Unit 2: Mixtures and Solutions

Disciplinary Core Ideas, Science & Engineering Practices, Crosscutting Concepts, Supporting Standards:

• 5-PS1-1

• 5-PS1-2

• 5-PS1-3

• 5-PS1-4

• 3-5-ETS1-1

• 3-5-ETS1-2

• 3-5-ETS1-3

Concepts & Skills:

1. Mixtures: Students make mixtures of water and solid materials and separate the mixtures with screens and filters. They find that water and salt make a special kind of mixture, a solution, which cannot be separated with a filter but only through evaporation. Students are challenged with a problem: how to separate a mixture of three dry solid materials.

Students will know:

• A mixture is two or more materials intermingled.

• An aqueous solution is a mixture in which a substance disappears (dissolves) in water to make a clear liquid.

• Mixtures can be separated into their constituents by using screens, filters, and evaporation.

• The mass of a mixture is equal to the mass of its constituents.

• Possible solutions to a problem are limited by available materials and resources (constraints).

• The success of a designed solution is determined by considering the desired features of a solution (criteria).

2. Models & Conservation of Mass: Students make multi-sensory observations of sealed black boxes in an effort to determine what is inside. They develop models and try to reach consensus with other students who investigated the same boxes. Students construct physical models of black boxes in an effort to replicate the behaviors of the original black boxes. Students investigate melting and freezing in terms of models and conservation of mass and clarify the difference between the processes of melting and dissolving.

Students will know

• Models are explanations of objects, events, or systems that cannot be observed directly.

• Models are representations used for communicating and testing.

• Developing a model is an iterative process, which may involve observing, constructing, analyzing, evaluating, and revising.

• Dissolving is an interaction between two (or more) substances: a solute which dissolves, and a solvent, which does the dissolving and into which the solute disappears.

• Melting is a change in a single substance from solid to liquid caused by heat (energy transfer).

• The amount of matter is conserved when it changes form.

3. Solutions: Students observe and compare soft-drink solutions that differ in the amount of powder (water held constant) and in the amount of water (powder held constant) in order to develop the concept of concentration. They make salt solutions of different concentrations and compare them, using a balance. Students determine the relative concentrations of three mystery solutions made from the same solid material by comparing the mass of equal volumes of the solutions. Finally, students layer salt solutions to determine their relative concentrations.

Students will know

• Concentration is the amount of dissolved solid material per unit volume of water.

• Solutions with a lot of solid dissolved in a volume of water are concentrated; solutions with little solid dissolved in a volume of water are dilute.

• When equal volumes of two salt solutions are weighed, the heavier one is more concentrated.

• Density is mass per unit volume.

• More concentrated salt solutions are denser.

• Solutions form layers based on density.

4. Solubility: Students make a saturated solution by adding salt to water until no more salt will dissolve. They also make a saturated Epsom salts solution. Using a balance, they compare the solubility of the two solid materials by comparing the mass of the salt and Epsom salts dissolved in the saturated solutions. Students use the property of solubility to identify an unknown material. They analyze local water samples, using separation techniques and design a way to remove salt from ocean water.

Students will know

• A substance is a single, pure material.

• Solutions are composed of a solvent (liquid) and a solute (solid), which is dissolved in the solvent.

• Solubility is the property that indicates how readily a solute dissolves in a solvent.

• A solution is saturated when as much solid material as possible has dissolved in the liquid.

• Solubility varies from substance to substance.

• Substances form predictable, identifiable crystals.

• Engineers plan designs, select materials, construct products, evaluate results, and improve ideas.

5. Chemical Reactions: Students make three solutions with water, calcium chloride, baking soda, and citric acid. They systematically mix pairs of those solutions and observe changes that occur. The changes (formation of a gas and a white precipitate) are identified as evidence of a chemical reaction. Students repeat the reactions in sealed zip bags to observe the volume of gas produced.

Students will know

• Some mixtures result in a chemical reaction.

• During reactions, starting substances (reactants) change into new substances (products).

• A gas or precipitate is evidence of a reaction.

• Some products of reactions are soluble and can be identified by crystal structure after evaporation.

• Calcium carbonate reacts with acid.