Mixtures, Solutions & Solubility: Fifth Grade Science
When two or more substances combine physically without undergoing a chemical reaction, they create mixtures, which can be heterogeneous blends or uniform, dissolved solutions.
What is a Mixture?
A mixture is a physical combination of two or more substances in which each substance retains its own individual chemical identity and properties. Because no chemical bonds are formed or broken during mixing, the components of any mixture can be separated using physical techniques. Mixtures can exist in any proportion; for example, a handful of sand and gravel can have 10 percent sand or 90 percent sand and still be classified as a sand-and-gravel mixture.
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| HETEROGENEOUS VS. HOMOGENEOUS MIXTURES |
+-----------------------+-------------------------+------------------------+
| Characteristic | Heterogeneous Mixture | Homogeneous Mixture |
+-----------------------+-------------------------+------------------------+
| Particle Distribution | Non-uniform, unevenly | Completely uniform, |
| | distributed throughout | identical throughout |
| Visibility of Parts | Distinct individual | Blended at molecular |
| | parts easily visible | level; parts invisible |
| Light Scattering | Often scatters light or | Usually transparent or |
| | settles over time | uniform in color |
| Classic Examples | Garden salad, pizza, | Salt water, sweet tea, |
| | trail mix, muddy water | brass, clean air |
+-----------------------+-------------------------+------------------------+
Heterogeneous Mixtures
In a heterogeneous mixture, the different ingredients are not distributed evenly, and individual components can easily be seen with the naked eye or a magnifying lens. A bowl of cereal with milk, a scoop of rocky soil, and vegetable soup are classic heterogeneous mixtures. In many liquid heterogeneous mixtures (suspensions), such as muddy pond water or Italian salad dressing, dense solid particles settle out to the bottom over time due to gravity.
Homogeneous Mixtures (Solutions)
In a homogeneous mixture, the substances are blended so evenly down to the molecular level that the mixture looks like a single, uniform substance. Even under a high-power light microscope, you cannot see individual boundaries between the components. Homogeneous mixtures are scientifically known as solutions.
The Anatomy of a Solution
Every chemical solution consists of two fundamental parts: the solute and the solvent.
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| ANATOMY OF A LIQUID SOLUTION |
| |
| [ SOLUTE ] [ SOLVENT ] [ SOLUTION ] |
| (Sugar Crystals) + (Pure Water) = (Sweet Water) |
| |
| Substance that is Substance that does Uniform, clear |
| dissolved (smaller the dissolving (larger homogeneous |
| quantity). quantity). molecular blend.|
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Solute and Solvent Defined
- Solute: The substance that dissolves. It is typically the component present in the smaller amount. Examples include salt in seawater, sugar in tea, and carbon dioxide gas in carbonated soda.
- Solvent: The substance in which the solute dissolves. It is the dissolving medium and is typically present in the larger amount. Water is known as the "universal solvent" because its polar molecular structure allows it to dissolve more substances than any other liquid on Earth.
Solutions in Different States of Matter
While we usually think of solutions as liquids, solutions can exist in any state of matter:
- Gas solutions: The Earth's atmosphere is a gaseous solution consisting of 78% nitrogen gas (the solvent) dissolving 21% oxygen gas, 1% argon, and trace amounts of carbon dioxide (solutes).
- Solid solutions (Alloys): Brass is a solid solution made by melting copper (solvent) and dissolving zinc (solute) into it; bronze is copper dissolving tin; steel is iron dissolving carbon.
Solubility and Saturation
Solubility is the maximum amount of a solute that can dissolve completely in a given volume of solvent at a specific temperature.
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| STAGES OF SOLUTION SATURATION |
| |
| UNSATURATED SATURATED SUPERSATURATED|
| +-----------------+ +-----------------+ +----------------+
| | . . . . | | . . . . . . . . | | . . . . . . . .|
| | . . . . | | . . . . . . . . | | . . . . . . . .|
| | . . . . | | . . . . . . . . | | . . . . . . . .|
| | | | [][][] Undissolved | [][][][][][] |
| +-----------------+ +-----------------+ +----------------+
| Can dissolve more Holds maximum solute Holds more than |
| solute easily. at this temperature. normal; unstable.|
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The Three Saturation Levels
- Unsaturated solution: Contains less dissolved solute than the solvent is capable of holding. If you add another teaspoon of salt, it will dissolve completely upon stirring.
- Saturated solution: Contains the absolute maximum amount of dissolved solute possible at that specific temperature. If you add even one more pinch of salt, it will not dissolve; it will sink to the bottom as un-dissolved crystals.
- Supersaturated solution: An unstable, high-energy condition where a solution temporarily holds more dissolved solute than normal saturation allows. This is created by heating a solvent to dissolve excess solute and then cooling it down very gently without disturbance. If a single seed crystal is dropped into a supersaturated solution, the excess solute crystallizes out instantly.
Factors That Accelerate Dissolving Rates
To make a solid solute dissolve faster into a liquid solvent, you can apply three physical techniques:
- Stirring or shaking: Agitation sweeps newly dissolved solute molecules away from the crystal surfaces, allowing fresh solvent molecules to contact the un-dissolved crystal.
- Increasing temperature: Heating the solvent makes its molecules move faster and collide more energetically with solute crystals, breaking solute bonds more rapidly.
- Crushing into smaller particles (Increasing Surface Area): Crushing a large candy crystal into fine powder exposes vastly more surface area to immediate contact with solvent molecules.
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| SURFACE AREA AND DISSOLUTION DYNAMICS |
| |
| ONE LARGE CUBE (4 cm) 64 SMALL CUBES (1 cm each) |
| |
| +---------+ +---+ +---+ +---+ +---+ |
| / /| | | | | | | | | |
| +---------+ | +---+ +---+ +---+ +---+ |
| | | | +---+ +---+ +---+ +---+ |
| | | + | | | | | | | | |
| +---------+ +---+ +---+ +---+ +---+ |
| |
| Total Surface Area: 96 cm² Total Surface Area: 384 cm² |
| Slow dissolving rate. 4x faster contact with solvent! |
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Exercises and Solutions
Exercise 1: Identifying Solute, Solvent, and Concentration. A chef stirs 25 grams of salt into 500 milliliters of warm chicken broth until all salt disappears. Identify the solute and the solvent in this solution. Then describe how the chef could decrease the concentration of salt without removing any soup.
Solution 1: The solute is the salt (25 grams), because it is the substance being dissolved. The solvent is the liquid broth (primarily water), because it is the medium doing the dissolving. To decrease the concentration of salt without removing any soup, the chef can add more unsalted water or unseasoned broth (increasing the volume of solvent), which dilutes the ratio of dissolved salt particles relative to the total liquid volume.
Exercise 2: Investigating Dissolution Rates. An experimenter has two identical 5-gram sugar cubes and two beakers containing 200 mL of water at 20°C. In Beaker A, the experimenter drops the solid sugar cube and does not stir. In Beaker B, the experimenter crushes the cube into a fine powder and stirs vigorously. Predict which beaker will dissolve the sugar faster and provide three scientific reasons based on particle theory.
Solution 2: Sugar will dissolve significantly faster in Beaker B. First, crushing the sugar cube into fine powder dramatically increases the total surface area exposed to the water, allowing far more water molecules to interact with sugar molecules simultaneously. Second, stirring provides active kinetic agitation, moving water molecules rapidly across the sugar grains. Third, stirring prevents a concentrated layer of dissolved sugar from stagnating around the crystals, continually replacing it with fresh, unsaturated water molecules to accelerate dissolution.