Dissolving Teacher Guide

Unit

Dissolving

Subject

Chemistry

Grade Level

HS

Activity Name(s)

Dissolving Salt in Water

Dissolving and Heat

Mixing with water

Being Prepared

The "Dissolving and Heat" activity will require:

• Temperature sensor (fast response)
• Two cups
• Lukewarm tap water
• 30 g (2 tablespoons) salt
• 30 g (2 tablespoons) sugar

The activities "Dissolving Salt in Water" and "Mixing with Water" only require a computer. Having one or two students per computer is ideal. Plan significantly in advance of these activities to ensure you have access to computers (state testing requisitions, other teacher use, etc.)

Liquids can be placed lower than computers, in "Dissolving Salt in Water", so that if there is any accidental spill, then it would not spoil the computers. Also be sure to run the activity using the equipment that the students will be using to ensure everything runs smoothly.

Getting Started

The controls for the simulations are not always intuitive to new users. However, they always have very descriptive assistance provided in the text preceding the simulations. Encourage frustrated students to review the written instructions before attempting to interact with the model further.

There is no equipment required beyond a computer with sufficient power to run the simulations. In activity two, temperature sensors are used which can be hooked to the computer

Suggested Timeline

The three activities are well connected and together can fit into either two 45-50 minute period or a 90 minute block. The three activities could all be delivered separately also as about half hour period for each activity . Rest of the period could be used to involve students in a group discussion to gaze the understanding of each activity and the underlying concept.

Thinking about the Discovery Questions

This unit is motivated by the discovery questions:

• Is dissolving salt in water a chemical reaction?
• When a material dissolves in water, does it produce heat?
• What type of molecules mix easily with water?

Misconceptions

Dissolving is not a straightforward concept for students to grasp. The common misconception that students have about dissolution is they would consider it to be a chemical process since they are 'mixing' substances. For Example, when salt dissolves in water, solid salt ceases to exist. But what they do not realize is nothing new is produced in the process. So it cannot be a chemical reaction.

Learning Objectives

• NGSS
  • Performance Expectations
    • HS-PS2-6. Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
  • Disciplinary Core Ideas
    • HS-PS2: Motion and Stability: Forces and Interactions
      • PS2.B: Types of Interactions
        • Attraction and repulsion between electric charges at the atomic scale explain the structure, properties, and transformations of matter, as well as the contact forces between material objects. (HS-PS2-6),(secondary to HS-PS1-1),(secondary to HS-PS1-3)
  • Practices
    • Developing and using models
      • Develop and/or use a model to predict and/or describe phenomena.
      • Develop a model to describe unobservable mechanisms.
      • Develop and/or use a model to generate data to test ideas about phenomena in natural or designed systems, including those representing inputs and outputs, and those at unobservable scales.
    • Planning and carrying out investigations
      • Collect data to produce data to serve as the basis for evidence to answer scientific questions or test design solutions under a range of conditions.
    • Analyzing and interpreting data
      • Analyze and interpret data to provide evidence for phenomena.
    • Constructing explanations and designing solutions
      • Construct an explanation that includes qualitative or quantitative relationships between variables that predict(s) and/or describe(s) phenomena.
      • Construct an explanation using models or representations.
    • Obtaining, evaluating, and communicating information
      • Communicate scientific and/or technical information (e.g. about a proposed object, tool, process, system) in writing and/or through oral presentations.
  • Crosscutting Concepts
    • Structure and function
      • Students model complex and microscopic structures and systems and visualize how their function depends on the shapes, composition, and relationships among its parts. They analyze many complex natural and designed structures and systems to determine how they function. They design structures to serve particular functions by taking into account properties of different materials, and how materials can be shaped and used.
• NSES
  • NSES Physical Science – Chemical Reactions
    • Chemical reactions occur all around us, for example in health care, cooking, cosmetics, and automobiles. Complex chemical reactions involving carbon-based molecules take place constantly in every cell in our bodies.
    • Chemical reactions can take place in time periods ranging from the few femtoseconds (10-15 seconds) required for an atom to move a fraction of a chemical bond distance to geologic time scales of billions of years. Reaction rates depend on how often the reacting atoms and molecules encounter one another, on the temperature, and on the properties - including shape - of the reacting species.
  • NSES Physical Science – Conservation of Energy and the Increase in Disorder
    • Heat consists of random motion and the vibrations of atoms, molecules, and ions. The higher the temperature, the greater the atomic or molecular motion.
  • NSES Physical Science – Structure of atoms
    • Matter is made of minute particles called atoms, and atoms are composed of even smaller components. These components have measurable properties, such as mass and electrical charge. Each atom has a positively charged nucleus surrounded by negatively charged electrons. The electric force between the nucleus and electrons holds the atoms together.
  • NSES Physical Science – Structure and Properties of Matter
    • Atoms interact with one another by transferring or sharing electrons that are furthest from the nucleus. These outer electrons govern the chemical properties of the element.
    • Bonds between atoms are created when electrons are paired up by being transferred or shared. A substance composed of a single kind of atom is called an element. The atoms may be bonded together into molecules or crystalline solids. A compound is formed when two or more kinds of atoms bind together chemically.

