What makes indicators change color

Solution First, you should estimate the pH at the equivalence point, at which the solution is 0. Colors of an Indicator Solution Indicators change color gradually at various pH. Questions There are numerous natural indicators present in plants. The dye in red cabbage, the purple color of grapes, even the color of some flowers are some examples. What is the cause for some fruits to change color when they ripen? Choose the true statement: All weak acids are indicators.

All weak bases are indicators. Weak acids and bases are indicators. All indicators are weak acids. An acid-base conjugate pair has different colors. Any indicator changes color when the pH of its solution is 7. There may be other reasons too. Recall that pH indicators are not only natural dyes but also weak acids. The dissociation of the weak acid indicator causes the solution to change color.

It is important here to note that the equation expressed in figure 4 is in equilibrium, meaning Le Chatelier's principle applies to it. For example, common indicators such as phenolphthalein, methyl red, and bromothymol blue are used to indicate pH ranges of about 8 to 10, 4. On these ranges, phenolphthalein goes from colorless to pink, methyl red goes from red to yellow, and bromothymol blue goes from yellow to blue.

For universal indicators, however, the pH range is much broader and the number of color changes is much greater. See figures 1 and 2 in the introduction for visual representations. Usually, universal pH indicators are in the paper strip form. Additionally, pH indicators can be found in nature, so therefore their presence in plants and flowers can indicate the pH of the soil from which they grow.

Nature contains several natural pH indicators as well: for example, some flower petals especially Roses and Hydrangeas , certain fruits cherries, strawberries and leaves can change color if the pH of the soil changes. See figure 7. Refer to the table of Universal Indicator Color change figure 1 in the introduction for clarification.

The household detergent contained a concentrated solution of sodium bicarbonate, commonly known as baking soda. As shown, the pH paper turns a dark blue: baking soda in solution is basic. Here is a closer look of the pH papers before and after dipping them in the lemon juice and cleaning detergent Figure 10 :. The phenolphthalein indicator has two different structures based on whether it is in an alkali pink or acid colorless solution. Both structures absorb light in the ultra-violet region, a region not accessible for the human eye.

However, the pink form also absorbs in the visible light spectrum. The reason for the visible light absorption is the structure of the pink form of the phenolphthalein indicator. Due to ionization, the electrons in the molecule are more delocalized than in the colorless form. Briefly, delocalization is when electrons in a molecule are not associated with a single atom, and instead are spread over more than one atom.

An increase in delocalization shifts the energy gap between molecular orbitals. Less energy is needed for an electron to make the jump into a higher orbital. The absorption of energy is in in the green region, nanometers, of the light spectrum. The human eye perceives a pink hue in the solution. Mar 22, Explanation: The video below shows an experiment using an indicator derived from boiling red cabbage.

Other common indicators include: bromothymol blue thymol blue methyl orange bromocresol green methyl red phenol red Hope this helps!

Related questions How do you determine ph of a solution? How do you determine pH at an equivalence point?