Alkalis are basic substances that can turn blue litmus paper red. Litmus paper contains a dye that is sensitive to pH and can be used to determine if a solution is acidic or basic. Blue litmus paper turns red in the presence of an alkali because alkalis have a high pH.
What is an alkali?
An alkali is a basic, or alkaline, substance that has a pH greater than 7. Some common alkalis are sodium hydroxide (lye), potassium hydroxide (potash), calcium hydroxide (limewater), ammonia, and sodium bicarbonate (baking soda). Alkalis are often soluble bases that dissociate into ions and increase the concentration of hydroxide ions (OH-) in aqueous solutions.
How do alkalis work?
When an alkali dissolves in water, it produces hydroxide ions (OH-). For example, when sodium hydroxide (NaOH) dissolves in water it dissociates into sodium ions (Na+) and hydroxide ions:
NaOH(aq) → Na+(aq) + OH-(aq)
The hydroxide ions make the solution alkaline, meaning it can neutralize acids. This gives alkalis the ability to turn indicators like litmus paper a different color.
What is litmus paper?
Litmus paper is filter paper that has been treated with a natural water-soluble dye obtained from lichens. The dye undergoes a color change in response to the pH of the solution it is exposed to. There are two types of litmus paper:
- Red litmus paper – turns blue in alkaline solutions (pH > 7)
- Blue litmus paper – turns red in acidic solutions (pH
By using both types of litmus paper, you can determine if a solution is acidic, basic, or neutral.
Why does alkali turn blue litmus paper red?
When blue litmus paper is exposed to an alkali, the high concentration of hydroxide ions (OH-) causes the dye in the litmus paper to change from blue to red. Litmus dye contains a type of chemical group called a chromophore that is sensitive to protons (H+ ions). In acidic solutions where H+ concentrations are high, the chromophore accepts protons and turns red. In alkaline solutions where OH- concentrations are high, the chromophore loses H+ ions and turns blue.
Acid-base indicators
Litmus is an example of an acid-base indicator – a weak acid that has different colors in its protonated (acid) and deprotonated (base) forms. Other naturally occurring acid-base indicators include:
| Indicator | Color in Acid | Color in Base |
|---|---|---|
| Litmus | Red | Blue |
| Phenolphthalein | Colorless | Pink |
| Methyl orange | Red | Yellow |
These indicators exhibit a color change over a certain pH range which allows them to be used to determine if a solution is acidic or basic.
How to test if an alkali turns blue litmus paper red
A simple way to test if an alkali can turn blue litmus paper red is to:
- Obtain blue litmus paper
- Cut a strip of blue litmus paper
- Place a drop of the alkali solution onto the litmus paper strip
- Observe if the litmus paper changes color to red
Some alkalis to test are sodium hydroxide, calcium hydroxide, or ammonia solution. As soon as the drop makes contact with the litmus paper, the paper should turn red, indicating the solution is alkaline.
Real-life examples
There are several real-life examples where alkalis turn blue litmus paper red:
- Drain cleaners contain sodium hydroxide which would turn litmus paper red.
- Ammonia-based household cleaners are alkaline so would also turn litmus red.
- Antacids like milk of magnesia contain magnesium hydroxide which would turn litmus red.
- Detergents are often alkaline and would turn litmus paper red.
In each case, the alkali dissociates into hydroxide ions that cause the color change in the litmus paper.
Other observations
In addition to the color change, some other observations when testing an alkali with litmus paper include:
- The color change happens quickly upon contact.
- The stronger the alkali, the faster and more intense the color change.
- Only the area of litmus paper in contact with the alkali will change color.
- The litmus paper remains red as long as it is exposed to the alkali.
- The color change is not reversible – the litmus paper will remain red even when removed from the alkali.
Conclusion
In summary, alkalis are basic substances that contain a high concentration of hydroxide ions. When blue litmus paper comes into contact with an alkali solution, the hydroxide ions cause the dye in the litmus paper to change from blue to red. This color change is fast and irreversible. Therefore, alkalis can reliably be identified by their ability to turn blue litmus paper red.
