Physical Science Final Review: Solutions

Fundamentals of Solutions

Definition of a Solution: A mixture that possesses the exact same composition, color, and density throughout all of its parts. It can be characterized as one substance dissolved into another.
Solute: This is the substance that is being dissolved in a solution.
Solvent: This is the substance that performs the dissolving process. It is generally the substance present in the greatest quantity.
Example: Air: Air is a solution consisting of: * $78\%$ Nitrogen. * $21\%$ Oxygen. * <1\% Argon, $CO_2$ , and other gases. * The Solvent in Air: Nitrogen is the solvent because it makes up the majority of the mixture.

Water: The Universal Solvent

Status: Water is frequently identified as the "universal solvent," though it is more accurate to describe it as the best solvent available.
Capabilities and Limitations: While water cannot dissolve every substance, it is capable of dissolving more substances than any other known liquid.
Molecular Mechanism: Water is highly effective because it is a small molecule. This small size allows water molecules to slip between the particles of other compounds to split them apart.

Types of Solutions: Dissolving Liquids, Gases, and Solids

Dissolving Liquids

Liquid dissolved in a Solid: Dental amalgam, which consists of liquid mercury dissolved in solid silver.
Liquid dissolved in a Liquid: Alcohol dissolved in water.
Liquid dissolved in a Gas: Water in the air, commonly known as humidity.
Miscibility: The ability of two liquids to dissolve into each other. Examples include: * Water and vinegar. * Water and milk.

Dissolving Gases

Gas dissolved in a Solid: Hydrogen dissolved in palladium.
Gas dissolved in a Liquid: Sodas (carbonated beverages), which consist of $CO_2$ dissolved in soda water.
Gas dissolved in a Gas: Air (primarily nitrogen and oxygen).
Note: Every mixture containing only gases is classified as a solution.

Dissolving Solids

Solid dissolved in a Solid: Brass, which is an alloy composed of copper and zinc.
Solid dissolved in a Liquid: Salt water.
Solid dissolved in a Gas: Smoke.
Alloys: A solution that remains solid at room temperature. These usually include at least one metal, though not always. To create an alloy, the component substances must typically be melted before they are mixed.

Molecular Composition of Solutes

Ionic Solute: Composed of a positive and a negative atom pulled together. Both atoms in the bond are already charged.
Polar Solute: Consists of multiple atoms sharing the same electron, but the electron is held closer to one atom than another. * The atoms themselves are neutral (not charged). * The resulting molecule has a distinct positive side and a negative side.
Nonpolar Solute: Consists of multiple atoms sharing an electron equally. * The atoms are neutral. * The molecule does not have positive or negative sides.

The Mechanics of Dissolving

There are three primary ways a solution is formed:

Without Ions: Dissolution: This process separates molecules from one another but does not split individual molecules into separate atoms.
With Ions: Ionization: Breaking apart polar covalent compounds to create new ions.
With Ions: Dissociation: Separating the existing positive and negative atoms within an ionic compound.

The Dissolution of Sugar in Water

Molecular Structure: Solid sugar is made of sugar molecules ( $C_{12}H_{22}O_{11}$ ) stacked together like bricks in a wall.
Step 1: Water molecules (which are polar) surround the sugar molecules (which are also polar).
Step 2: The positive ends of the water molecules align with the negative ends of the sugar molecules, and vice versa.
Step 3: Once a sugar molecule is completely surrounded by water, it can no longer stick to other sugar molecules.
Step 4: The sugar molecule eventually floats away from the solid stack.
Completion: Eventually, all individual sugar molecules are separated and spread out evenly through the solvent, completing the dissolution process.

Ionization vs. Dissociation

Ionic Solution: A solution containing charged particles called ions. Note: Not all solutions contain ions.
Dissociation: The process of making an ionic solution by separating the positive and negative atoms in an already ionic compound. This causes existing bonded ions to stop associating.
Ionization: The process of making an ionic solution by breaking apart polar covalent compounds. This forces a shared electron to "pick a side," thereby creating new ions out of non-ions.

Summary Table of Dissolving Types

Type of Dissolving	Starts With	Ends With
Dissolution	Any solute	Uncharged solution
Ionization	Polar solute	Ionic solution
Dissociation	Ionic solute	Ionic solution

Rules of Solubility: "Like Dissolves Like"

To dissolve a solute, the solvent must match the solute's type.

Polar Solvents (e.g., Water): * Can dissolve polar solutes. * Can dissolve ionic solutes (via dissociation). * CANNOT dissolve nonpolar solutes.
Nonpolar Solvents (e.g., Oil): * Can dissolve nonpolar solutes. * CANNOT dissolve polar solutes.
Essential Knowledge: Water is polar; oil is nonpolar.

Dissociation in Polar Solvents

When a polar solvent dissolves an ionic solute:

Positive and negative ions are pulled apart by attraction to the corresponding + and - ends of the solvent.
Solvent particles surround and separate the ions.
Dissolved particles spread out evenly to form a solution.

Ionization in Polar Solvents

When a polar solvent dissolves a polar solute to create an ionic solution:

The positive and negative ends of the covalent molecules are broken apart by the polar solvent molecules to FORM new ions. The atoms were uncharged before dissolving but are charged after.
The newly formed ions spread evenly through the solvent.

