Ministry Expectations / SNC2D (chemical reaction0
C2.6 Plan and conduct an inquiry to classify some common substances as acidic, basic, or neutral (eg. Use acid-base indicators or pH test strips to classify common household substances)
C3.6 Describe the process of acid-base neutralization (ie. An acid reacts with a base to form a salt and often water)
C3.7 Describe how the pH scale is used to classify solutions as acidic, basic, or neutral (eg. A solution with a pH of 1 is highly acidic; a solution with a pH of 7 is neutral)
Introduction and Background / In this demonstration, a tube with a rainbow of colors at different pH will be made. Universal indicator is a mixture of different pH indicators that produce characteristic colors at different pH regions.
Materials / Safety goggles, long tube (open at both ends), rubber stoppers, water (60 mL), strong acid (0.1 M HCl—10 mL) and strong base (0.1 M NaOH—10mL) of equal molarity, Universal indicator.
Safety / Materials are fairly safe. Sodium hydroxide is corrosive. Hydrochloric acid is hazardous. Do not ingest or inhale any of these materials. For personal protection you should wear goggles. Both solutions can be poured down the sink with excess water. Ensure there is no leak in the tube and rubber stopper fits securely.
Procedure
/ 1. Ensure that all glassware are clean of contaminants
2. Fill a beaker with 60 mL water, add 15 drops of indicator, and transfer to the tube
3. Add acid to one end of the tube and invert
4. Add base to the other end of the tube
5. Invert a few times until the spectrum is visible
Results / This demonstration produces reds, oranges, and yellows colours on the acid end; purples and blues on the basic end; and green in the middle where acid and base ions meet in equal concentrations to neutralize each other.
Explanation / -The universal indicator is an acid-base indicator that turns blue in the presence of strong base and red in the presence of strong acid. Actually a variety of other colors are displayed that correspond to different acid/base pH values.
-The liquid in the tubeat the beginning contained a neutral water solution. Adding hydrochloric acid, which is an acid that causes the indicator to turn red. Sodium hydroxide is a base and caused the solution to become purple. The mixing of ions causes different colour to appear in the tube.
-Diffusion means to blend and become all one color. If the tube is left undisturbed, the rainbow will last as long as a couple of weeks, before becoming all one color. This demonstrates the very slow rate of diffusion in liquids.
Tips for Success/Teacher Notes / -Remember the 5 P’s of demonstrations. “Prior Practice Prevents Poor Presentations”
-Leave about a two centimeter “air” bubble in the tube to aid in the mixing process
- Make sure both stoppers are secure during the demonstration. It is necessary to hold both stoppers in place while the demonstration is taking place.
- Having a white background available, to act as a background screen, is helpful to emphasize the rainbow of colors at the end of the demonstration.
References / http://www.vwreducation.com/nsta/4997-71_RainbowTube.pdf (Scholar Chemistry)
2009, ScholAR Chemistry
Making Connections / Make links to Acid Rain (Environmental Chemistry Issue-SNC1D)
Can make links to Plants in the natural environment (SBI3U-F2.3)
Rainbow in a Tube Student Worksheet
1. What is an acid? A base? What is the pH scale? What are the pH values for each?
2. Why is the water green?
3. Why is the whole spectrum visible though only one type of acid and base is added? Answer with a help of an eqution.
4. What are the colours of the spectrum?
5. What is the name given to a chemical substance that reveals whether a substance is an acid or a base? Give two examples.
6. What would happen to the color of the solution in the demonstration tube if it was shaken vigorously?
6. How would the reaction be different if sodium bicarbonate is used instead?
Answer key: Rainbow in a tube
1. What is an acid? A base? What is the pH scale? What are the pH values for each?
· An acid is a substance that donates a proton and base is a substance that accepts a proton. The pH scale measures the “acidity” of substances. It runs from 0-14 with acids being substances with a pH<7 and bases are substances with pH>7, and neutral substances measuring a pH = 7
2. Why is the water green?
· Solution is green because it is neutral.
3. Why is the whole spectrum visible though only one type of acid and base is added? Answer with a help of an equation.
HCl + NaOHà NaCl + H2O
· The contents of the tube have not mixed very well. The strong acid (HCl) was added to one end and the strong base (NaOH) to the other. Since these are “strong” acids and bases, the universal indicator shows a different color at low and high pH. As the ions migrate away from the ends, the concentration of the ions decrease and the universal indicator registers this with colors that suggest less acid or base ions are present. When the acid and base ions meet in the middle in equal concentrations, the universal indicator should reveal the pH color which approximates neutral (green).
4. What are the colours of the spectrum?
· Red, yellow, green, purple
5. What is the name given to a chemical substance that reveals whether a substance is an acid or a base? Give two examples.
· The name given to these types of substances is “indicator”. Common to the laboratory are “two-color indicators” (litmus red, phenolphthalein, phenol red, bromothymol blue, etc.) and “wide-range indicators” like the universal indicator used in this demonstration
6. What would happen to the color of the solution in the demonstration tube if it was shaken vigorously?
· Shaking the tube should thoroughly mix the solution and reveal the closet approximate pH of the neutralized reaction solution
Universal Indicator Chart
Universal indicator chart above