Discussion: Setting the Stage

• Is dissolving salt in water a chemical reaction?

  Students at this might think it to be a physical process as explained above.

• When a material dissolves in water, does it produce heat?

  For ionic compounds, it depends on the difference in the energy required to break ionic bonds (heat absorbed) and the heat released while hydrating the ions.

• When you dissolve salt in water, what happens to the temperature?

  Temperature goes down.

• What type of molecules mix easily with water?

  Ionic Compounds.

• What every day substances do not mix with water?

  Oil.

What about when you dissolve sugar in water? Is it the same or different?

Discussion: Formative Questions

• Describe how the inter-atomic attractions are responsible for salt dissolving in water.

  When the model is run where water is added to the salt crystal, using click "Charge" button clearly shows how positives and negatives are drawn towards each other. Then on clicking "interatomic interactions" shows the attractive forces between the atoms.

• Describe why a crystal of salt formed as the water evaporated from a drop of salt solution.

  When the salt solution evaporates, the solid salt is left behind.

• What happened to the temperature when the salt dissolved?

  It decreases.

• What happened to the temperature when the sugar dissolved?

  It increased.

• What happened to the temperature with salt and sugar combined?

  No measurable change because salt dissolved in water cools the water down (endothermic) and sugar in water would increase the temperature almost neutralizing the whole effect.

• Are these reactions endothermic or exothermic?

  Salt in water is endothermic process whereas sugar in water is exothermic.

• Which would you expect to exert strong chemical forces, a polar molecule or a non-polar molecule?

  A polar molecule would be expected to exert strong chemical forces as the intermolecular forces are stronger in them

Discussion: Wrapping Up

• Is dissolving salt in water a chemical reaction?

  By now students would be able to understand that no new substance is produced during dissolution.

• What type of molecules mix easily with water?

  Like dissolves like. Since water is a polar molecule, it would be easier to dissolve ionic compounds and polar molecules. But some non polar molecules like sugar would dissolve too because of hydrogen bond formation.

Additional Background

Since dissolving and energy change due to it depends on making and breaking of bonds, it is recommended to cover the unit on bonding including intermolecular forces, bond strength and polarity.

Analysis

Dissolving Salt in Water

1. Would salt dissolve in water if the water molecules did not have charges?

   Like sugar salt might dissolve in water as long as their is attraction between the molecules than within.

2. Why do some substances like salt and sugar dissolve in water while others, such as oil and pepper, do not?

   Oil, pepper and water molecules are attracted to their own molecules than each other. Whereas salt and sugar molecules are attracted to water molecules more than their own molecules.

Dissolving and Heat

1. What happened to the temperature when the salt dissolved? How much did it change?

   The temperature should decrease.

2. What happened to the temperature when the sugar dissolved? How much did it change?

   The temperature should decrease.

3. What happened to the temperature with salt and sugar combined?

   The temperature should decrease.

4. Are these reactions endothermic or exothermic?

   Endothermic.

Mixing with Water

1. Is the statement "all liquids are able to be mixed to create a solution" true? Explain your answer.

   Answers will vary.

2. Does solubility of solids dissolving in liquids increase or decrease with rising temperatures?

   Increases.

3. Explain how attractive forces between molecules and random motion determine if one substance will dissolve in another.

   Answers will vary.

Further Investigation

• Try the same experiment with various water temperatures from ice water to hot tap water. How does this affect the dissolving process.

  From ice water to hot tap water, the dissolution would be faster.

• Try the same experiment with different quantities of salt or sugar. Does doubling the quantity double the effect?

  Doubling the quantity would not bring any changes.

• What dissolves in water? What dissolves in oil? Salt, sugar and benzene.

  Salt and sugar would dissolve in water because of the attraction between the molecules. Benzene would dissolve in oil because both are non polar.