CHEMISTRY NOW: Aqueous Systems (Grades 5-8) Print

Objective:

Students will be able to distinguish between solutions, colloids, and suspensions on the basis of their physical properties. Students will design and carry out an investigation to classify household substances on the basis of their physical properties. Students will observe and describe the Tyndall effect.


Introduction Notes:

CHEMISTRY NOW: Aqueous Systems

 

Subject Area: Chemistry 

Grade Level:  Middle School Chemistry

Lesson Title:  Aqueous Systems

National Science Education Standards: 
Science as Inquiry: 5–8

Properties and Changes of Properties: 5

 

Suggested Prior Knowledge: concepts of mixtures, pure substances

 

Purpose: To give students an understanding of the properties of different types of aqueous mixtures: solutions, colloids, and suspensions

Key Vocabulary:  

aqueous solution—homogeneous mixture in which a solute is dissolved in a solvent; the solute cannot be filtered out and does not settle out. These do not exhibit the Tyndall effect

colloid—heterogeneous mixture in which the dispersed particles are too large to dissolve but are large enough to be filtered out of the dispersing medium.These exhibit the Tyndall effect

heterogeneous—matter with parts that do not appear uniform (the same) in composition throughout

homogeneous—matter with parts that do appear uniform in composition throughout

mixture—two or more substances physically mixed together

suspension—heterogeneous mixture in which the dispersed particles are so large that they not only can be filtered out, but will also settle out of the dispersing medium. These will also exhibit the Tyndall effect

Tyndall effectscattering of light by minute particles in a mixture that results in a visible light beam showing in the mixture

Objectives:

  1. Students will be able to distinguish between solutions, colloids, and suspensions on the basis of their physical properties.
  2. Students will design and carry out an investigation to classify household substances on the basis of their physical properties.
  3. Students will observe and describe the Tyndall effect.

 

Materials:  

- Safety goggles

- Samples of known colloids, solutions and suspensions (list attached)

- Unknown aqueous samples to classify (list attached)

- Flashlight

- Clear glass containers for mixtures (small glass jars with lids work best – when closed, they can be shaken)

- Filter paper (small cone-shaped paper coffee filters work well)

- Funnels

- Ringstand with ring or clamp to hold funnel

- 250 ml beaker to use when filtering

- Distilled water to dilute mixtures as necessary

Procedure:

1. Discuss with students three basic types of aqueous systems: solutions, colloids and suspensions. Review aqueous systems, pure substances, and mixtures. Discuss the difference between homogeneous mixtures and heterogeneous mixtures.

   What kind of investigation can we design to determine which types of aqueous mixtures exhibit specific characteristic properties?

   What are some characteristics of these known solutions, colloids and suspensions?

 

2. Lab protocols should be followed incorporating safety equipment. Goggles must be worn at all times.

 

3. Guide students to design an investigation that is valid and will allow them to observe the physical properties of the previously classified mixtures. They will use their observations of these known mixtures to develop guidelines for differentiating between the three basic aqueous systems. Once the students have used the known mixtures to develop their guidelines, they can work to classify the unknown mixtures as solution, colloid, or suspension.

 

4. Some questions to help focus students’ plans and guide them include:

   Does a solution appear the same as a colloid or suspension? [A solution and a colloid appear uniform while a suspension does not.]

   In what ways does a colloid appear different from a solution?[Colloids will exhibit the Tyndall effect while solutions will not.]

   In what ways does a suspension behave differently from a solution or colloid? [Dispersed particles will settle out of a suspension, but not a solution or colloid.]

   In what ways does a solution behave differently from a colloid or a suspension? [The dispersed particles of a solution cannot be filtered out of the solvent.]

 

5. General Procedure to use with each mixture:

a. Obtain and wear goggles at all times.

b. Obtain samples of the known solutions, colloids, and suspensions.

c. Students may want to dilute any mixtures that are very thick with some distilled water.

d. For each known solution, colloid, and suspension (listed at the end of the lesson), carry out these same basic procedures and record your observations. Students might want to shake each mixture in a closed jar so that all particles are well dispersed.

   Appearance: Does a sample of the mixture appear uniform (the same throughout) or can different phases been seen?

   Settling: Allow a sample of the mixture to sit undisturbed for at least 10 minutes. Does the mixture separate? Does part of the mixture settle to the bottom (or top) of the system?

   Filtration: Pour a sample of the mixture through the filter paper nested in a funnel. Does the mixture separate, leaving the particles in the filter and the liquid in the 250 ml beaker under the funnel?

   Tyndall effect: Shine a flashlight beam through a sample of the mixture in a clear glass container (large beakers work well). Is the light beam visible in the system?

e. On the basis of these observations, students should develop their own guidelines for determining if an aqueous mixture is a solution, colloid, or suspension. Students can then carry out the procedure above with the unknown mixtures and use their observations to classify each as a solution, colloid, or suspension.

 

6. After the students have finished their investigations, they may present their findings to their classmates using a slide show or poster. Presentations should clearly illustrate the similarities and differences in these aqueous systems. Students might include a property for each kind (solution, colloid, and suspension) that makes it unique and an example of each that is familiar to the student.

 

7. As a follow-up to this investigation, have students bring in additional examples of solutions, colloids, and suspensions from home to share with classmates. For each example, students can explain which kind of aqueous system it represents and give evidence to support their classification.

 

8. Encourage students to compare their results with classmates.

 

Additional Resources:

         http://www.flinnsci.com/Sections/Safety/safety.asp

         http://garrisonwaterside.com/history.html

         http://en.wikipedia.org/wiki/Tyndall_effect

         http://chemistry.about.com/od/lecturenotesl3/a/colloids.htm

 

 


Classification of Dispersions

 

Mixture

Classification

Oil and vinegar salad dressing

Suspension

Gelatin

Colloid

Sodium borate in water

Solution

Starch in water1

Colloid

Smoke in water2

Suspension

Clay or silt in water

Suspension

Salt in water

Solution

Food coloring in water

Solution

Carbon dioxide in water

Solution

Steam in air

Suspension

Table sugar in water

Solution

Homogenized milk

Colloid

Tea

Solution

Whole milk

Suspension

Household bleach

Solution

Ammonia in water

Solution

 

1 This mixture can easily be made using water in which pasta was cooked.

2 Burn a birthday candle in a large glass jar and then cover the jar tightly. Candle will burn out and leave plenty of smoke in the closed jar.

 

 

 

 


Student Worksheet for Investigation of Aqueous Systems

 

 

Experiment Title: _____________________Date: __________Name: ___________________

 

Student Hypothesis: __________________________________________________________

 

Materials: _________________________________________________________________

 

_________________________________________________________________________

 

Procedure:

 

Wear Safety Goggles for all lab work.

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

 

 

 


Data and Observations:

 

Mixture

Observations

Appearance

uniform?

Filtration

separates?

Settling?

 

Tyndall effect?

Classification

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Analysis of Data: 

_________________________________________________________________________

_________________________________________________________________________

_________________________________________________________________________

_________________________________________________________________________

 

Conclusion: 

_________________________________________________________________________

_________________________________________________________________________

_________________________________________________________________________

 

What are some properties of solutions?

_________________________________________________________________________

_________________________________________________________________________

_________________________________________________________________________

 

What are some properties of colloids?

_________________________________________________________________________

_________________________________________________________________________

_________________________________________________________________________

 

What are some properties of suspensions?

_________________________________________________________________________

_________________________________________________________________________

_________________________________________________________________________

_________________________________________________________________________

 

 

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