Freezing
Liquid to solid on cooling.
Freezing
Freezing
What you'll learn
- Freezing — liquid changes to solid when cooled to its freezing point.
- For water, freezing point = 0 °C — same temperature as melting point of ice.
- How refrigerators and freezers use freezing to preserve food.
- NCERT Looking Around 5, Chapter 7 (Experiments with Water) and Chapter 4 (Mangoes Round the Year) — storing food.
Key concepts
Level 1 — Liquid to solid
Verbal: When liquid water loses heat to 0 °C, it turns to ice — particles lock into fixed positions.
Symbolic: Liquid --(cool)--> Solid at freezing point.
| Substance | Freezing point | Example |
|---|---|---|
| Water | 0 °C | Ice tray in freezer |
| Cooking oil | Below room temp (approx.) | Cloudy in cold winter |
| Mercury | Very low (not in home) | Thermometer liquid |
NCERT link: Ch 4 — storing mangoes; Ch 7 — water becoming ice in freezer.
Level 2 — Reverse of melting and daily life
Verbal: Freezing and melting are reverse changes at the same temperature for pure water.
Real-life: Kulfi set in freezer; ice packs for injury; frozen peas retain nutrients.
| Application | Benefit |
|---|---|
| Freezer at home | Slows food spoilage |
| Ice cream | Frozen dessert |
| Mountain streams | Night freeze → morning melt |
Worked example
Water at 5 °C placed in freezer (−18 °C). Describe changes.
Step 1 — Water cools: 5 °C → 0 °C (still liquid).
Step 2 — At 0 °C, freezes to ice.
Step 3 — Ice may cool below 0 °C in freezer.
Answer: Liquid → solid ice.
Ice left on table at 25 °C room — what happens?
Step 1 — Heat enters ice.
Step 2 — At 0 °C, melts to water (melting — reverse of freezing).
Answer: Freezing reversed → melting occurs.
Common mistakes
| Mistake | Why it happens | Fix |
|---|---|---|
| Freezing and cooling same thing | Temperature vs state change | Cooling lowers temp; freezing = liquid→solid at FP |
| All liquids freeze at 0 °C | Water-centric | Each substance has its own FP |
| Freezer removes all bacteria | Spoilage slowed not always killed | Many microbes survive cold — cooking still important |
| Ice is colder than 0 °C always | Freezer ice can be below 0 | Formation at 0 °C; can stay colder afterward |
Quick check
- What is freezing?
- Freezing point of water (°C)?
- Is freezing the reverse of melting?
- Stretch: Why does salt spread on roads in snowy countries? (Lowers FP — ice melts.)
Revision tip: Label home freezer temperature (often −18 °C on sticker) — connect to how long milk stays fresh.
Open the Practice tab for graded questions on Freezing.
Key Takeaways (TL;DR)
- What you'll learn
- Key concepts
- Worked example
- Common mistakes
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