Solvent Calculators

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A solvent is a substance that dissolves a solute to form a homogeneous solution, typically present in the largest amount. The most important solvent in biology is water — a polar molecule with exceptional dissolving power for ionic and polar substances. In chemistry laboratories, organic solvents (ethanol, acetone, DMSO, hexane, chloroform) are used to dissolve non-polar compounds, extract lipids and hydrophobic molecules, and prepare reaction media. The principle 'like dissolves like' summarizes solubility: polar solvents dissolve polar and ionic solutes; non-polar solvents dissolve non-polar solutes.

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Like Dissolves Like

Polar solvents (water, DMSO, ethanol) dissolve polar and ionic compounds through dipole-dipole interactions and H-bonding. Non-polar solvents (hexane, chloroform, diethyl ether) dissolve non-polar compounds through dispersion (London) forces. The driving force for dissolution: the solvent-solute interactions must overcome solvent-solvent and solute-solute interactions; dissolution is favorable when ΔG_mix < 0.

Water as Universal Solvent

Water is the premier biological solvent because: (1) High polarity (large dipole moment) and H-bonding solvate ions and polar molecules effectively. (2) High dielectric constant (ε = 78.4) reduces electrostatic forces between ions ~80-fold compared to vacuum, allowing ionic compounds to dissociate. (3) Cohesive (H-bonds between water molecules) and adhesive (H-bonds with other polar surfaces). (4) Broad liquid range at biologically relevant temperatures (0–100°C at 1 atm).

Common Laboratory Solvents

  • DMSO: Polar aprotic; excellent for dissolving drug compounds; penetrates cell membranes; cryoprotectant at 10%
  • Ethanol (70% or 100%): Polar protic; disinfectant; precipitates nucleic acids; denatures proteins
  • Acetonitrile: Polar aprotic; HPLC mobile phase; low UV absorbance
  • Chloroform: Non-polar; used in phenol-chloroform DNA/RNA extraction (caution — carcinogen)
  • Hexane: Non-polar; lipid extraction

Solvent Polarity and Miscibility

Polarity quantified by dielectric constant (ε): water ε = 78.4; DMSO ε = 47; ethanol ε = 24; acetone ε = 21; chloroform ε = 4.8; hexane ε = 1.9. Water and ethanol are fully miscible (both polar); water and hexane are immiscible (polar vs. non-polar).

Glossary

Solvent
The substance present in the largest amount in a solution that dissolves the solute; water is the universal biological solvent; organic solvents (DMSO, ethanol, hexane) used for non-polar compounds.
Dielectric Constant (ε)
A measure of solvent polarity: water ε = 78.4; DMSO ε = 47; hexane ε = 1.9; high ε solvents effectively solvate ions and reduce electrostatic interactions between charged species.
Like Dissolves Like
The principle that polar solvents dissolve polar and ionic solutes; non-polar solvents dissolve non-polar solutes; reflects the thermodynamic requirement that solvent-solute interactions must be energetically favorable.

Frequently Asked Questions

A solvent is the substance present in the largest amount that dissolves the solute. A solute is the substance being dissolved. In a 5% NaCl solution: water is the solvent, NaCl is the solute. In biology, water is the dominant solvent for virtually all cellular processes. In laboratory chemistry, the choice of solvent depends on the solubility of the compound of interest — polar solutes require polar solvents; non-polar solutes require non-polar solvents ('like dissolves like').

Water's effectiveness as a biological solvent comes from: (1) High polarity — large dipole moment and ability to form H-bonds with other polar molecules and ions. (2) High dielectric constant (ε = 78.4) — reduces electrostatic attraction between ions ~80-fold, allowing ionic compounds to dissociate. (3) H-bond capacity — water can donate and accept H-bonds, solvating charged groups on proteins and nucleic acids. (4) Forming hydration shells around ions and polar molecules that keep them dispersed. Non-polar molecules are excluded from water (hydrophobic effect), driving protein folding and membrane formation.

DMSO (dimethyl sulfoxide, (CH₃)₂SO) is a polar aprotic solvent with exceptional dissolving power for both polar and many non-polar compounds. Key uses: (1) Vehicle for dissolving hydrophobic drug compounds (typically 0.1–0.5% DMSO final concentration in cell assays). (2) Cryoprotectant (10% DMSO in freezing medium). (3) Carrier for topical drug delivery (penetrates skin). Cautions: DMSO penetrates rapidly through skin and can carry dissolved compounds with it; cytotoxic to cells above ~1% final concentration; should not be heated (reacts vigorously with strong oxidants). Store at room temperature (freezes at 18.5°C).

Solvent selection depends on: (1) Solubility — 'like dissolves like': polar solutes need polar solvents; non-polar solutes need non-polar solvents. Use the polarity index or dielectric constant as a guide. (2) Reactivity — the solvent must be inert toward reactants and products. (3) Boiling point — must be compatible with reaction temperature and easy removal during workup. (4) Safety — minimize use of carcinogens (benzene, chloroform), flammable solvents, and reproductive toxins. (5) Downstream compatibility — HPLC mobile phases require UV-transparent solvents; biological assays require non-toxic solvents at the concentrations used.