Rotting food is an example of odor development as a result of a chemical change. An example of a color change signaling a chemical reaction can be observed when iron reacts with oxygen to produce iron oxide, such as when an iron nail is left outside, and it develops a reddish-brown rust. A chemical cold pack in a first aid kit is an example of a chemical reaction that absorbs heat energy resulting in cooling. Burning wood is an example of a reaction that releases excess energy as heat. Alternatively, a reaction may require energy from the environment in order to take place, causing heat to be absorbed, and leading to a decrease in temperature. When the chemical bonds of the reactants are broken, sometimes excess energy is released, causing heat to be discharged, and leading to an increase in temperature. In a chemical alteration, the temperature change occurs as a result of the breaking or formation of chemical bonds. To help determine whether there has been a reaction, chemists consider the basic indicators that a reaction has occurred, such as a change in temperature, a change in color, the development of an odor, the formation of a precipitate, or the formation of a gas. Sometimes it is difficult to tell if a chemical reaction has taken place. This results in the rearranging of atoms in substances to form the products of a chemical reaction, which are brand new molecules that cannot be easily reverted back to their original state. What Is a Chemical Change? A chemical change occurs when the composit ion of a substance is changed, which requires the breaking and forming of chemical bonds during a chemical reaction. The chemical structure of water is the same whether it is a solid (ice), liquid, or gas (steam). Think about ice melting into water, and then water being heated up and turning into steam. Similarly, when a material changes phase, it only changes physically the substance is still the same. A piece of metal may be heated in a fire until it glows, but the metal is the same material before heating and after cooling. The wood itself has not changed during sanding to become a new material, only the texture of the surface changed. For instance, a block of wood may feel rough when you run your finger across it but rubbing the wood with sandpaper smooths the surface so it no longer feels rough. A change in the texture of a substance is a change in the way it feels. Types of some physical changes are texture, shape, temperature, and a change in the state of matter. What Is a Physical Change? In a physical change, the material involved in the change is structurally the same before and after the change. Chemical changes, on the other hand, are not reversible: A log burned in a fire turns to ashes, but the ashes cannot be changed back into a log. Physical changes in matter are often reversible: An ice cube can melt into liquid water, and then the liquid water can be frozen back into an ice cube. Find the length of chord \( A B \).Matter is capable of undergoing changes, which are classified as either physical or chemical. If p, q are real and p≠q, then show that the roots of the equation $(p-q)x^2 5(p q)x-2(p-q)=0$ are real and unequal.Give two examples for each of the following cases:(a) Physical changes which are reversible.(b) Physical changes which are not reversible.(c) Chemical changes.(a) What is a physical change? Give two examples of physical changes.(b) What is a chemical change? Give two examples of chemical changes.What are the five examples of Chemical Changes?.
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