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Solubility is an ability of a substance to dissolve.

In high school chemistry, unless you otherwise, we are dealing with solubility in water. However, in other situations we may talk about how something dissolves in some other liquid, such as alcohol.

Vocabulary

Solute – The substance being dissolved

Solvent – The liquid or gas in which the solute is dissolved

Solution – A mixture of solute and solvent

Example 

When we add sugar into water, some of the sugar will dissolve. In this process:

  • sugar is the solute
  • water is the solvent

One of the characteristics of table sugar is its solubility in water

That was a definition of solubility as it is used in a common language. Now let’s see solubility as chemists understand it:

Solubility

Solubility = maximum amount of solute that dissolves in a solvent, at equilibrium.

Equilibrium is a state where reactants and products reach a balance – no more solute can be dissolved in the solvent in the set conditions (temperature, pressure).

Such a solution is called a saturated solution.

Example: If you take one litre of water and you start dissolving table salt in it (NaCl) and:

  • temperature of water is 25oC
  • pressure is 1 ATM (Atmosphere – standard pressure in the open air on Earth)

On could dissolve up to 357.00 grams, but not a gram more.

The rest of the salt will stay on the bottom as residue and will not dissolve.

Solubility of salt in water is therefore 357.00g/L.

This is where the solution reaches equilibrium.

Every chemical substance which dissolves in water has a fixed solubility.

If a substance does not dissolve then its solubility is zero.

Here are two cases where the idea of solubility is not applicable:

Miscible and immiscible substances

Some substances, like water and alcohol, can be mixed together and create a homogenous phase in any proportion. A solubility measure cannot be applied to such two substances. Such substances are called miscible.

If two substances cannot be mixed together (like water and oil), they are immiscible.

– – – – – – – – – – – – – – –

* Like dissolves like

*Water as a universal solvent

*Electrolytes are compounds that are pulled apart by the charges on water.

* Surface tensions

* soaps and surfactants

* Okay, water isn’t really a universal solvent. Acetone works for some chemicals.

* Measuring the concentration of a solution

* Mass Percent (w/w)

* Mass/Volume percent (% w/v) format.

* Volume percent (%v/v). On this bottle it says, “Alcohol 10% by vol.”
moles/liters

* supersaturated solutions

Online textbook

Chapter 5: The Water Molecule and Dissolving

Students investigate the polarity of the water molecule and design tests to compare water to less polar liquids for evaporation rate, surface tension, and ability to dissolve certain substances. Students also discover that dissolving applies to solids, liquids, and gases.

  1. Water is a Polar Molecule
  2. Surface Tension
  3. Why Does Water Dissolve Salt?
  4. Why Does Water Dissolve Sugar?
  5. Using Dissolving to Identify an Unknown
  6. Does Temperature Affect Dissolving?
  7. Can Liquids Dissolve in Water?
  8. Can Gases Dissolve in Water?
  9. Temperature Changes in Dissolving

Lesson on Water, Solubility, Concentration, from Ken Costello

Learning Standards

College Board Standards for College Success: Science

LSM-PE.2.2.3 Observe patterns in the concentration of molecules of a solution (e.g., dye in water, tea) or across a membrane. Construct a model of the observed patterns.

LSH-PE.2.2.3 Explain why cells of organisms swell when placed in water and why they shrink when placed in a solution of salt water. Evaluate other student explanations of the same phenomenon. Construct a representation that generalizes the phenomenon to all organisms.

Objective PS.3.2: Physical and Chemical Changes of Matter

PS-PE.3.2.3 Given a solid (e.g., sugar, table salt) and water:

[BOUNDARY: Students are not expected to write chemical formulas or to balance chemical reactions.]

PS-PE.3.2.3a Determine the amount of solid that can dissolve in the water at room temperature.

PS-PE.3.2.3b Construct a representation of the change that takes place when the solid dissolves in the water.

PS-PE.3.2.3c Predict, based on the starting mass of the substances, the ending mass. Justification includes a discussion of the law of conservation of mass and of the particulate nature of matter.

C-PE.2.2.9 Investigate why the dissolution of a salt (e.g., ammonium chloride, sodium acetate) in water is difficult to classify as either a chemical or physical change.

C-PE.2.2.6 Construct atomic–molecular level representations of the solution process for both ionic and molecular species. Describe, using these representations, the process of dissolving a solute in a solvent. Compare and contrast the solution of an ionic compound in water and the solution of a polar molecular compound (e.g., sucrose) in water. Predict, using molecular structure, the conductivities of the resulting solutions.

C-PE.2.2.8 Investigate, from a set of given samples, the principles involved in carrying out ionic reactions in solution.

C-PE.2.2.8d Construct atomic–molecular level representations of reactions in solution, and use these representations to explain the processes that must occur when two ionic substances dissolve in water and react.

Enduring Understanding 2A

Matter can be described by its physical properties. The physical properties generally depend on the spacing between the particles (atoms, molecules, ions) and the forces of attraction between the particles. There is a relationship between the macroscopic properties of solids, liquids and gases and the structure of the constituent particles of those materials on the molecular and atomic scale…. Solutions are an important class of mixtures; of particular importance is a conceptual understanding of the molecular level of the structure and composition of a liquid solution. In addition, the energetics of solution formation can be understood qualitatively through consideration of the interactions and structure of the components before and after the creation of the solution

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