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Boyle's Law states that when temperature and moles are constant, pressure and volume are inversely proportional. As volume decreases, pressure increases, and vice versa. The product of pressure and volume remains constant under these conditions, expressed as P₁V₁ = P₂V₂ for two different states.

Gay-Lussac's Law describes a direct relationship between pressure and temperature when volume and moles remain constant. As temperature increases, gas particles move faster and collide more frequently, increasing pressure. The ratio P/T equals a constant, so P₁/T₁ = P₂/T₂. Temperature must always be expressed in Kelvin.

Charles's Law shows that volume and temperature are directly proportional when pressure and moles are held constant. When temperature rises, gas particles move faster, creating more collisions that push the balloon walls outward. The ratio V/T remains constant, so V₁/T₁ = V₂/T₂, with temperature in Kelvin.

Avogadro's Law states that volume and number of moles display a direct relationship when pressure and temperature are constant. Adding more gas molecules increases collisions, forcing the container to expand. The ratio V/n equals a constant, expressed as V₁/n₁ = V₂/n₂ for two different conditions.

The ideal gas law combines expressions from Boyle's, Gay-Lussac's, Charles's, and Avogadro's laws into a single equation: PV = nRT. Here, R is the universal gas constant (8.314 J/mol·K or 0.08206 L·atm/mol·K). This equation relates all four gas properties and allows calculation of any unknown variable when three others are known.

Ideal gases follow the ideal gas law under conditions of relatively low pressure and high temperature. Real gases deviate from ideal behavior due to intermolecular forces and molecular volume, which become significant at high pressures or low temperatures. Understanding real gases effects of intermolecular forces and molecular volume helps explain when ideal assumptions break down.

Gas laws require Kelvin temperature because pressure and volume are directly proportional to absolute temperature. Using Celsius would produce incorrect ratios since zero Celsius does not represent zero molecular motion. The Kelvin scale starts at absolute zero, ensuring that temperature ratios accurately reflect changes in particle kinetic energy and collision frequency.