Evidence of a Chemical Change

1 Describe five types of evidence detectable by human senses that usually indicate a chemical change. 

Physical and chemical changes were introduced in Section 2.3. 

In Chapter 2 we stated that a chemical change occurs when the chemical identity of a substance is destroyed and a new substance forms. Particles of matter are literally changed. The number and type of atoms that make up molecules are the same before and after a chemical change, but the number and type of molecules changes. If we could observe matter at the particulate level, we would have a simple method for detecting chemical change. Of course, we cannot directly see what happens at the particulate level, so we must rely on indirect evidence of particulate-level rearrangements. 

Another visible form of evidence of a chemical change is the formation of a solid product when clear solutions are combined (Fig. 8.2). Substances in each of the reacting solutions combine to form the solid product. Formation of a gas, as evidenced by bubbles forming in a liquid, is another form of visible evidence of a chem- 

Are any of the following a chemical change Give evidence for each answer. 

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Process Put in the bottle a few drops of neutral water and a piece of blue litmus paper. Adjust the platinum terminals so that they are near the bottom of the bottle and from 3 mm. to 6 mm. apart. Connect the platinum wires with the coil, and pass sparks through the apparatus for about twenty minutes, or until there is definite evidence of a chemical change revealed by the litmus paper. 

Make sure that deteriorating containers, or containers in which evidence of a chemical change in the contents is apparent (e.g., appearance of peroxide crystals in a bottle of an ether), are inspected and handled by someone experienced in the possible hazards inherent in such situations. 

Evidence of a Chemical Change Evolution of a Chemical Equation... 

Each of these photos illustrates evidence of a chemical reaction. Q Reactions that happened when the marshmallow was burned are obvious by the color change. 

Evidence of a chemical reaction How can you tell when a chemical reaction has taken place Although some chemical reactions are hard to detect, many reactions provide physical evidence that they have occurred. A temperature change can indicate a chemical reaction. Many reactions, such as those that occur during the burning of wood, release energy in the form of heat and light. Other chemical reactions absorb heat. 

A A change in temperature due to absorption or emission of heat is evidence of a chemical reaction. This chemical cold pack becomes cold when the barrier separating two substances is broken. 

The main evidence of a chemical reaction indudes a color change, the formation of a solid, the formation of a gas, the emission of Ught, and the emission or absorption of heat. 

Figure 8.1 Color change as evidence of a chemical reaction. When a solution containing hydroxide ion is added to crystal violet dye, the intensity of the color decreases with time until it disappears. The product of the reaction is colorless in water solution. 

Figure 8.6 Heat transfer as evidence of a chemical reaction. Heat can be either released or absorbed by a chemical system undergoing a chemical change. When heat is absorbed, as in this instant cold compress, your skin feels cold because you are the source of heat that drives the reaction. 

Table 8.1 summarizes the five types of evidence that indicate the possibility of a chemical change. 

A chemical reaction always involves chemical change because atoms of the reacting substances form new combinations with new properties. During a chemical change, there is usually visual evidence that a chemical reaction has taken place (see Figure 8.2). For example, a chemical reaction occurs when an antacid tablet is dropped into a glass of water. The NaHCOs and citric acid (C6Hg07) in the tablet react to form bubbles (gas) of carbon dioxide (CO2). Other types of visible evidence of a chemical reaction are listed in Table 8.1. 

There is supporting evidence in the literature for the validity of this method two cases in particular substantiate it. In one, tests were made on plastics heated in the pressure of air. Differential infrared spectroscopy was used to determine the chemical changes at three temperatures, in the functional groups of a TP acrylonitrile, and a variety of TS phenolic plastics. The technique uses a film of un-aged plastic in the reference beam and the aged sample in the sample beam. Thus, the difference between the reference and the aged sample is a measure of the chemical changes. 

In short, we may say that there is evidence that the deviations from the relations predicted by the characteristic equations may be due to chemical changes in the substances, which are not taken into consideration in the kinetic deduction of the equations. Weinstein loc. cit.) considers that it is possible to deduce an equation which takes account of these chemical changes as well. It will be sufficient here to re-einphasise the fact that there is at present no characteristic equation known which agrees accurately with the behaviour of a single substance, let alone various substances, over a wide range of temperature. 

Mechanical treatment alone may be sufficient to induce significant decomposition such processes are termed mechanochemical or tribo-chemical reactions and the topic has been reviewed [385,386]. In some brittle crystalline solids, for example sodium and lead azides [387], fracture can result in some chemical change of the substance. An extreme case of such behaviour is detonation by impact [232,388]. Fox [389] has provided evidence of a fracture initiation mechanism in the explosions of lead and thallium azide crystals, rather than the participation of a liquid or gas phase intermediate. The processes occurring in solids during the action of powerful shock waves have been reviewed by Dremin and Breusov [390]. 

The sodium hydrogen carbonate underwent a chemical change. What evidence do you have of this change ... 

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