Which one of the following acids is secreted by leaves of Nettle that causes painful stings ?
Methanoic acid
Understanding Nettle Stings and Secreted Acids
Nettle plants are well-known for their stinging hairs, which can cause irritation, pain, and itching upon contact. This unpleasant sensation is due to chemicals injected into the skin by specialized structures on the leaves and stems.
Identifying the Acid in Nettle Stings
The stinging hairs of Nettle leaves act like tiny needles. When brushed against, the tips break off, and the sharp edges pierce the skin. This allows the liquid stored in the hair to be injected. This liquid contains several irritating chemicals, including histamine, acetylcholine, serotonin, and crucially, an acid.
The primary acid responsible for the painful sting from Nettle leaves is Methanoic acid.
- Methanoic acid is also commonly known as Formic acid. Its chemical formula is \( \text{HCOOH} \).
- This acid is relatively simple but acts as a strong irritant to human skin, causing the characteristic burning and itching sensation associated with Nettle stings.
Analyzing the Options
Let's look at the other options provided and why they are not the acid responsible for Nettle stings:
- Citric acid: Found in citrus fruits like lemons and oranges. It is a weak organic acid (\( \text{C}_6\text{H}_8\text{O}_7 \)). It is not secreted by Nettle leaves.
- Tartaric acid: An organic acid found in grapes, bananas, and tamarinds. It is also a weak acid (\( \text{C}_4\text{H}_6\text{O}_6 \)) and not related to Nettle stings.
- Acetic acid: The main component of vinegar (\( \text{CH}_3\text{COOH} \)). While an organic acid, it is not found in significant amounts in Nettle stinging hairs to cause the primary pain sensation.
Therefore, the acid secreted by the leaves of Nettle that causes painful stings is Methanoic acid.
Revision Table: Common Acids and Sources
| Acid Name | Common Source(s) | Chemical Formula |
|---|---|---|
| Methanoic acid (Formic acid) | Nettle stings, Ant stings | \( \text{HCOOH} \) |
| Citric acid | Citrus fruits (lemons, oranges) | \( \text{C}_6\text{H}_8\text{O}_7 \) |
| Tartaric acid | Grapes, Tamarinds, Cream of tartar | \( \text{C}_4\text{H}_6\text{O}_6 \) |
| Acetic acid | Vinegar | \( \text{CH}_3\text{COOH} \) |
Additional Information on Nettle Stings
While Methanoic acid is a key component, the mix of chemicals in Nettle stings works together to produce the strong reaction. Histamine causes allergic reactions like itching and swelling, acetylcholine affects nerve endings, and serotonin also plays a role in the pain response. Traditionally, rubbing the area with dock leaves (which often grow near nettles) is said to neutralize the sting, though the effectiveness is debated and may be more mechanical or psychological.
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