From NCERTAsked by Anonymous from London, United Kingdom24 Oct 2026, 6:09 AM ISTUpdated
In one line
Mercury is dangerous in old thermometers because it is a poisonous metal that can harm your body and the environment if it leaks out.
Mercury (element) — Chemical element with atomic number 80 (Hg) Mercury, the shiny silver liquid inside old thermometers, is poisonous if touched or breathed in. Image: WikipediaThermometer — Device to measure temperature Old glass thermometers often used mercury to show temperature, but are now being replaced by safer types. Image: Wikipedia
The rule
Mercury is a toxic (poisonous) metal that can enter the body through the skin, breathing, or swallowing, and cause serious health problems.
Let's understand it simply
Imagine you drop an old thermometer and it breaks. Inside, you see a shiny, silver liquid—that's mercury. It looks like fun to touch, but it's actually very dangerous. Mercury can turn into a gas at room temperature. If you breathe this gas, it can go into your body and make you sick. Even a small amount can be harmful over time.
How it works
Old thermometers use mercury because it expands and contracts evenly with temperature changes.
If the thermometer breaks, mercury spills out as a liquid.
Mercury can slowly turn into an invisible gas (vapour) at room temperature.
If you touch it or breathe in the vapour, mercury enters your body.
Mercury can harm your brain, nerves, kidneys, and even your lungs.
The story behind it
Mercury thermometers have been used for over 300 years. People liked mercury because it is shiny, does not stick to glass, and shows temperature changes clearly. But scientists later found that mercury is very poisonous. Now, most countries are replacing mercury thermometers with safer ones.
Where you see it in real life
Old clinical thermometers: Used to check fever at home—if broken, mercury can spill out.
School science labs: Old lab thermometers may still contain mercury.
Broken blood pressure machines: Some old machines use mercury.
Old switches and bulbs: Some electrical items used mercury inside.
Spilled mercury in the market or home: Sometimes, people play with mercury without knowing the risk.
Why it matters
Health risk: Mercury can cause memory loss, shaking, and trouble walking if it gets into your body.
Danger to children: Kids are more likely to touch or play with spilled mercury.
Pollutes water and soil: Mercury can get into rivers and fields, harming fish and crops.
Safer alternatives: Digital thermometers and alcohol thermometers do not use mercury and are safe.
Common mistakes
Thinking mercury is safe because it looks cool—it's actually very poisonous.
Trying to clean up spilled mercury with a broom or vacuum—this spreads the mercury vapour more.
Not washing hands after touching mercury—this can let the poison enter your body.
Remember it
If a thermometer breaks and you see silver liquid, stay away and tell an adult—mercury is poison, not a toy.
Check yourself
What is the shiny liquid inside old thermometers called?
Mercury, a poisonous metal.
Why should you not touch or play with spilled mercury?
Because mercury is poisonous and can harm your body.
What should you do if a mercury thermometer breaks?
Stay away and tell an adult, because mercury is dangerous.
Why are digital thermometers safer than mercury ones?
They do not contain poisonous mercury, so they are safe if broken.
Quick recap
Mercury is a shiny, silver liquid metal used in old thermometers.
It is poisonous if touched, breathed in, or swallowed.
Mercury can harm your brain, nerves, and kidneys.
Never touch or play with spilled mercury.
Digital and alcohol thermometers are safer alternatives.
