The process of forming a solution can be affected by a number of variables. Larger pieces of material versus smaller pieces may dissolve in a shorter or longer period of time. The temperature of the solvent may affect the rate at which a material dissolves. Finally, the amount that the solvent is stirred may also change the rate at which a solution forms. In this lab you will design experiments to explore the effects of these variables and answer the following objective questions.
Be prepared to make graphs for all of these objectives. In each case the rate of dissolution is your dependent variable and belongs on the y-axis. The variable you are testing is your independent variable and belongs on the x-axis.
A solution is a homogeneous mixture of two or more substances. These are most often liquids in which a solvent (e.g., water) dissolves a solid solute (e.g., kool-aid powder). Certain substances will dissolve readily in another and it is said that “like dissolves like.” This saying refers to polarity. Water is a polar substance and will easily dissolve other polar substances such as salt and sugar. Water will not dissolve a non-polar substance like vegetable oil—no matter how much you stir, the oil will remain in suspension. Would you expect oil to dissolve salt or sugar?
A molecule or substance is said to be polar when atoms form a covalent bond in which the electrons are not shared equally. This causes one end of the molecule to be slightly positive, and the other to be slightly negative. We can think of this molecule as a mini-magnet with two poles. When water dissolves another polar substance, opposites attract—the negative side of the water molecule is attracted to the positive side of the solute molecule or the positive side of the water molecule is attracted to the negative side of the solute molecule. As water molecules come into contact with a substance the charged poles of the H2O molecules will pull the poles of other molecules until all the two substances are evenly mixed and dispersed into a solution. The more often water molecules strike the molecules in the solid and the harder they strike them the faster the solid dissolves.
This process can happen at various speeds depending on the conditions. The purpose of this lab is to explore how and why the rate of dissolution can be affected by variables such as the size of a chunk, agitation (or stirring) and temperature.
The procedure for this lab is mostly up to you. For each objective you must design and carry out experiments to answer the objective question(s). Keep the following in mind as you plan your work:
The lab is broken up into sections on the following pages. Complete each section and check in with your teacher to get his/her initials.
Here are a few things you need to know how to do in order to be successful.
This lab generally presents few hazards except when heating water with the burner. Nevertheless, please follow the safety precautions below:
A formal lab report is required. See the last page of this lab packet for guiding questions for the Analysis section of your report.
Even substances very soluble in water, such as sugar, will not dissolve very much when not stirred. In order to answer the question about whether chunk size (sugar tablet, granulated, ground-up) affects the rate of dissolving simply do experiments for part two on all three sizes. If chunk size affects dissolving rate you should see that in your data comparing stirring rates. For a comparison between stirred water and un-stirred water measure out an amount of sugar and add it to water you have allowed to come to rest in a beaker and do not stir it. Once this is done move directly to Objective Two. Record your observation of this beaker at the end of the lab period and be sure to refer to it in your report. Try placing about 15 g granulated sugar into a beaker containing about 200 mL of water. Let the water come to rest before adding the sugar and do not stir it. Write what you see has happened in this beaker at the end of the lab period.
Write your observations here:Stirring the water has a measurable effect on the amount of time it takes for the sugar to dissolve. Shaking counts as agitation, too, but may be difficult to do with the lab equipment available. Design a quantifiable way to stir the water: controlling how many times around per minute may be a way to start. Don’t forget to control for chunk size as well. Will it help to stir when you drop the whole tablet in? Does it help proportionally more when the chunk size is smaller? Use at least three different stirring speeds and three different chunk sizes (tablet, granulated and ground up).
Write down what you are holding constant in the space below. Specify amounts.
Write down the steps of your experiment below and show them to your teacher before proceeding.
Create a data table in the space below. Beneath your data table record your observations and notes about your procedure. Use a separate sheet of graph paper to make a graph of your data.
When you have completed your work in this section check in with your teacher. This is a required part of the lab and your teacher’s initials are required before you can move on to the next objective. Initials will be given for good data, observations and procedural notes.
Sugar may dissolve at different rates in cool, warm or hot water. Is there a quantifiable relationship between temperature and rate of dissolution? Try at least three different temperatures (iced water, room temperature, and hot). Do stir your mixtures but be sure to do so in the same way for each one. If there is time to do so then try different sizes of chunk to see whether size or temperature is more important.
Write down what you are holding constant in the space below. Specify amounts.
Write down the steps of your experiment below and show them to your teacher before proceeding.
Create a data table in the space below. Beneath your data table record your observations and notes about your procedure. Use a separate sheet of graph paper to make a graph of your data.
When you have completed your work in this section check in with your teacher. This is a required part of the lab and your teacher’s initials are required before you can move on to the next objective. Initials will be given for good data, observations and procedural notes.
Answer the following questions in numbered question and answer format. Turn in these answers with your data tables and lab notes. Re-write your data so that it is clear and easy to read.
Analysis: