Course goals related: demonstrate qualitative knowledge and understanding of the
ID: 1042432 • Letter: C
Question
Course goals related: demonstrate qualitative knowledge and understanding of the basic laws of nature upon which all chemical principles are based, such as the law of conservation of mass and energy, the periodic law, and the gas laws [Scientific Literacy] 1. A beaker containing a green liquid is left uncovered in a laboratory for one week., After the liquid evaporates, the beaker contains a dry green solid. Was the original liquid in the beaker an element, a compound, or a mixture? Please explain your choice. Answer Explanation: Sweet-tasting grape juice is left unrefrigerated for several months, and turns into a sour-tasting liquid, even though no liquid evaporated. Why is this process considered to be a chemical change rather than a physical change? 2. Answer: During a laboratory experiment, a student added a few milliliters of hydrochloric acid solution to a test tube, and then added small piece of zinc metal to the acid. The mixture in the test tube began to bubble furiously, and became warm. 3. Is the process that took place in the test tube exothermic or endothermic? Please explain your choice. a) Answer Explanation Would the results of only this experiment be enough to establish a scientific law that acids react with metals in general? Explain your answer b) Answer ExplanationExplanation / Answer
1.The original liquid in the beaker is a mixture.
The components of the mixture can be separated by physical means but element and compound can be separated by physical means.
Between the constituents of the mixture,only physical forces exist.
They are not combined chemically.
In the given example , the solvent of the solution(mixture)evaporates and leaves the solute in the beaker.
2. This is an example of chemical change.
In a Chemical change new substance is formed. The chemical composition of the original substance changes.
Where as in a physical change no new substance is formed.
There is a change only in the physical properties of the substance.
But in the given question, grape juice contains sugars ( carbohydrates).Bacteria in the atmosphere reacts with them and changes them into acids which are sour in taste .
Chemical composition of the original substance is changed.Hence, it is a chemical change.
3. a)Reaction between Zn and HCl is as shown below:
Zn (s) + 2HCl(aq) -> ZnCl2(aq) + H2(g) + energy
The given reaction is an example of exothermic reaction because heat energy is released during the reaction due to this the solution became warm.
b) Law describes an observation.
Yes , scientific law explains what happens in a particular phenomenon.
So, this observation can be used to establish a scientific law.
Different metals react in different ways.
For example copper does not release hydrogen gas on reaction with dilute acids.
Generally most of the active metals react with dilute acids and release hydrogen gas.
The given reaction describes that. So it can be used to state the law but it is alone not sufficient to draw the generalisation.