Changing State

Substances can change from one state to another

• Physical changes don’t change the particles-just their arrangement or their energy   * When a solid is heated, its particles gain more energy   * This makes the particles vibrate more, which weakens the forces that hold the solid together   * At a certain temperature, called the melting point the particles have enough energy to break free from their positions, called melting and turns a solid to an liquid   * When a liquid is heated, again the particles get even more energy   * This energy makes the particles move faster, which weakens and breaks the bonds holding the liquid together   * At a certain temperature, called the boiling point, the particles have enough to break their bonds     * This is evaporating, and the liquid becomes a gas   * As a gas cools, the particles no longer have enough energy to overcome the forces of attraction between them   * Bonds form between the particles   * At the boiling point, so many bonds have formed between the gas particles that the gas becomes a liquid, this is called condensing   * When a liquid cools, the particles have less energy so move around less   * There’s not enough energy to overcome the attraction between the particles, so more bonds form between them   * At the melting point, so many bonds have formed between the particles that they’re held in place     * The liquid becomes a solid and this is called freezing

Heating up

• Melting - When a solid is heated, it absorbs energy and it melts, turning into a liquid.
• Boiling - If the liquid is heated, it absorbs more energy and it boils, turning into a gas.
• These changes absorb energy from the surroundings so they are endothermic.
• Evaporating is when a liquid turns into a gas slowly, at temperatures that are below its boiling point. Puddles dry up because they evaporate – they don’t boil.

Cooling down

• Condensing - If a gas is cooled, it transfers energy to the surroundings, and turns into a liquid.
• Freezing - If the liquid is cooled, it transfers energy to the surroundings, and turns into a solid.
• These changes transfer energy to the surroundings so they are exothermic.

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