Question

Salt dissolves in water. But as salt is constantly added to the water, at some point

salt appears at the bottom of the vessel. Why does this happen?​

Answer 1

Answer:

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Answer 2

Explanation:

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Key Concepts

The polarity of water molecules enables water to dissolve many ionically bonded substances.

Salt (sodium chloride) is made from positive sodium ions bonded to negative chloride ions.

Water can dissolve salt because the positive part of water molecules attracts the negative chloride ions and the negative part of water molecules attracts the positive sodium ions.

The amount of a substance that can dissolve in a liquid (at a particular temperature) is called the solubility of the substance.

The substance being dissolved is called the solute, and the substance doing the dissolving is called the solvent.

Summary

Students will make a 2-D model of a salt crystal and use water molecule cut-outs to show how water dissolves salt. After seeing an animation of water dissolving salt, students will compare how well water and alcohol dissolve salt. They will relate their observations to the structure of salt, water, and alcohol on the molecular level.

Objective

Students will be able to explain, on the molecular level, why water can dissolve salt. Students will be able to identify the variables in their experiment. Students will also be able to explain why a less polar liquid, such as alcohol, is not good at dissolving salt.

Evaluation

Download the student activity sheet, and distribute one per student when specified in the activity. The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan.

Safety

Be sure you and the students wear properly fitting goggles. Isopropyl alcohol is flammable. Keep it away from flames or spark sources. Read and follow all warnings on the label. Alcohol should be disposed of according to local regulations.

Materials for Each Group

Construction paper, any color

Scissors

Tape or glue

Water

Isopropyl alcohol (70% or higher)

Salt

Balance

2 clear plastic cups

2 small plastic cups

Graduated cylinder

About the Materials

You may choose to laminate the water molecules, sodium ions, and chloride ions located on the last page of the activity sheet so that you can reuse them with your students next year.