Changes of Matter: Dissolving, Evaporating and Freezing
FoundationSugar and salt dissolve in water to form solutions, and stirring, heating and crushing speed dissolving; when water evaporates the salt crystallises; water freezes to ice and melts back — matter changes in cycles.
- Dissolving makes a solution: solute (sugar, salt) in solvent (water); the substance remains as tiny particles.
- Stirring, heating and crushing speed dissolving; saturation comes when no more dissolves.
- Evaporation turns liquid into gas in the air; drying clothes and making salt both use it.
文體小知識：物質變化實驗文帶我們在廚房做實驗，親眼看見溶解、蒸發、凝固這些物質變化。
§Text Passage
1Stir a spoonful of sugar into water: the grains disappear and the water becomes a clear, sweet liquid — dissolving. The mixture is a solution: the sugar is the solute and the water the solvent. The sugar has not vanished; it has broken into particles too small to see, spread through the water. Oil shows not everything dissolves: poured into water it floats on top and forms no solution.
2How can sugar dissolve faster? Three ways: stirring spreads the grains quickly; heating — warm water dissolves more and faster than cold; crushing — powdered sugar dissolves faster than a cube because of the larger contact surface. A cup of water can take only so much sugar: keep adding until grains lie undissolved at the bottom, and the solution is saturated.
3The states of matter change in cycles. Water slowly becomes invisible vapour and drifts into the air — evaporation: drying clothes on the line and the salt crust left on the beach after high tide are both evaporation at work. Heat salt water and, as the water evaporates, the salt crystallises on the dish — the very principle of making sea salt. Cooled to zero degrees Celsius, water freezes into ice; heated, the ice melts back to water. In dissolving, evaporating, freezing and melting, the matter itself does not become another substance — these are physical changes, going round and round.
4Little experiments at home show it best: pour salt water into a shallow dish and leave it in the sun for days; as the water evaporates, white salt crystals appear at the edges. Pour melted chocolate into a mould and chill it for an hour, and it sets solid again. Watching and recording how matter changes is the first step of scientific inquiry.
1Summary 重點摘要
Sugar and salt dissolve in water to form solutions, and stirring, heating and crushing speed dissolving; when water evaporates the salt crystallises; water freezes to ice and melts back — matter changes in cycles.
2Vocabulary cards Vocabulary
A substance breaking into particles spread through a liquid to form a solution.
The even mixture formed when a solute dissolves in a solvent.
The substance that dissolves, such as sugar in water.
The liquid that dissolves another substance, such as water.
A solution that can dissolve no more solute.
A liquid slowly turning into gas and drifting into the air.
Solute forming crystals from a solution, such as grains of salt.
A liquid cooling into a solid, such as water becoming ice.
A solid turning into a liquid when heated, such as ice becoming water.
3Step by step Steps
- Step 1: Meet dissolvingSome substances spread out in water into tiny particles invisible to the eye; this is dissolving. For example, stir a spoonful of salt into one hundred millilitres of water; the grains vanish, the water stays clear but turns salty, because the salt is spread as tiny particles, forming a salt solution. Dissolved material has not disappeared; it can be brought back by methods such as evaporation — a very important point.
- Step 2: Tell what dissolves from what does notNot everything dissolves in water, and we should learn to tell them apart. For example, sugar and salt dissolve in water; sand sinks to the bottom undissolved; oil floats on top and does not dissolve, clearly forming two layers on standing. By watching what happens after a material enters the water — whether it vanishes, sinks or floats — we can judge whether it dissolves, and then choose the most suitable way to handle it.
- Step 3: Experiment to find faster dissolvingDissolving can be fast or slow, and comparison experiments show how to speed it up. For example, prepare three cups of equal cold water: a sugar cube in the first left untouched, a sugar cube stirred in the second, powdered sugar in the third; the stirred cup and the powdered cup finish first, proving that stirring and crushing both speed up dissolving. Using hot water instead of cold makes it faster still, because the warmer it is, the faster the particles move.
- Step 4: Meet saturated solutionsWater can only dissolve a limited amount of a substance, and at the limit the solution is saturated. For example, keep adding sugar until grains lie undissolved at the bottom — the solution is saturated; heat it and more sugar dissolves, and the extra sugar reappears on cooling. The warmer the water, the more sugar it can hold — a property often used in making drinks and processing food.
- Step 5: Watch evaporationWhen warmed, water slowly turns into vapour and escapes into the air; this is evaporation. For example, drop equal amounts of water on two glass slides, one in the sun and one in the shade; the sunny one dries first — the warmer, the faster the evaporation. A breezy place also speeds evaporation. Hanging washing in a sunny, well-ventilated spot uses exactly this principle, and a puddle drying up after rain shows the same process.
- Step 6: Watch crystallisationMaterial dissolved in water does not vanish; when the water evaporates it appears again as crystals. For example, pour saturated salt water into a shallow dish by the window for days; when the water has evaporated, white salt crystals appear at the bottom and edges — the salt has come back. Sea-salt pans by the coast use this method of evaporation and crystallisation to make salt from seawater, and the rows of pans are a splendid sight.
- Step 7: Watch freezing and meltingThe states of matter change in cycles: a liquid turning solid when cooled is freezing, and a solid turning back to liquid when warmed is melting. For example, water in the freezer becomes ice — freezing; the ice taken out melts back to water at room temperature — melting. Melted chocolate set solid again in the fridge is freezing too. Through all these changes the total amount stays the same and only the form changes — the truth that matter is never lost.
4Diagrams Diagrams
5Worked example Example
Example: Yan wants to separate the salt from salt water. Design the experiment steps and explain the science of each step.
- Pour the salt water into a shallow dish, measure and record the amount, and leave it in a sunny, airy place to speed evaporation.
- Wait several days: the water keeps evaporating into vapour, and the level in the dish slowly falls.
- When all the water has gone, white salt crystals remain at the bottom and edges — the salt is separated.
- The science: salt's boiling point is far above water's, so it does not evaporate with the water; as the water evaporates the salt crystallises — the principle of sea-salt pans.
- Answer: Evaporation removes the water and the salt crystallises: pour the salt water into a shallow dish, sun it for several days, and crystals remain — exactly the principle used in sea-salt pans by the coast.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is the mixture called when sugar dissolves in water, and what are the sugar and the water?
Q2What three methods speed up dissolving?
Q3What is a saturated solution?
Q4What principle does making sea salt use?
Q5At what temperature does water freeze into ice, and do ice and water differ in substance?
8Recap Recap
- Dissolving makes a solution: solute (sugar, salt) in solvent (water); the substance remains as tiny particles.
- Stirring, heating and crushing speed dissolving; saturation comes when no more dissolves.
- Evaporation turns liquid into gas in the air; drying clothes and making salt both use it.
- Water freezes at zero degrees Celsius and the ice melts back; no matter is lost.
- Dissolving, evaporating, freezing and melting are physical changes, cycling again and again.