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Diffusion, dilution and solutions
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Simple experiments show that matter is made of tiny moving particles. Solutions have their own key words.
Diffusion of gases
Put cotton wool soaked in ammonia solution at one end of a long glass tube and cotton wool soaked in hydrochloric acid at the other. The gases diffuse along the tube and react to form a white ring of ammonium chloride.
The ring forms nearer the hydrochloric acid end. Ammonia molecules (M_r 17) are lighter than hydrogen chloride molecules (M_r 36.5), so they move faster.
A brown gas such as bromine also slowly spreads through a gas jar of air by diffusion.
The ring forms nearer the hydrochloric acid end. Ammonia molecules (M_r 17) are lighter than hydrogen chloride molecules (M_r 36.5), so they move faster.
A brown gas such as bromine also slowly spreads through a gas jar of air by diffusion.
Dilution of coloured solutions
Dissolve a crystal of purple potassium manganate(VII) in water and dilute it again and again.
The colour gets paler but is still seen after many dilutions. This shows the crystal contains a huge number of very small particles that spread out among the water particles.
The colour gets paler but is still seen after many dilutions. This shows the crystal contains a huge number of very small particles that spread out among the water particles.
Solution words
Solvent: the liquid that does the dissolving (such as water).
Solute: the substance that dissolves.
Solution: the mixture formed when a solute dissolves in a solvent.
Saturated solution: a solution in which no more solute can dissolve at that temperature.
Solute: the substance that dissolves.
Solution: the mixture formed when a solute dissolves in a solvent.
Saturated solution: a solution in which no more solute can dissolve at that temperature.
In the ammonia and hydrogen chloride experiment, the white ring forms closer to the hydrochloric acid. Explain why.
- Ammonia has a lower M_r (17) than hydrogen chloride (36.5).
- Lighter particles move faster, so ammonia diffuses further in the same time.
Answer: Ammonia particles are lighter so they diffuse faster and travel further.
Gases diffuse because their particles move randomly. Lighter molecules diffuse faster, so the ammonium chloride ring forms nearer the hydrochloric acid. Dilution shows particles are tiny and numerous. Solvent dissolves solute to make a solution; a saturated solution can dissolve no more.
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