1. Mixing two colorless liquids yields a colorless liquid.
QUESTION:
Which of the following observations implies that a chemical change has definitely not occurred?
Mixing two colorless liquids yields a colorless liquid.
A piece of paper, held next to a flame, catches fire and leaves a black residue.
Both
Neither
Which of the following observations implies that a chemical change has definitely not occurred?
A….Mixing two colorless liquids yields a colorless liquid.
B….A piece of paper, held next to a flame, catches fire and leaves a black residue.
C…Both
D…Neither
PAUSE
The correct answer is D…. Neither.
HIGHLIGHT choice D
Burning or combustion, which is the production of heat and light, is a definite sign that a chemical change has occurred.
CLICK
Therefore, choice B is wrong.
CROSS OUT Choice B
Choice A is also wrong.
CLICK The lack of a visible change does not necessarily mean that no chemical reaction has occurred.
In other words, the lack of an evidence of a reaction does not mean that it’s not happening.
One counter example is the reaction between aqueous solutions of sodium hydroxide
HIGHLIGHT NaOH
and hydrochloric acid
HIGHLIGHT HCl
The aq labels here,
HIGHLIGHT labels
mean that these substances are dissolved in water. Sodium hydroxide and hydrochloric acid
are both colorless in water.
CALLOUT “colorless” pointing to NaOH(aq) and HCl(aq)
HIGHLIGHT products
Both products are also colorless.
CALLOUT “colorless” pointing to NaCl(aq) and H2O(l)
Thus, in this example, there is no visible evidence that a reaction has occurred.
However, a solution of sodium hydroxide is bitter….
CALLOUT “bitter” pointing to NaOH(aq)
Whereas a solution of hydrochloric acid is sour…
CALLOUT “sour” pointing to HCl(aq)
The product… sodium chloride… is salty.
CALLOUT “salty” pointing to NaCl(aq)
The change in taste is a clear indication that a chemical change has occurred.
Warning…. Do not taste anything in the laboratory!
CLICK PAUSE END RECORDING
Burning or combustion (production of heat and light) is a definite sign of a chemical change.
Lack of visible evidence for a reaction does not mean that no reaction has occurred. Counter example:
NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l)

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Johuan Gasery Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08