Indicator Sponge – A Discrepant Event Demonstration from Flinn Scientific

The discrepant event of placing a red sponge in a red solution and having it turn blue is sure to capture your students’ attention and stimulate a lively discussion of possible explanations.

The indicator sponge is saturated with congo red solution. Congo red is a dye, a biological stain, and a pH indicator. It has been used as a direct fabric dye for cotton to produce a bright red fabric. Biologists use Congo red as a general contrast stain for cellulose. Congo red is also used as a pH indicator. The colour transition is between pH 3.0 and 5.0. Below a pH of 3.0 (very acidic solutions), the indicator is blue. Above pH 5.0, the indicator is red. When a cellulose sponge is soaked in a Congo red solution, the dye becomes permanently bonded to the cellulose fibers. The active acid/base sites on Congo red are still available and the sponge now becomes an indicator sponge for acids. It can also be used to check for acid spills on counters after students have used acids.

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pH Rainbow Tube


In this demo, the teacher creates a beautiful rainbow of colours in a demonstration tube using universal indicator and a dilute
acid and base.


Universal indicator can be used to illustrate an entire range of pH conditions because it is made up of a mixture of different
indicators that change colour at different pH values. As an acid is diluted with water, its pH increases—but never above pH 7.
Likewise, as a base is diluted, its pH decreases—but, again, never below pH 7. Continue reading

Cabbage Patch Indicator

pH scale

Red cabbage extract will change colour depending on acidity and act as an indicator of a potential chemical change.  Acidity is measured on a pH scale where pH stands for “potential for Hydrogen”. Acidity is caused by hydrogen atoms that have lost their electrons and are roaming free in water [H+]. The scale goes from 0 to 14. The colour scale represents the concentration of these positively charged hydrogen ions [H+]. Acid, or low pH, gives foods a sour taste, and base or high pH is usually bitter.

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