Physical & Chemical Changes - Assignment Example

Physical & Chemical Changes. Physical and chemical changes are everywhere, but these processes sometimes are so simple and common, that we may not even notice them. For example, if we cut an apple into two sections and ten minutes later observe what happens to the inside color, we will notice that it has become darker. Why The oxygen in the air reacts chemically with the apple and causes a color change. Another example is the burning of a candle. The wax candle melts down into liquid wax. This is a physical change. At the same time, some of the liquid wax changes to a gas which burns. This burning of the wick and gaseous wax is an example of a chemical change. Thus, a chemical change (or chemical reaction) is a change in which one kind of matter is changed into a different type of matter. Some other examples of chemical changes include the rusting of your car, setting your shoe on fire or digesting food. All of these materials combine chemically with another material , and cannot be separated by any physical means. A physical change is a change in the form of matter but not in its identity. The dissolving of one thing into another thing is an illustrative. For instance, dissolving sugar into water. The water and the sugar retain their chemical identities and can be separated by physical means. One more example - ice melting to water. Ice and water are both H20. The identity of the matter is not changed, just the state that it is in. Remember: a physical change is when no new chemical substance is formed. As it has already been mentioned, a chemical reaction occurs when two or more chemical substances are mixed together and change into new substances. For this to happen, the bonds between atoms and molecules must break up and then re-form in different ways. Because the bonds can be strong, energy (usually in the form of heat), is often needed to start a reaction. A reaction, which gives out heat energy, is known as exothermic. A reaction, which takes in heat energy, is endothermic. Fireworks are exothermic. Frying eggs is an endothermic reaction. The new substances (products) have different properties from those of the original substances (reactants). Remember: when a new chemical substance is produced, the change is known as a chemical change. This does not mean that new elements have been made. In order to make new elements, the nuclear contents must change, but there are magnitudes of difference in the amounts of energy in ordinary chemical reactions compared to nuclear reactions. The alchemists, in their efforts to change less expensive metals to gold, did not have the fundamental understanding of what they were attempting to do to appreciate the difference. What happens to matter when it undergoes chemical changes According to the law of conservation of mass, atoms are neither created, nor destroyed, during any chemical reaction. Thus, the same collection of atoms is present after a reaction as before the reaction. The changes that occur during a reaction just involve the rearrangement of atoms (Chemtutor, 2003; Gallagher, 2001). Chemical reactions are represented on paper by chemical equations. For example, hydrogen gas (H2) can react (burn) with oxygen gas (O2) to form water (H20). The chemical equation for this reaction is written as: The '+' is read as 'reacts with' and the arrow means 'produces'. The chemical formulas on the left represent the starting substances, called reactants. The substances produced by the reaction are shown on the right, and are called products. The numbers in front of the formulas are called coefficients (the number '1' is usually omitted). Because atoms are neither created nor destroyed in a reaction, a chemical equation must have an equal number of atoms of each element on each side of the arrow (i.e. the equation is said to be 'balanced'). Here are some more examples of chemical reactions with the corresponding chemical equations: a silver spoon tarnishes. The silver reacts with sulfur in the air to make silver sulfide, the black material we call tarnish: 2 Ag + S Ag2S. An iron bar rusts. The iron reacts with oxygen in the air to make rust: 4 Fe + 3 O2 2 Fe2O3 (Blaber, 1996). There are several different types of chemical change. Synthesis reaction. In a synthesis reaction two or more simple substances combine to form a more complex substance. Two or more reactants yielding one product is another way to identify a synthesis reaction. For example, simple hydrogen gas combined with simple oxygen gas can produce a more complex substance - water. The chemical equation for this synthesis reaction looks like: (reactant + reactant -------> product). To visualize a synthesis reaction look at the following cartoon: In the cartoon, the skinny bird (reactant) and the worm (reactant) combine to make one product, a fat bird. Decomposition reaction. In a decomposition reaction a more complex substance breaks down into its more simple parts. One reactant yields 2 or more products. Basically, synthesis and decomposition reactions are opposites. For example, water can be broken down into hydrogen gas and oxygen gas. The chemical equation for this decomposition reaction looks like: (reactant -------> product + product). To visualize a decomposition reaction look at the following cartoon: In this cartoon the egg (the reactant), which contained the turtle at one time, now has opened and the turtle (product) and egg shell (product) are now two separate substances. Single replacement reaction. In a single replacement reaction a single uncombined element replaces another in a compound. Two reactants yield two products. For example when zinc combines with hydrochloric acid, the zinc replaces hydrogen. The chemical equation for this single replacement reaction looks like: (reactant + reactant ---------> product + product). To visualize a single replacement reaction look at the following cartoon: Notice, the guy in the orange shirt steals the date of the other guy. So, a part of one of the reactants trades places and is in a different place among the products. Double replacement reaction. In a double replacement reaction parts of two compounds switch places to form two new compounds. Two reactants yield two products. For example, when silver nitrate combines with sodium chloride, two new compounds - silver chloride and sodium nitrate are formed because the sodium and silver switched places. The chemical equation for this double replacement reaction looks like: (reactant + reactant ---------> product + product). To visualize a double replacement reaction look at the following cartoon: (Westbroek, 2000) So, these are the basic ideas connected with physical and chemical change. Certainly, these processes are much more complex and go with each other in real life. Thus, the distinction between physical and chemical change is not absolute, and some important types of change, such as the dissolving of salt in water, could be regarded as either being chemical or physical, or both. References. 1.Blaber, M. (1996), Stoichiometry: Chemical Formulas and Equations. General Chemistry I: A Virtual Textbook. Retrieved February 27, 1996 from http://wine1.sb.fsu.edu/chm1045/notes/Stoich/Equation/Stoich01.htm. 2.Chemtutor. (2003). Reactions. Chemtutor, LLC. Retrieved October 27, 2003, from http://www.chemtutor.com/react.htm. 3.Gallagher, L. (2001). Chemical and Physical Changes. Chemical and Physical Changes for 11 to 14-years old. Retrieved August 2, 2001, from http://www.lcc.ukf.net/KS3Chem/chemphychanges.htm. 4.Westbroek, G. (2000). To react or not to react THAT is the question! Utah State 8th Grade Integrated Science. Retrieved August 7, 2000, from http://www.usoe.k12.ut.us/CURR/Science/sciber00/8th/matter/sciber/chemtype.htm. Read More