Transcript notes
Physical vs. Chemical Changes Get ready to take notes What is a Physical Change? A physical change is a change that alters the form or appearance of material but does not convert the material into new substances. For example water can freeze, melt, or evaporate. These are physical changes because they are reversible! Evidence of a Physical Change Observable evidence of a physical change includes a change in shape, size, and phase. Water is the most notable compound that changes phase but you can also get other compounds that change phase and shape. For example Dry Ice is frozen carbon dioxide which when it is in it’s gas form makes your soda bubble. States of Matter-Water You should know by now there are 4 states of matter that substances can be in-solid, liquid, gas, or plasma. (plasma is not very common) The graph to the right shows what happens to the states of water as it is heated up over time. 1000 C O0 C Energy and the Motion of Molecules Molecules in a solid are tightly packed together. There is very little movement but there is some. Molecules in a liquid are packed closely together so they can flow. Molecules in a gas are much further apart. KINETIC Energy—which has more? Shape when put into a container Solids do not take the shape of the container they are put in. Liquids and Gases assumes the shape of the container that they are placed in. (This INFO) Examples of Physical Properties A physical property is something that can be observed or measured. The following is a list of physical properties! Mass, Weight, Volume, Density, Phase, Size, Shape, Color, Texture, Hardness, Odor, Flexibility, Solubility, Malleability (ability to hammer into thin sheets), Ductility (ability to make substance into wire), and Thermal Conductivity Identify the Physical Properties What are the physical properties for this Lion? Identify the Physical Properties What are the physical properties for a glass of water? What is a Chemical Change? A chemical change is a change in matter that forms one or more new substances. The new substances are made of the same elements as the original substances, but are now in different combinations. Chemical reactions can not be changed or reversed to get the same exact substances. The key characteristic is the production of new materials that are chemically different from the starting materials. Evidence of Chemical Changes A color change is often a sign that chemical change has occurred. For example when leaves change colors in the fall this is a chemical change in the leave that can not be reversed. Evidence of Chemical Changes Precipitation is another example of evidence from a chemical change. Two clear solutions can react when mixed forming a yellow precipitate. The presence of the precipitate tells you a new substance has been formed. Two liquids give you a solid Everyday examples: milk that curdles, soap scum, egg white when cooked Carbon Snake Video Evidence of Chemical Changes Another way to tell if a chemical change has taken place is if there is a change in odor. For example when food spoils like an egg rots or your bread is moldy or old it rots. This rotting process is a chemical change. Carbon snake video Evidence of Chemical Changes Gas Production is another line of evidence that shows chemical change! Oxygen is formed during photosynthesis and collect on the leaves of plants. This oxygen is a product of the reaction between carbon dioxide and water. Colored Fire Evidence of Chemical Changes A drastic change in temperature is another sign of a chemical reaction. When natural gas (methane) is burned this is a chemical reaction that creates heat. This is how gas stoves work because of heat created from the burning of the gas. Evidence of Chemical Changes A change in heat, light, or sound can indicate a chemical change. For example the explosion of fireworks or a crackling fire. Heat may be taken in or given off. For example striking a match or mixing vinegar and an antacid! Keep in mind that heat, light, and sound are all different forms of energy. So when chemical changes take place energy is being added or taken away. Chlorate Fire REACTIONS and PHASE CHANGES: ENDOTHERMIC = heat is absorbed EXOTHERMIC = heat is realeased video Examples of Chemical Properties A chemical property is a characteristic that has to be tested to be known and describes the ability of the substance to change into a new substance. The following are examples of chemical properties that can be altered when a chemical change takes place. Flammability, nonflammable, reacts with oxygen, reacts with water, pH, reactivity, and toxicity. Chemistry in Fireworks Sodium in Water Identify Chemical Properties What are the chemical properties for an apple? Identify Chemical Properties What are the chemical properties for lemon juice? Chemical Reactions involving Atmospheric Oxygen Sometimes chemical reactions can be caused by the atmospheric oxygen we breath in. For example fires need oxygen to burn. Without oxygen that chemical reaction could not take place. Chemical Reactions involving Atmospheric Oxygen Rusting is a general term that we use when iron reacts with oxygen is in the presence of water or air moisture. Without oxygen things wouldn’t rust! Oxygen once again is a key part of a chemical reaction. Chemical Reactions involving Atmospheric Oxygen Respiration and Photosynthesis are two chemical reactions that rely upon oxygen in the atmosphere. Oxygen is a byproduct in photosynthesis as plants try to get glucose for energy. Oxygen is the component we breath in which gives life to our aerobic cells. These cells can then perform certain life cycles so they can create useable energy for our body. Chemical Changes can affect Physical Properties Sometimes you can have chemical changes that even affect physical properties. For example when you cook an egg it raises the temperature of the egg and causes a chemical change which can not be reversed. This also changes the physical texture and phase of the egg. Chemical Changes can affect Physical Properties Once again we look back at rusting! This is a chemical reaction but causes changes in the overall appearance of the metal that is being oxidized. The point is chemical changes can and often do cause physical properties to change. Difference Between Physical and Chemical Changes Physical changes only effect Chemical changes will effect physical properties. Physical changes produce no energy. Physical changes produce no new substances the atoms are arranged the same way in the products and reactants. Physical changes are generally easy to reverse. both physical and chemical properties. Chemical changes produce energy generally in the form of heat, light, or sound. Chemical changes produce new substances the atoms rearrange and form new compounds. Chemical changes are not easily reversible without an additional chemical reaction. Reactants vs. Products Reactants are the elements or materials that go into the reaction. Products are the elements or materials that come out of the reaction Usually a chemical reaction will reach a state of equilibrium where the reactants and the products equal one another. Notice that the same atoms that are reactants rearrange and become the products. Conservation of Mass No matter how many reactants and products are involved, all the atoms present at the start of a reaction are present at the end. The amount of matter in a chemical reaction does not change. Therefore, the total mass of the reactants must equal the total mass of the products. This is the principle called the conservation of mass. Matter is neither created nor destroyed!