list of “typical” steps that scientists may use to solve a problem. Keep in mind, however, that the strategy and the order of steps may vary greatly from problem to problem.
Typical Steps in the Scientific Method
- Define a problem or ask a question—A clear statement of the problem or question is a crucial step in beginning an investigation.
- Make observations about the problem—Gathering as much information as possible on the problem will be helpful in writing a plausible hypothesis and in designing a good experiment.
- Develop a hypothesis—This is a possible answer or tentative explanation to the problem or question. It should be based on the facts and observations, and should be capable of being tested.
- Design and implement an experiment—Experimental testing will provide evidence which either supports or contradicts the hypothesis.
- Record and analyze data—Data, such as observations and measurements, are recorded and then analyzed. If the data support the hypothesis, then the conclusion would state that the hypothesis is correct. If the data contradict the hypothesis, then a new hypothesis must be made and tested.
- Draw a conclusion—Scientists base their conclusions on observations made during experimentation. When a hypothesis has been tested many times and has proven to be correct, it becomes a theory. However, a theory is still not a law. Continued testing and acceptance by the scientific community leads to a theory becoming a scientific law.
For an experiment to be considered valid, it must meet two important requirements:
- Only one variable is tested at one time and it is the only one that will produce the observed results. All other factors in the experiment must be held constant.
- It is designed in such a way that any other scientists can repeat the experiment and get similar results.
Variables are the factors that influence the outcome of an experiment. Only one factor should be varied during an experiment. All other factors should remain the same throughout an experiment. The factors that remain the same are called constants. A controlled experiment is an investigation in which the factors that influence the outcome are kept the same except for the one whose effect is being studied. The independent variable is the variable that is intentionally changed or manipulated by the experimenter ( graphed on the x -axis); whereas, the dependent variable is the variable being measured or watched, sometimes called the outcome or the responding variable ( graphed on the y -axis). This activity uses the scientific method to determine the solubility of sodium chloride (salt). Solubility is the maximum amount of solute that will dissolve in a given amount of solvent under a fixed set of conditions. There are many variables which affect the solubility rate of a solid in a liquid such as temperature, particle size, stirring and amount of solute already dissolved.
Experiment Overview
Using either a self-designed or a standard procedure, determine the factors that affect the solubility rate of a solid in a liquid by following the steps of the scientific method.
Materials
Sodium chloride, NaCl, fine crystals, 12 g Sodium chloride, powdered, NaCl, 12 g Sodium chloride, rock salt, NaCl, 12 g Water, tap Balance, 0.1-g precision Beaker, 250-mL Beaker, 600-mL (or any large beaker) Graduated cylinder, 100-mL Graph paper Hot plate Stirring rod Stopwatch or timer Thermometers, 2 Tongs, beaker Weighing dishes, 3
Prelab Questions
- Choose one variable (such as temperature, particle size, stirring or amount of solute already dissolved) to address the problem statement “what factors affect the solubility rate of a solid in a liquid?”
- Record the variable on the Solubility Races Prelab Worksheet.
- Make an informed guess (hypothesis) about how this factor will affect the rate of solubility of a solid in a liquid.
- Record the hypothesis on the Solubility Races Prelab Worksheet.
- Make a preliminary list of materials to be used to test the hypothesis.
- List materials on the Solubility Races Prelab Worksheet. Write a specific list of all materials needed, including sizes, amounts, etc.
- Design an experiment, using common household and laboratory materials, to test how that factor affects the solubility rate of a solid in a liquid. Design an effective experiment which will test your hypothesis. Include potential hazards as well as safety precautions in the laboratory procedure.
- Outline the experimental procedure on the Solubility Races Prelab Worksheet. Write a clear, specific, step-by-step, repeatable procedure. Note: Include the independent variable (the one factor which is intentionally changed between the setups), the dependent variable (the factor which will be measured), and draw a rough sketch of the experiment. Also include any necessary safety procedures.
- Construct an empty data table on the Solubility Races Prelab Worksheet where results from this experiment may be written if this experiment is performed. Clearly label all data columns. Include units where necessary. Leave space to write data for three trials and leave a column for averages.
Safety Precautions
Wear chemical splash goggles and use caution when working with a hot plate and hot glassware. Wash hands thoroughly with soap and water before leaving the laboratory.
Procedure
Note: Consult the instructor about using the student-designed procedure or using one of the standard procedures before beginning the laboratory activity.
Student-Designed Procedure
- Gather the items needed to perform the experiment which your group designed in the Prelab Activity section. (Obtain teacher approval before proceeding.)
- Run the student-designed experiment.
- Record your data in the data table. Note: This is the student-designed table on the Solubility Races Prelab Worksheet.
