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Acid-Base Neutralization
Prior knowledge

Indicators are chemicals that change color in acids or bases. Universal indicator has been engineered to give a wide range of colors, in a pattern that is easy to remember because it follows the same sequence of colors found in a rainbow. 
 
red   orange   yellow
green
blue    violet
acidic
basic
pH < 7
pH = 7
pH > 7
                   
By looking at the color of a solution, you can easily tell whether the substance being tested is an acid, a base or neutral. You can also determine the relative pH. For example, if a substance turns red with universal indicator, it is more acidic than a solution that turns orange or yellow.
 
Materials
 
  • Magnetic stirrer with magnet
  • Universal indicator
  • Pipet containing 1.0 M HCl
  • Pipet containing 1.0 M NaOH
  • Pipets containing unknowns A, B, C, D and E
 
Procedure
 
  1. Obtain a 250 mL beaker and fill it approximately half way with water.  Add 5 drops of universal indicator to obtain a green color. Water, which is nearly neutral, displays a green color with universal indicator.
      
    Make note of this fact- universal indicator is green in a neutral solution.
 
  1. Place the magnet into the solution. Turn on the stirrer. Add HCl, drop by drop, until the color turns red. As a solution is acidified, the color of universal indicator changes from orange, to yellow, to red. 
 
Take note of this fact- universal indicator is red, yellow or orange in an acid. Red  indicates a greater degree of acidity. 
 
  1. At this point the solution should be red. Add NaOH, drop by drop, to neutralize the acid. Wait for the solution to completely mix between adding drops. Eventually, the color should change to green as the solution becomes neutral, then it will turn blue and violet as excess NaOH is added. 
 
Take note of this fact- universal indicator is violet or blue in a basic solution. 
 
  1. At this point, the solution is violet. You want to turn the solution back to neutral, by adding acid, so that you begin the next part of the lab with a green solution.
 
  1. If the acid (HCl) and the base (NaOH) are equal in strength, then equal volumes of each should neutralize each other. To test this out, begin with a green solution. Add 5 drops of HCl. Next, add 5 drops of the NaOH. If the solution becomes green again, it indicates that the acid and base are of equal strength. 
 
What if the solution turns red? This indicates that the acid is stronger than the base. If the solution turns blue, then the base is stronger than the acid. 
 
Make note of this: Comparing the number of drops of acid needed to neutralize the base gives you an idea of how strong the acid is compared to the base. 
 
     
Part II- Unknown Analysis
 
You have 5 unknowns. The objective of Part II  is for you to determine everything you possibly can about them. Determine whether the unknowns are acidic, basic, or neutral. If the unknown is an acid, find out how strong it is compared to the 1.0 M NaOH. If the    unknown is a base, find out how strong it is compared to the 1.0 M HCl. The more you learn, the higher your score will be.
 
 
what you learned
supporting evidence
unknown A
 
 
unknown B
 
 
unknown C
 
 
unknown D
 
 
unknown E
 
 



E Saylor

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