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Chemical Reactions and Equations Learning objectives To observe physical and chemical properties associated with chemical changes To be able to give evidence for the occurrence of a chemical reaction To write a balanced chemical equation To identify areaction as a combination, decomposition, precipitation, single or double replacement, or combustion reaction ntroduction When a substance undergoes a physical change, it changes its appearance but not its composition. For example, when ice melts to form water, it undergoes a physical change from solid to liquid. In a chemical change, the composition of a substance is changed to give a new substance with different physical and chemical properties. For example, when silver tarnishes, the shiny silver (Ag) changes to a dull gray silver sulfide (AgaS). Not only has the chemical formula changed from Ag to Agus, but also the physical properties (shininess) of the substance have changed. Some examples of physical and chemical changes are listed in Table 1 below. Table 1. Comparison of Physical and Chemical Changes Examples of Physical changes Examples and Evidence of Chemical Changes Change in state (melting, freezing, evaporating Formation of a gas (bubbles) subliming) Formation of a solid (precipitate) Change in size Tearing Change in Color Heat is given off or absorbed A solid dissolving in a solvent In a chemical reaction, atoms in the reactants are rearranged to produce new combinations of atoms in the products. However, the total number of atoms of each element in the reactants is equal to the total number of atoms in the products. This is true according to the Law of Conservation of Mass, which states that the total mass remains constant during a chemical reaction. In other words, matter cannot be created or destroyed in a chemical reaction. In a chemical equation, the reactants are shown on the left and the products are on the right. An arrow in between indicates that a chemical reaction occurs. Finally, the physical state of each compound is designated in parentheses after the chemical formula. Solid, liquid, gas, and aqueous (dissolved in water) are shown as(s,(n,(g), and (aq), respectively.

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1. Which characteristic below describes ONLY a solid (not liquid or gas)?
a) definite shape b) particles in contact c) definite volume d) completely fills container
2. When iron rusts, it forms a reddish-brown solid. This is:
a) a physical change b) a chemical change c) no change at all
3. What is the name of FeSO4?
a) iron (I) sulfate b) iron (II) sulfate c) iron sulfite d) iron sulfide
4. A bottle label states that the bottle contains âdrinking water with minerals added for flavor. This would be an example of:
a) an element b) a compound c) a heterogeneous mixture d) a homogeneous mixture
5. Which answer has the correct number of significant figures for: length = 2.0 m + 0.2100 m?
a) 2.2 b) 2.21 c) 2.210 d) 2.2100
Place the answer with the correct number of significant figures on the lines provided below.
6. To drive one kilometer, how many centimeters should you drive?
_______________cm
7. A 10.0 gram block of gold has a density of 19.32 g/cm3. What is its volume?
______________cm3
8. What is the formula of sodium sulfide?
_________________________
9. What is the name of H2S?
_______________mm
10. How many neutrons are in 35Cl?
_______________ neutrons
For problems 11 through 13, show your work to receive full credit and use the correct number of significant figures in your final answer.
11. The density of nickel is 8.91 g/cm3. Calculate the mass of a nickel ball with a radius of 0.50 mm in grams.
(Ï = 3.1416; Vsphere = 4Ïr3/3)
12. The recommended adult dose of Elixophyllin , a drug used to treat asthma, is 6.00 mg/kg of body mass. Calculate the dose in milligrams for a 134-lb person.
13. It is 273 K outside. Do you need a jacket? Why or why not? (show your work for full credit)
14) Element X has three naturally occurring isotopes. The masses (amu) and % abundances of the isotopes are given in the table below. What is the average atomic mass of the element in amu?

isotope38 abundance of 5.07,mass of 37.919

isotope39x,abundance of 15.35,mass of 39.017

isotope 42,abundance of 79.85,mass of 42.111

15. Classify each of the following elements as alkali, alkaline earth, halogen or a noble gas:
a) He ________________________
b) F ________________________
c) Na ________________________
d) Ca ________________________
e) Br ________________________

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