When you stir sugar into water, it seems to disappear — yet the water tastes sweet. This is the science of solutions. Solutions are everywhere: in our blood, the ocean, the air we breathe, and the medicines we take.
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Key Concepts and Definitions
Solution: A homogeneous mixture of two or more substances. It looks the same throughout and does not settle on standing.
Solute: The substance that dissolves in a solution (present in smaller amount). Example: sugar in sugared water.
Solvent: The substance that does the dissolving (present in larger amount). Example: water in sugared water. Water is called the universal solvent because it dissolves more substances than any other liquid.
Solubility: The maximum amount of solute that can dissolve in 100 g of solvent at a given temperature. Units: g per 100 g of solvent.
Saturated solution: No more solute can dissolve at that temperature. Unsaturated: can still dissolve more. Supersaturated: contains more dissolved solute than the solubility limit (unstable).
Concentration: The amount of solute dissolved in a given amount of solution. Formula:
Concentration (g/L) = mass of solute (g) / volume of solution (L)
Effect of temperature: Solubility of most solids in water increases with temperature. Solubility of gases in water decreases with temperature (fizzy drinks go flat when warm).
Miscible and Immiscible liquids: Liquids that mix completely are miscible (alcohol and water). Liquids that do not mix are immiscible (oil and water).
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Worked Examples
Identify solute and solvent in a solution of 5 g of salt dissolved in 100 mL of water.
Salt (5 g) is the solute (smaller amount). Water (100 mL) is the solvent (larger amount). The solution is a dilute, unsaturated salt solution.
At 25°C, 36 g of NaCl dissolves in 100 g of water. If 40 g of NaCl is added to 100 g of water at 25°C, what happens?
Only 36 g dissolves; the remaining 4 g of NaCl remains undissolved at the bottom. The solution is saturated.
Calculate the concentration of a solution made by dissolving 10 g of glucose in 500 mL (0.5 L) of water.
Concentration = 10 g / 0.5 L = 20 g/L
A cold glass of soda goes flat quickly after being opened and left in a warm room. Explain.
Carbon dioxide (CO2) is dissolved in the soda under pressure. When opened, pressure drops. Warming further decreases gas solubility, so CO2 escapes as bubbles — the drink goes flat.
Why does stirring and heating help dissolve sugar faster?
Stirring brings fresh solvent into contact with undissolved sugar. Heating increases the kinetic energy of water molecules, allowing them to break apart sugar crystals more quickly. Both increase the rate of dissolving (not solubility in the case of stirring alone).
Classify these as miscible or immiscible: (a) water and ethanol, (b) water and oil.
(a) Water and ethanol mix in all proportions — miscible. (b) Water and oil form two layers — immiscible.
A solution contains 25 g of sugar in 200 g of water. Express the concentration as mass fraction (mass of solute / total mass of solution).
Total mass of solution = 200 + 25 = 225 g. Mass fraction = 25/225 ≈ 0.111 (or about 11.1%).
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Key Formulas
Key formulas
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Common mistakes
> Many students think solubility and rate of dissolving are the same. Stirring increases the rate but NOT the solubility. Also, the solvent is always the larger-quantity component — do not confuse it with the solute. Remember: gases become LESS soluble on heating, opposite to most solids.
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Summary
A solution is a homogeneous mixture of solute (dissolved substance) and solvent (dissolving medium). Solubility depends on temperature: solids dissolve more at higher temperatures; gases dissolve less. Concentration measures how much solute is in a solution. Liquids can be miscible or immiscible. Water is the universal solvent.