Thermal Properties of Matter notes

Thermal Properties of Matter explains how matter responds to heat. Temperature tells the degree of hotness and decides the direction of heat flow, while heat is energy transferred due to temperature difference. The chapter covers temperature scales, heat capacity, thermal expansion of solids and liquids, anomalous expansion of water, calorimetry, specific heat, change of state, latent heat and heat transfer by conduction, convection and radiation. Newton's law of cooling explains how hot bodies cool in surroundings. For NEET, this chapter is highly scoring because questions are usually formula-based, graph-based or application-based, especially from calorimetry, thermal expansion, latent heat, heat conduction and cooling curves.

Temperature is the physical quantity that tells how hot or cold a body is and decides the direction of heat flow. When two bodies are in thermal contact, heat flows from the body at higher temperature to the body at lower temperature until thermal equilibrium is reached. Heat is energy in transit due to temperature difference, while internal energy is the total microscopic kinetic and potential energy of molecules of a body. Temperature can be measured using Celsius, Kelvin and Fahrenheit scales. Kelvin is the SI scale and begins at absolute zero. Heat capacity is the heat required to raise the temperature of a body by one kelvin, while specific heat capacity is per unit mass.

Most materials expand when heated and contract when cooled because their particles vibrate with greater average separation at higher temperature. In solids, expansion can be measured as linear expansion, superficial expansion or volume expansion. Linear expansion describes change in length, superficial expansion describes change in area and volume expansion describes change in volume. Their coefficients are α, β and γ respectively, and for isotropic solids β ≈ 2α and γ ≈ 3α. Water behaves unusually between 0°C and 4°C: it contracts on heating from 0°C to 4°C and expands above 4°C, making its density maximum at 4°C. This anomalous expansion is important for aquatic life in winter.

Specific heat capacity is the heat required to raise the temperature of unit mass of a substance by one kelvin. Heat capacity is the heat required for the whole body, while molar heat capacity is the heat required to raise the temperature of one mole by one kelvin. Calorimetry is the measurement of heat exchange. The principle of calorimetry states that if no heat is lost to surroundings, heat lost by hotter bodies equals heat gained by colder bodies. This principle is used in mixture problems, finding final temperature, and determining specific heat. Water has high specific heat, so it absorbs or releases large heat for small temperature change, affecting climate and cooling systems.

Matter commonly exists as solid, liquid and gas. A change of state occurs when matter changes from one state to another due to heat exchange. Melting changes solid to liquid, freezing changes liquid to solid, vaporisation changes liquid to gas, condensation changes gas to liquid and sublimation changes solid directly to gas. During a phase change at constant pressure, temperature remains constant even though heat is supplied or removed. This hidden heat is called latent heat because it changes internal molecular arrangement rather than temperature. Latent heat of fusion is used in melting and freezing, while latent heat of vaporisation is used in boiling and condensation. Heating and cooling curves help visualize these processes.

Heat transfer occurs by conduction, convection and radiation. Conduction is heat transfer through a material without bulk movement of matter, mainly due to molecular collisions and free electrons. It is important in solids and is described by thermal conductivity. Convection is heat transfer by bulk motion of fluid; hot fluid rises and cold fluid sinks, producing convection currents. Radiation is heat transfer by electromagnetic waves and does not require a material medium. A black body is an ideal absorber and emitter of radiation, while emissivity measures how effectively a real body emits compared with a black body. Everyday examples include cooking, sea breeze, thermos flasks, warm clothes, room heaters and cooling fins.

Newton's law of cooling states that the rate of loss of heat or fall of temperature of a body is directly proportional to the temperature difference between the body and its surroundings, provided the temperature difference is small and surrounding temperature remains constant. Mathematically, dT/dt = -k(T - Ts). The negative sign shows that body temperature decreases during cooling. The cooling curve is exponential: the body cools quickly at first when temperature difference is large and slowly later as it approaches room temperature. The law is used to estimate cooling of hot liquids, forensic time of death, thermometer response and heat loss from objects. Experimental verification involves recording temperature at equal time intervals.