Every solution is an acid, a base, or neutral. Acids and bases are chemical opposites of each other. Solutions that are neither acids or bases are neutral. Chemists use numbers to indicate the strengths of acids and bases. The numbers go from 1 to 14. Strong acids have low numbers and strong bases have high numbers. Neutral solutions are in the middle.

Chemists use a solution called Universal Indicator to identify acids and bases. Universal Indicator changes color when mixed with an acid or base. The Universal Indicator Color Guide shows that Universal Indicator turns red when it is added to a strong acid, it turns purple when it is added to a strong base, and it turns yellowish-green when it is added to a neutral solution.

UNIVERSAL INDICATOR COLOR GUIDE


All acids in the range of 1 to 4 turn the indicator red. All bases in the range of 11 to 14 turn the indicator purple. Today you will learn how to test if one acid is stronger than another even if they both turn the indicator the same color.

PART 1: READING THE SCALE

1a. Which acid is stronger -- one that turns Universal Indicator orange or one that turns Universal Indicator yellow?
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1b. Which base is stronger -- one that turns Universal Indicator blue or one that turns Universal Indicator purple?
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All solutions are acids, bases, or neutral. You can use pH paper and a pH Color Chart to test whether a solution is an acid, a base or neutral.

Part 1: READING THE pH SCALE
To practice using the pH paper:

* Squeeze 6 drops of Solution X into one of the measuring cups. Gently swirl the cup.
* Take one strip of pH paper out of the bag, and dip it into Solution X.
* Remove the strip from the cup and quickly observe the color of the pH paper. Be sure to look at the color right away, because it will change quickly. The first color shows the correct pH.


1a. What is the color of the pH paper right after you dipped it into Solution X? __________

1b. What number on the pH Color Chart goes with this color? ______________

1c. Look at the chart below. Is Solution X an acid, a base, or neutral? ________


 

Figure 4.Portions of the student notebooks for two assessments on the same concept, acids and bases, generated by independent teams of developers with a shell whose directions were vague. (Stecher, Klein, Solano-Flores, McCaffrey, Robbyn, Shavelson, & Haertel, 1999). The notebooks reflect different interpretations for the same set of directions prescribed by the shell.