The kinetic theory of matter states that atoms and molecules are in constant motion and that the higher the temperature around them, the greater their speed will be. In other words, increased heat energy will make atoms and molecules move faster. This ties in to the states or phases of matter. In the solid phase, atoms and molecules are close together and vibrate in random directions. In the liquid phase, they move around and collide with each other. Then, in the gas phase, they move around much faster and spread out or expand. Energy is necessary for a phase change, often in the form of heat energy.
To show your kids how this works, do an experiment. Try making tea, using a mug of hot water and one of cold water to compare it to. Based on what was said in the previous paragraph about kinetic theory and molecule movement, will the hot or the cold water work best to mix the tea molecules? The hot water molecules collide much more rapidly with the tea, mixing the molecules up faster. You might want to try this with other substances as well: food coloring, salt, and sugar, for example. Does boiling water work better than hot tap water for mixing the substances, or are they same? Point out to your young kids the steam coming from boiling water - this is the result of water molecules reaching boiling point and changing from liquid to gas.
When a solid reaches its melting point, it becomes a liquid. We see this when solid ice melts into liquid water. Then, when the liquid is heated enough, it reaches boiling point and changes from liquid to gas phase. The melting and boiling points vary, depending on the properties of the elements that form the molecules. Water has a low melting point (32 °F), but many elements have much higher ones. Iron's melting point is 2777 °F!
Chemical change affects the type of atoms in a molecule; physical change, on the other hand, does not change the atomic makeup of a molecule. When you take a piece of paper and hold the end to a flame, the result - burned paper - is a chemical change. The paper cannot become "unburned." When you get a plain piece of paper wet, though, the change is physical. You can dry out the paper, often without leaving much damage - the water will evaporate. See if your kids can identify each of these changes as chemical (non-reversible) or physical (reversible): a nail getting rusty, juice freezing into a popsicle, wood burning in a fire, and sugar being mixed with water.
Bonds join different atoms or molecules together. Atoms have "shells" of electrons around them, which attract other atoms - when the outer shell has an incomplete number of electrons, it will attract another incomplete atom. A covalent bond occurs when atoms share electrons. Water (H2O) has very strong covalent bonds between hydrogen and oxygen atoms. A compound with covalent bonds will typically not conduct electricity when dissolved in water. (100% pure water does not conduct electricity.)
Ionic bonds typically form between metal and nonmetal compounds. A cation is an atom that has lost an electron and has a positive charge; an anion is an atom that has gained an electron and has a negative charge. When an ionic compound is dissolved in water, the water will conduct electricity.
In a reaction, the original bonds between atoms break down and the atoms form new bonds. Energy is used to break the old bonds and energy (usually in the form of heat) is given off or absorbed as the new bonds are formed. Exothermic reactions produce heat, whereas endothermic reactions take heat in. Next time you bake a cake, consider this. The cake dough is not really a cake, but when it's heated in the oven, an endothermic chemical reaction occurs and new bonds are formed, creating the compound of flour, sugar, butter, eggs, salt, and everything else that makes the cake.
Oxidation reactions occur when a chemical substance loses electrons to another substance. Combustion is an example of this. Gasoline contains carbon and hydrogen, which "oxidize" to produce water (H2O) and carbon dioxide (CO2) when it burns. Corrosion is another oxidation reaction, occurring when metal is in contact with oxygen. The reaction causes a compound, called an oxide, to form on the surface of the metal. Rust (iron oxide) is a common example of this.
In a reduction reaction, one chemical substance gains electrons from another substance. An example of this is plant photosynthesis: the plant cells use energy from the sun to turn water and carbon dioxide into glucose and oxygen.