- Repeat the experiment three times and average the results.
- Construct a graph of the averaged data. Note: The independent variable should be plotted on the x-axis and the dependent variable on the y-axis. Label the axes with the variables and the units as needed. Draw a best-fit line, if appropriate.
- Write a conclusion based on the data gathered from the experiment. The data will either support or contradict the hypothesis.
- List the possible sources of error in this experiment and list ways that the procedure may be improved to give more reliable data. Note: During any experiment, it is good practice to record if and where any errors or inconsistencies may have occurred and whether they are equipment-related, user-related, or procedure-related.
Standard Procedure—Particle Size
- Observe the physical characteristics of the three samples of NaCl—powder, fine crystal, coarse crystal (rock salt).
- Make as many observations about the samples as possible. Observe color, texture, crystal size, etc.
- Record these observations on the Solubility Races Worksheet.
- Using the information just gathered about the three samples, make an informed guess (hypothesis) about how particle size will affect the solubility rate of the solid in a liquid.
- Record the hypothesis on the Solubility Races Worksheet.
- Using a graduated cylinder, measure precisely 100 mL of tap water into a 250-mL beaker. Water temperature should be approximately room temperature (20 °C). Using a thermometer, measure the actual water temperature and record this temperature in the Solubility Races Worksheet Data Table.
- Use a balance to weigh out 2.0 grams of powdered sodium chloride.
- Add the sodium chloride to the water, starting the stopwatch as it is added.
- Stir the sodium chloride and water with a stirring rod at an even rate and record the length of time needed for all of the solid to dissolve in the Solubility Races Worksheet Data Table.
- Repeat steps 6–9 two more times using the powdered sodium chloride.
- Record the data on the Solubility Races Worksheet. Average the three trials and record in the table.
- Repeat steps 6–11, this time using the fine crystal sodium chloride.
- Record your data in the Solubility Races Worksheet Data Table. Average the three trials and record in the table.
- Repeat steps 6–11, this time using the coarse crystal sodium chloride.
- Record your data in the Solubility Races Worksheet Data Table. Average the three trials and record in the table.
- List the independent variable, dependent variable, and all constants on the Solubility Races Worksheet.
- Construct a bar graph of the averaged data. Note: The independent variable should be plotted on the x-axis and the dependent variable on the y-axis; Label the axes with the variables and the units as needed.
- Write a conclusion on the Solubility Races Worksheet based on the data gathered from the experiment. Was the hypothesis correct? The data will either support or contradict the hypothesis.
- List the possible sources of error in this experiment and list ways that the procedure may be improved to give more reliable data. Note: During any experiment, it is good practice to record if and where any errors or inconsistencies may have occurred and whether they are equipment-related.
Standard Procedure—Temperature
- Fill a large beaker with approximately 500 mL of tap water.
- Turn on the hot plate and heat the water to 75 °C.
- Make an informed guess (hypothesis) as to how temperature will affect the solubility rate of the solid in a liquid.
- Write the hypothesis on the Solubility Races Worksheet.
- Using a graduated cylinder, measure precisely 100 mL of tap water into a 250 mL beaker. Water temperature should be approximately 75 °C. Using a thermometer, measure the actual water temperature and record it on the Solubility Races Worksheet.
- Use a balance to weigh out 2.0 grams of powdered sodium chloride.
- Add the sodium chloride to the water, starting the stopwatch as it is added.
- Stir the sodium chloride and water with a stirring rod at an even rate and record the length of time needed for all of the solid to dissolve in the Solubility Races Worksheet Data table.
- Repeat steps 5–8 two more times using the powdered sodium chloride.
- Average the three trials and record in the table.
- Repeat steps 5–10, this time using the fine crystal sodium chloride.
- Average the three trials and record in the table.
- Repeat steps 5–10, this time using the coarse crystal sodium chloride.
- Average the three trials and record in the table.
- List the independent variable, dependent variable, and all constants on the Solubility Races Worksheet.
- Construct a bar graph of the averaged data. Note: The independent variable should be plotted on the x-axis and the dependent variable on the y-axis; Label the axes with the variables and the units as needed.
- Write a conclusion based on the data gathered from the experiment. Was the hypothesis correct? The data will either support or contradict the hypothesis. Note: In order to examine the effect of temperature on solubility, look at the data from only one type of NaCl at a time and compare room temperature data to the hot water data.
- List the possible sources of error in this experiment and list ways that the procedure may be improved to give more reliable data. Note: During any experiment, it is good practice to record if and where any errors or inconsistencies may have occurred and whether they are equipment-related.
- Consult your instructor for appropriate disposal procedures.
|