Nonpolar Solvents and Specialized Cases

Applications: Nonpolar solvents are used by dry cleaners to dissolve greasy stains that water (a polar solvent) cannot remove. The term "dry" signifies that no water is used.
Universal Components: Solvents like soap and alcohol can dissolve both polar and nonpolar solutes. This is because they possess an ionic "head" and a nonpolar hydrocarbon "tail."
Nonpolar Solvent Uses: Breaking down waxes, paints, and oils; "dry" cleaning alternatives.
Nonpolar Solvent Drawbacks: They do not dissolve polar substances and are often flammable or toxic.

Factors Affecting the Rate of Dissolving

Surface Area (Size Matters)

Breaking a solute into smaller pieces increases the total surface area, causing it to dissolve faster.
Dissolving occurs at the surface where the solute touches the solvent.
Mathematical Example of Surface Area (Page 20): * A large cube with a side of $12\,cm$ . * Total surface area listed as $6\,cm^2$ (initial scale reference). * Dividing the cube into smaller cubes ( $1\,cm$ on a side) results in $1,728$ cubes and $10,368$ faces. * Example Progression: Surface area increases from $864\,cm^2$ to $1,728\,cm^2$ and finally to $10,368\,cm^2$ as particles get smaller.

Stirring and Shaking

For Solids in Liquids: Stirring/shaking increases solubility by moving the solute and solvent around, bringing the solute into contact with fresh, unused solvent.
For Gases in Liquids: Stirring/shaking decreases solubility. For example, shaking a soda causes $CO_2$ to form tiny bubbles which expand and escape when the container is opened.

Temperature

For most solutions: Solubility increases as the temperature of the solvent increases. Higher temperature means solvent particles move faster and hit the solute more frequently.
For Gas-Liquid solutions: Solubility decreases as temperature increases. If gas particles move too fast, they break the bonds holding them to the solvent and escape.

Pressure

General Effect: Pressure has little impact on most solutions.
Gases in Liquids: High pressure increases the solubility of gases in liquids. Gas must push out harder than the external pressure to escape; if external pressure is high enough, gas cannot escape.

States of Saturation

Saturated Solution: Contains the maximum amount of solute that a solution can hold at a specific temperature. All available solvent is used.
Unsaturated Solution: A solution that can still dissolve more solute at a given temperature. There is "leftover" solvent.
Supersaturated Solution: Contains more solute than a saturated solution at the same temperature. This occurs as certain solutions cool down to room temperature.
Examples in Daily Life: * Northern Sweet Tea: Adding sugar to cold tea often results in sugar piling at the bottom because solubility is lower at lower temperatures. * Honey: If honey sits too long, water evaporates. With less water (solvent) to hold the sugars, the sugars eventually "fall out" and crystallize.

Colligative Properties

Physical properties that are affected by the number of solute particles present in a solution.

Freezing Point Depression: The freezing temperature is lowered when more solute particles are added. * Ex: Adding antifreeze to a car radiator. * Mechanism: Solute particles interfere with the specific repeating pattern particles try to form when freezing, making it harder to freeze.
Boiling Point Elevation: The boiling temperature increases when the number of solute particles increases. * Ex: Adding salt to water for cooking. * Mechanism: Liquid particles must move past solute particles to escape into the air. This difficulty requires a higher temperature to achieve boiling.

Electrolytes

Conductivity: A solution must contain ions to conduct electricity. * Many ions = Good conductor. * Few ions = Okay conductor. * No ions = Bad conductor / Good insulator.
Electrolyte: A solute compound that creates ions when dissolved. * Strong Electrolyte: Produces many ions; highly conductive (e.g., Salt/ $NaCl$ ). * Weak Electrolyte: Produces few ions; somewhat conductive (e.g., Acetic acid in vinegar). * Nonelectrolyte: Does not produce ions; not conductive.

Practice Problems & Review Questions

Deep-sea diver's gas (5% $O_2$ in $He$ ): Solute = Oxygen ( $O_2$ ); Solvent = Helium ( $He$ ).
Steel ( $C$ , $Mn$ , $P$ in $Fe$ ): Solutes = Carbon, Manganese, Phosphorus; Solvent = Iron ( $Fe$ ).
Sweet tea (sugar and water): Solute = Sugar; Solvent = Water.
Baby diaper (urine and polymer): Solute = Urine (liquid); Solvent = Polymer (solid).
Juice mixture (apple/grape): Liquid/Liquid solution.
Reaction Check: If $Ca_3(PO_4)_2$ breaks into $Ca^{2+}$ and $(PO_4)^{3-}$ , the process is Dissociation. Reasoning: It ends with charged particles and the particles were already charged within the compound.
Saturated Solution (KNO3): According to the solubility chart (p. 692), to make a saturated solution at $40^{\circ}\text{C}$ , one must add $60\,g$ of potassium nitrate to $100\,g$ of water.
Classification (Sodium Chlorate): If $25\,g$ of sodium chlorate are dissolved in $100\,g$ of water at $70^{\circ}\text{C}$ , the solution is considered dilute (based on standard concentrations for that substance at that temperature).
Refreezing Ice: Salt added to ice forms a saltwater solution. This may not refreeze because the solute particles lower the freezing point below the ambient temperature of the environment.
Electrolyte Identification: Cooking oil would NOT be classified as an electrolyte because it is nonpolar and does not produce ions.