Summary

In this demonstration, students will observe salt dissolving in water and participate in a think-pair-share activity using teacher-led questions. It is intended to be an introduction to solutions, particularly saturation.

Grade Level

High School

NGSS Alignment

This demonstration will help prepare your students to meet the performance expectations in the following standards:

• Scientific and Engineering Practices:
  • Developing and Using Models
  • Constructing Explanations and Designing Solutions

Objectives

By the end of this demonstration, students should be able to

• Define saturated, unsaturated, and supersaturated.
• Predict how changes in quantities of solute and solvent will affect solution concentration.

Chemistry Topics

This demonstration supports students’ understanding of

• Solutions
• Concentration
• Saturation
• Solute & Solvent
• Solubility

Time

Teacher Preparation: 5 minutes

Lesson: 10-25 minutes (may vary depending on depth of discussion)

Materials

• Large beaker or cup
• About a cup (300 g) of table salt, NaCl
• Stirring rod
• Tap water, enough to nearly fill the beaker or cup
• Notebook paper if students are drawing particle diagrams

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wear proper safety gear during chemistry demonstrations.
• Safety goggles and lab apron are required.

Teacher Notes

Activity overview:

• The intent of the demonstration is for students to use accurate vocabulary to explain a common process, salt dissolving in water.
• Carry out the demo as indicated below. Note that amounts are approximate and can easily be scaled up or down.
  • Fill a large beaker three-quarters of the way with room-temperature tap water. Place it where students can observe it.
  • Add a spoonful of sodium chloride, NaCl, and stir it until it is dissolved. (It is important that all of the salt dissolves.)
  • Add a cup of sodium chloride and stir it for about a minute. (It is important that a visible amount of salt remains at the bottom of the beaker.)

• Teachers should present one question at a time for students to think about individually (~1 minute), talk about in pairs or small groups (~2 minutes), then volunteer or be randomly selected to share with the class.
• Questions are included in a bulleted list below. Each may be presented verbally, written on the board, or projected with a document camera.
• Questions:
  • Is the solution saturated, unsaturated, or supersaturated? Was there a point when it was something different? If so, when?
  • In your own words, define saturated, unsaturated, and supersaturated.
  • Draw a particle diagram of the components in the beaker after the demonstration.
  • Average ocean salinity is reported as 3.5% (mass salt/mass ocean water x 100%). A saturated NaCl solution is 28% sodium chloride by mass at 25^oC.
    Is ocean water saturated? Why or why not?
  • Discuss 3 reasons that 3.5% is an average and may vary from sample to sample.
  • What would happen if water was added to a saturated solution? What would happen if water was added to the solution at the front of the room?
  • What would happen if some of the solvent, water, evaporated? Draw a particle diagram and compare it to your original drawing.
  • Global warming is reducing the size of the polar ice caps. A student makes the following claim: With the melting of polar ice caps, ocean salinity increases. Do you agree or disagree with the claim? Justify your reasoning.
• There is no specific student handout for the activity. Students should have their notebooks out in order to write down information that is new to them and to draw particle diagrams. Otherwise, only verbal answers to questions are necessary.
• Depending on students’ prior knowledge, some of the content may be familiar to them; however, their use and accuracy of vocabulary terms may not be sufficient. The teacher should listen for common misconceptions during the think-pair-share and address them as necessary.
• Teachers can use just a few questions or all questions depending on lesson scope and time limits.
• An optional formative assessment is to choose a question from the discussion to have each student answer on a sheet of notebook paper and submit for checking.

Prior knowledge:

• Students should be familiar with the terms solute, solvent, solution, and concentration prior to the demo. If they have not learned these terms previously, introduce them before the demo and identify salt as the solute and water as the solvent.
• While it is not necessary for students to understand percent concentration, it will be introduced as a means of comparison of concentration.
• Two of the questions assume that students know that ionic compounds dissociate when dissolved.
• Some vocabulary terms/definitions that will likely be part of discussions are listed below.
  • Solubility: Amount of solute that can be dissolved in a given amount of solvent at a certain temperature.
  • Saturated solution: A solution where the maximum amount of solute has been dissolved in a given amount of solvent at a certain temperature.
  • Unsaturated solution: A solution where less than the maximum amount of solute has been dissolved in a given amount of solvent at a certain temperature.
  • Supersaturated solution: An unstable mixture (not a true solution) where it appears that more than the typical maximum amount of solute has been dissolved in a given amount of solvent at a certain temperature. (Supersaturated solutions can be prepared by heating a solution, dissolving excess solute, and cooling it slowly without disturbing. When disturbed, a supersaturated solution will yield a saturated solution with excess solute at the bottom.)

• Answers to Questions:
  • At the end of the demo, is the solution saturated, unsaturated, or supersaturated? Was there a point when it was something different? If so, when?Support your answer with evidence.
    The solution is saturated and there is excess solute at the bottom. (Common misconception is that the solution is supersaturated, but that would only be true if the solution was heated, extra solute dissolved, and then it was cooled back to room temperature without disturbing.)
    
    Yes, when only a spoonful of salt was added, the solution was unsaturated.The evidence is the more solute was able to be dissolved (some of the large amount of salt that was added did dissolve).
  • In your own words, define saturated, unsaturated, and supersaturated.
    Answers will vary, but definitions are above in teacher notes.Commonly, students will leave out portions of the definitions, such as in a given amount of solvent, and at a certain temperature.
  
  • Draw a particle diagram of the components in the beaker after the demonstration. You do not need to show water molecules.
    The dissolved salt should be dissociated (broken apart into its ions) and the excess solute should be represented as its crystal lattice (difficult to show in 2-D).
  • Average ocean salinity is reported as 3.5% (mass salt/mass ocean water x 100%).A saturated NaCl solution is 28% sodium chloride by mass at 25^oC. Is ocean water saturated? Why or why not?
    Ocean water is unsaturated because 3.5% is less concentrated than the maximum, 28%.
  • Discuss 3 reasons that 3.5% is an average and may vary from sample to sample.
    Answers will vary. Potential responses include: temperature differences, geography (proximity to fresh water runoff), climate (rainfall, evaporation rates), depth of sample (cold, dense salt water is located nearer the bottom).
    
    Fun facts: Salinity is not limited to sodium and chloride ions, though they are the most prevalent; sulfate, magnesium, potassium, and calcium ions also contribute. The Atlantic Ocean has higher salinity than other oceans due to greater rate of evaporation than rainfall/river runoff.
  • What would happen if water was added to a saturated solution?What would happen if water was added to the solution at the front of the room?
    The solution would become unsaturated.
    
    More solute would dissolve.The solution would remain saturated (assuming excess solute remains), though the total volume of solution increases.
  
  • What would happen if some of the solvent, water, evaporated?Draw a particle diagram and compare it to your original drawing.
    If solvent was evaporated, more NaCl will precipitate out (since less solute can be dissolved in proportionally less solvent). The solution itself will remain saturated.
    
    The 2^nd particle diagram should have a lower solution volume, with a greater excess solute volume.As before, the dissolved salt should be dissociated and the excess solute should be represented as its crystal lattice. (Quantitatively speaking, the number of dissolved ions should be proportional to the remaining volume to show that the concentration of the saturated solution remains unchanged.)
  • Global warming is reducing the size of the polar ice caps. A student makes the following claim:With the melting of polar ice caps, ocean salinity increases. Do you agree or disagree with the claim? Justify your reasoning.
    Answers will vary. If the ONLY consideration is melting of polar ice caps, the salinity would decrease because polar ice caps are mostly fresh water.