2.3 PROPERTIES OF MATTER REACTIVITY Chemical properties describes how describe the way one “excited”a substance substance reacts to is - form new substances. highly reactive elements or substances ex., burns easily, combine with other substances easily, or explodes in contact with air. might explode or burn very easily. PHYSICAL PROPERTIES MELTING POINT characteristic that The temperature at describes without a which a substance change into a new changes from a solid substance. to a liquid. ex. color, size, In other words, how melting point, whether hot it has to be before or not it conducts it melts! electricity or is magnetic BOILING POINT FREEZING POINT The temperature at The temperature at which a substance which a liquid changes changes from a liquid into a solid, or freezes. into a gas. The freezing point and (Boils!) the melting point of a For water, it’s 212 substance are the same Fahrenheit, or temperature, because the temperature is a 100 Celsius threshold or boundary between solids and liquids. 1 DENSITY Density Basically, how tightly The mass per packed together into a unit volume of space the atoms are - very similar to how heavy or a substance. light something is - metals (how much stuff is are dense because their crammed into how much atoms are really close space) together, cotton candy is . light and fluffy because the sugar molecules are spread out. FINDING DENSITY DENSITY OF WATER Find the mass in grams, and For water, divide it by the volume, in ml or 1 gram = 1 ml = 1 cm 3 cm 3 So if you have a cubic Answer will be in: centimeter box filled with water, it will have a mass 3 of 1 gram, and a volume g/ml or g/cm of 1 millimeter. If a substance has a density less Density of Water, continued than “1” it will float on water. Since 1 gram of water has a volume of 1 ml, which would fill a cm 3 container, the density of water is 1 g/ml = 1g/cm 3. 2 If a substance has a density greater BUOYANCY than “1” it will sink in water. The force with which a more dense fluid pushes a less dense substance upward. i.e., the less dense stuff floats on top of the denser stuff! Chemical Changes Examples of Chemical Changes One substance Cooking changes into another substance. food. Usually requires burning, heat, explosion. Burning i.e., ENERGY! wood. Chemical Change – new substance! Physical Changes Doesn’t change the kind of substance! Liquid water is H 2O Boiling water is still H2O Frozen water is too! 3 Dissolving is a physical change! Electrolysis of Water For example, big crystals of sugar have – Electric current millions of molecules – as they are broken decomposes water into hydrogen and oxygen apart into smaller and smaller chunks, they gas seem to “disappear” in the solution. – Volume of hydrogen produced is twice that of oxygen – Water = H 20 Electrolysis of Water Schematic diagram of the electrolysis of water 4.