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Solutions are homogeneous mixtures with concentration expressed in multiple units. Molarity (M = mol/L solution) is the most commonly used but is temperature-dependent because volume changes with temperature. Molality (m = mol/kg solvent) is temperature-independent and preferred for colligative property calculations. Mole fraction (x = mol component / total moles) is dimensionless and useful for Raoult's law and Henry's law. Mass percentage (w/w%) and parts per million (ppm) are mass-based and temperature-independent. Key interconversion: M to m requires density (d): m = 1000M / (1000d - MM_s). For dilute aqueous solutions, m ≈ M since density ≈ 1 g/mL and solute mass is negligible. Mole fraction to molality: x_2 ≈ mM_1/1000 (dilute limit). Molarity from density and mass%: M = 10dw%/M_s. Normality = M x n-factor. The choice of concentration unit matters for calculations: use molality for delta_Tb and delta_Tf, mole fraction for RLVP and Raoult's law, and molarity for osmotic pressure. Misusing units is a common error in JEE problems — always check which property uses which concentration unit before calculating.