Boiling
Changes of State: Boiling
Boiling
Boiling: Fast Liquid to Gas
What you'll learn
- Define boiling and identify the boiling point of water.
- Distinguish boiling from slow evaporation.
- Give examples of boiling used in Indian cooking.
- Explain the use of boiling to make water safer to drink.
Key concepts
Level 1 - Meaning of boiling
Boiling is the fast change of a liquid into a gas when it is heated strongly. Bubbles rising throughout a pot of water on a hot stove are a sign that the water is boiling.
Level 2 - Boiling point
Water boils at 100 degrees Celsius at normal sea-level pressure. While boiling continues, the temperature of the water stays at 100 degrees Celsius until all of it turns into steam.
Level 3 - Boiling versus evaporation
Evaporation happens slowly, only at the surface of a liquid, even at room temperature. Boiling happens quickly, throughout the whole liquid, and needs strong heating to reach the boiling point.
Level 4 - Indian context
Rice, dal, and tea are cooked using boiling water in most Indian kitchens every day. Drinking water is sometimes boiled at home to kill germs and make it safer, especially during the monsoon season.
NCERT anchor: Looking Around 4, Ch 18 — Too Much Water, Too Little Water (boiling water for safe drinking)
Worked example
Boiling water for tea
Step 1 - Pour water into a pan and place it on a hot stove.
Step 2 - Heat the water steadily.
Step 3 - Watch bubbles form and rise throughout the water at 100 degrees Celsius.
Step 4 - Add tea leaves once the water is boiling.
Answer: The water reached its boiling point and changed rapidly into steam and bubbles, showing it was boiling.
Comparing evaporation and boiling
Step 1 - Leave a bowl of water on a table for a day; it slowly gets less, evaporating at the surface.
Step 2 - Heat the same amount of water in a pan on a stove instead.
Step 3 - Watch it reach 100 degrees Celsius and bubble rapidly throughout.
Step 4 - Compare how much faster the heated water turns to steam.
Answer: Evaporation is slow and only at the surface; boiling is fast and happens throughout the liquid.
Common mistakes
| Mistake | Why it happens | Fix |
|---|---|---|
| Boiling and evaporation are exactly the same | Both change liquid water into vapour | Evaporation is slow and only at the surface; boiling is fast and throughout the liquid at 100 degrees Celsius |
| Water gets hotter and hotter the longer it boils | Bubbles look more intense over time | Boiling water stays at 100 degrees Celsius; extra heat just makes more steam, not hotter water |
| Boiling water is only used for making tea | Tea is the most familiar use | Boiling water is also used to cook rice, dal, eggs, and to make drinking water safer |
| A covered and an uncovered pot boil at the same speed | Both are heated on the same stove | A covered pot traps heat and usually reaches boiling faster than an uncovered one |
Quick check
- What is boiling?
- What is the boiling point of water at sea level?
- How is boiling different from evaporation?
- Why do people boil drinking water at home?
- Stretch: Explain why a pressure cooker cooks food faster using boiling water.
Revision tip: Boiling turns a liquid quickly into a gas throughout the liquid once it reaches its boiling point (100 degrees Celsius for water).
Open the Practice tab for graded questions on Boiling.
Key Takeaways (TL;DR)
- What you'll learn
- Key concepts
- Worked example
- Common mistakes
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