The law of conservation of matter (which we learned about earlier) says that the total amount of matter doesn't change during chemical reactions. For most chemical reactions, the energy changes are small enough that we can't detect any change in mass. But in nuclear reactions, the energy changes are much bigger, and we can measure small changes in mass as energy is released or absorbed. We'll learn more about this in a later chapter on nuclear chemistry. Thermal Energy , Temperature, and Heat Thermal energy is a type of kinetic energy that comes from the random motion of atoms and molecules. Temperature is how we measure how hot or cold something is. When atoms and molecules move or vibrate faster, they have more kinetic energy on average and we say the object is hot. When they move more slowly., they have less kinetic energy, and we say the object is cold. If you add thermal energy to something (without causing a chemical reaction or phase change), its temperature will go up. If you take away thermal energy, its temperature will go down. Heat is thermal energy that moves from one object to another because of a temperature difference. When heat flows ,it increases the thermal energy of one object and decreases the thermal energy of the other Let's say we have a hot object (H)and a cold object (L). The atoms and molecules in H are moving faster than those in L. If we put H and L in contact, thermal energy will naturally flow from H to L.H will cool down (its molecules will slow down), and L will warm up (its molecules will speed up)This will continue until both objects are at the same temperature. Chemical reactions and physical changes can release or absorb heat. A change that releases heat is called an exothermic process. For example, when you use a blowtorch, the burning of the fuel is an exothermic process -it releases heat and light. A change that absorbs heat is called an endothermic process. When you use a cold pack on a sore muscle.the chemicals inside mix and absorb heat, making it feel cold. Energy used to be measured in calories. A calorie is the energy needed to raise the temperature of one gram of water by 1^circ C However, this can vary depending on air pressure and the water's starting temperature. The calorie is still often used, especially for food energy (though a food Calorie is actually a kilocalorie, or 1000 regular calories). The modern scientific unit for energy is the joule (I)One joule is the energy used when a force of I newton moves an object 1 meter. A kilojoule (kJ)is 1000 joules. For reference, 1 calorie equals 4.184 joules. Now let's look at two important concepts: heat capacity and specific heat.The heat capacity (C) of an object is how much heat energy it takes to raise its temperature by 1^circ C Heat capacity depends on both what the object is made of and how much of it there is. For example, a capacity depring pan has a higher heat capacity than a small iron frying pan because there's more iron to heat up. Specific heat capacity (c)often just called specific heat, is the heat capacity per gram of a substance. It's how much energy it takes to raise the temperature of I gram of the substance by