Prep.2 | Changes in the States of Matter | Unit 1 – Lesson 2 | Science

شرح ساينس الصف الثاني الاعدادي لغات – الفصل الدراسي الاول
شرح ساينس تانية اعدادي – ترم اول

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in this lesson:
The Change in the Physical State of Matter
• There is a continuous increase in the temperature from A to B.
• The substance is in its solid state.
• The particles of the solid substance vibrate faster in their positions
as a result of gaining thermal energy, which leads to increasing their kinetic energy.
• There is no change in temperature from B to C ,because the absorbed thermal energy is consumed in weakening the attraction forces between the particles of the solid substance till it completely changes (converts) into the liquid state.
• The substance in this interval exists in both solid and liquid states.
• Point B or C (this stable temperature) represents the melting point of the substance.
• There is a continuous increase in the temperature from C to D.
• The substance is in its liquid state.
• The speed of the liquid particles increases as a result of gaining thermal energy, which leads to increasing their kinetic energy.
• There is no change in temperature from D to E , because
the absorbed thermal energy is consumed in weakening
the attraction forces between the particles of the liquid substance
till it completely changes (converts) into the gaseous state.
• The substance in this interval exists in both liquid and gaseous states.
• Point D represents the boiling point of the substance.
• There is a continuous increase in the temperature from E to F
• The substance is in its gaseous state.
• The speed of the gas particles increases as a result of gaining thermal energy, which leads to increasing their kinetic energy.

Factors affecting the melting and the boiling points of the substances
Technological application – The pressure cooker (steam pot)
Its importance : Cooking food in a shorter time compared to cooking it in a normal cooking pot to save fuel (energy).
Its idea: – Trapping water vapour (steam) inside it during cooking, which increases the pressure inside it and raises the boiling point of water above 100°C, which reduces cooking time and saves fuel.

The conversions of matter from one state to another are reversible processes:
• Water turns into ice by cooling, and conversely, ice turns into water by heating.
So that, melting and freezing processes are reversible processes.

• Water vapour turns into water by condensation , and conversely, water turns into water vapour through evaporation.
So that, evaporation and condensation are two reversible processes. 

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