Solutions, Solutes, Solvents & Solubility: Sixth Grade Science
Solutions are specialized homogeneous mixtures where one substance is completely dissolved and dispersed at the molecular scale within another.
The Architecture of a Solution
A solution is a homogeneous mixture consisting of two essential components: the solute and the solvent. Because the solute particles are dissolved down to individual atoms, ions, or small molecules (smaller than 1 nanometer in diameter), a solution never settles out upon standing, cannot be separated by ordinary paper filtration, and appears transparent rather than cloudy.
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| ANATOMY OF A CHEMICAL SOLUTION |
| |
| [ SOLUTE ] + [ SOLVENT ] |
| (Substance that dissolves; (Substance that dissolves solute;|
| present in smaller amount) present in greater amount) |
| | |
| v |
| [ SOLUTION ] |
| (Homogeneous Molecular Mixture) |
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Solute Versus Solvent Identification
- The Solute: The substance that is dissolved and dispersed throughout the mixture. It is usually the component present in the lesser quantity. For example, when making iced tea, table sugar and tea extracts are solutes.
- The Solvent: The dissolving medium that surrounds, separates, and holds the solute particles. It is the component present in the greatest quantity. In liquid solutions involving water, water is the solvent.
Water: The Universal Solvent
Water is known in chemistry as the "universal solvent" because it dissolves more substances than any other liquid on Earth. Water's exceptional dissolving power stems from its molecular geometry: water (H2O) is a polar molecule. The oxygen atom pulls electrons toward itself, creating a partial negative charge on the oxygen end and a partial positive charge on the two hydrogen ends. When an ionic crystal (such as table salt, NaCl) is dropped into water, the negative oxygen ends attract positive sodium ions (Na+), while the positive hydrogen ends attract negative chloride ions (Cl-). Water molecules physically pull the ions out of the crystal lattice and surround them in a protective sphere of hydration, keeping them dissolved.
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| DISSOLUTION OF AN IONIC CRYSTAL |
| |
| Salt Crystal Lattice Water Hydration Spheres |
| |
| [Na+] [Cl-] [Na+] (H) (H) |
| [Cl-] [Na+] [Cl-] \ / |
| [Na+] [Cl-] [Na+] (O)- |
| | |
| v |
| Water molecules pull ions away: [ Na+ ] |
| ^ |
| | |
| (O)- |
| / \ |
| (H) (H) |
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Understanding Solubility and Saturation
Solubility is an intensive physical property defined as the maximum mass of solute (measured in grams) that can dissolve in a specific volume of solvent (usually 100 grams or 100 mL of water) at a specific temperature. Based on how much solute is dissolved, solutions are classified into three saturation levels:
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| SATURATION LEVELS OF SOLUTIONS |
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| Saturation State | Operational Definition and Behavior |
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| Unsaturated | Contains less dissolved solute than the maximum |
| | capacity; additional solute added will dissolve easily|
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| Saturated | Contains the exact maximum amount of dissolved solute |
| | possible; extra solute added falls to the bottom |
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| Supersaturated | An unstable state containing MORE dissolved solute |
| | than normal maximum; created by cooling a hot solution|
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Factors That Affect the Rate of Dissolution
The rate of dissolution (how quickly a solute dissolves) is different from total solubility (how much solute can dissolve). Three primary physical factors increase the rate of dissolution:
- Surface Area (Particle Size): Crushing a large solid crystal into fine powder exposes vastly more surface area to solvent collisions, allowing molecules to dissolve much faster.
- Agitation (Stirring or Shaking): Stirring moves freshly dissolved solute particles away from the solid surface and brings fresh, unbonded solvent molecules into direct contact with the solute.
- Temperature: Heating liquid solvent molecules increases their kinetic energy. Hot solvent molecules move faster and collide with the solute with greater force and frequency, tearing particles away rapidly.
Temperature Effects on Solids Versus Gases
Temperature affects the solubility of solid solutes and gaseous solutes in opposite ways:
- For most solid solutes (such as sugar or salt), increasing temperature increases solubility. Hot water can hold substantially more dissolved sugar than cold water.
- For gaseous solutes (such as oxygen or carbon dioxide dissolved in water), increasing temperature decreases solubility! As water heats up, gas molecules gain kinetic energy and escape from the liquid into the air. This explains why warm soda goes flat rapidly and why thermal pollution in rivers harms fish by driving out dissolved oxygen.
Exercises and Solutions
Exercise 1: Interpreting Solubility Curves. A student consults a solubility graph and observes that at 20 degrees Celsius, exactly 36 grams of sodium chloride (NaCl) can dissolve in 100 grams of water. (a) If the student stirs 25 grams of salt into 100 grams of water at 20 degrees Celsius, classify the solution as unsaturated, saturated, or supersaturated; (b) If the student adds an additional 20 grams of salt to that same beaker (total 45 grams) and stirs vigorously at 20 degrees Celsius, describe what will be observed in the beaker and state how many grams of salt remain undissolved.
Solution 1: (a) The solution is unsaturated because 25 grams is less than the maximum saturation capacity of 36 grams; the water can hold an additional 11 grams of salt before reaching saturation. (b) The solution will become saturated, and exactly 9 grams of solid white salt will settle undissolved at the bottom of the beaker. At 20 degrees Celsius, 100 grams of water can only hold 36 grams of NaCl. Subtracting 36 grams from the 45 grams added yields 9 grams of excess solute that cannot dissolve.
Exercise 2: Explaining Thermal Dissolution in Aquatic Ecology. A power plant discharges clean, warm cooling water at 38 degrees Celsius into a cold mountain stream where the natural water temperature is 12 degrees Celsius. Even though no toxic chemicals are discharged, fish downstream begin suffocating and dying. Using the principles of gas solubility and temperature, explain why the warm water caused the fish to suffocate.
Solution 2: Gaseous solutes, such as dissolved oxygen (O2) required by aquatic organisms for cellular respiration, have lower solubility in warm water than in cold water. When water temperature increases, the kinetic energy of the dissolved oxygen molecules increases, causing them to break free from hydrogen bonding attractions with water and escape into the atmosphere. The sudden discharge of 38-degree water causes thermal pollution, severely depleting the dissolved oxygen concentration in the stream below the biological threshold required for fish gills to extract oxygen, leading to asphyxiation.