Heating Curve

Brownian Motion & Diffusion

Brownian motion

• Brownian motion is defined as the random movement of particles in a liquid or a gas produced by large numbers of collisions with smaller, often invisible particles
• The observation of Brownian motion proves the correctness of the kinetic particle theory

Large particles show jerky and erratic movement caused by many collisions with smaller particles

Diffusion

• This is the process by which different gases or different liquids mix and is due to the random motion of their particles
• Diffusing particles move from an area of high concentration to an area of low concentration
• Eventually the concentration of particles is even as they spread out to occupy all of the available space
• Diffusion happens on its own and no energy input is required although it occurs faster at higher temperatures

Diffusion of potassium manganate (VI) in water. After a few hours the concentration of KMnO₄ is the same everywhere in the solutionExtended Only

Changes in State & Kinetic Theory

• When substances are heated, the particles absorb thermal energy which is converted into kinetic energy. This is the basis of the kinetic theory of matter
• Heating a solid causes its particles to vibrate more and as the temperature increases, they vibrate so much that the solid expands until the structure breaks and the solid melts
• On further heating, the now liquid substance expands more and some particles at the surface gain sufficient energy to overcome the intermolecular forces and evaporate
• When the b.p. temperature is reached, all the particles gain enough energy to escape and the liquids boils
• These changes in state can be shown on a graph which is called a heating curve
• Cooling down a gas has the reverse effect and this would be called a cooling curve
• These curves are used to show how changes in temperature affect changes of state

Heating & cooling curve for water with interconversions of state

Exam Tip

While changing state, the temperature of the substance remains the same as the heat energy is rapidly converted into kinetic energy. This is called latent heat and corresponds to the horizontal sections of a heating / cooling curve.