Solids, Liquids and Gases
FoundationMatter has three states: solids keep their shape and volume, liquids flow but keep their volume, and gases spread to fill the space. Heating and cooling change matter between states, but the matter itself never disappears.
- Matter has three states: solid, liquid and gas.
- Melting: solid to liquid with heat; freezing: liquid to solid on cooling.
- Evaporation: liquid to gas; condensation: gas to liquid on cooling.
文體小知識：趣味實驗說明文用容易做的小實驗講科學，廚房裏的材料就能看見物質的變化。
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1Most matter around us comes in three states. Solids keep a fixed shape and volume, such as stones, ice cubes and a wooden ruler. Liquids have no fixed shape and flow, but keep a fixed volume, such as water, milk and soy sauce. Gases have neither fixed shape nor fixed volume and spread through the whole space, such as air and water vapour.
2States can change into one another. Solids melt into liquids when heated: ice melts into water in a warm room, and chocolate goes soft in your palm. Liquids freeze when cooled: water in the freezer turns to ice. Liquids evaporate into gases when heated: wet clothes dry slowly in the sun, and boiling water gives off clouds of water vapour.
3Gases condense back into liquids when cooled. Stand a chilled can of soft drink on the table and tiny drops soon appear on the outside — water vapour in the air touching the cold surface and condensing. Dew on the grass at dawn is formed the same way, as night-time water vapour cools.
4Try a little experiment: put chocolate in a small bowl and stand the bowl in warm water. In a few minutes the chocolate softens and melts — solid becoming liquid. Pour the melted chocolate into a mould and chill it in the fridge for an hour, and it sets back into a solid. It is still the same matter; only the state has changed.
1Summary 重點摘要
Matter has three states: solids keep their shape and volume, liquids flow but keep their volume, and gases spread to fill the space. Heating and cooling change matter between states, but the matter itself never disappears.
2Vocabulary cards Vocabulary
The state of matter with fixed shape and volume, such as a stone or an ice cube.
The flowing state of matter with no fixed shape but a fixed volume, such as water.
The state of matter with no fixed shape or volume that spreads to fill a space, such as air.
A solid changing into a liquid when heated, such as ice melting into water.
A liquid changing into a solid when cooled, such as water turning to ice.
A liquid slowly turning into gas at its surface, such as wet clothes drying.
A gas turning into liquid when cooled, such as the drops on a chilled drink can.
Water in its gas form, present in the air and invisible to the eye.
The form in which matter exists, such as solid, liquid or gas.
3Step by step Steps
- Step 1: Learn the features of the three statesMatter commonly comes in three states: solid, liquid and gas. A solid keeps a fixed shape and size, like a wooden ruler that never changes by itself; a liquid has no fixed shape — water poured into different cups takes each cup's shape, though the amount stays the same; a gas drifts in every direction, so a puff of air freshener soon fills the whole room. Hold on to two points when judging — does it keep a fixed shape, and does it flow — and confusion becomes unlikely.
- Step 2: Sort everyday thingsOnce you know the features, use them to sort the things around you. Judge a stone, milk, oxygen, an eraser, orange juice and cooking gas one by one: the stone and eraser keep their shape, so they are solids; milk and orange juice flow and change shape with the container, so they are liquids; oxygen and cooking gas are invisible and drift about, so they are gases. With practice you will sort quickly and accurately. When you meet a substance that is hard to judge, note the question first and add the conclusion after asking a teacher or checking a book.
- Step 3: Watch meltingA solid melts into a liquid when it is heated to a certain temperature. Leave an ice cube on a plate at room temperature and it takes in heat from the air, melting into water within half an hour; hold chocolate in your palm for two minutes and your body warmth begins to soften it. Melting needs heat, which is why ice cream left out of the freezer melts so fast in summer. Different solids melt at different temperatures: ice melts near zero degrees Celsius, while iron needs great heat to become molten.
- Step 4: Watch freezingA liquid freezes into a solid when it cools to a certain temperature — the opposite of melting. Pour water into an ice tray and put it in the freezer; the water slowly gives out heat, and after two hours it has become hard ice, a liquid turned back into a solid. Melted candle wax is another example: it cools, hardens and keeps its new shape. Freezing gives out heat, so the process of turning to ice is actually heat leaving the water — the exact opposite of melting.
- Step 5: Watch evaporation and condensationA liquid slowly turns into a gas at any temperature — this is evaporation; water vapour turning back into liquid on meeting something cold is condensation. A wet handkerchief hung outside dries within an afternoon as its water evaporates into the air; pour a chilled soft drink into a glass and the vapour outside condenses on the cold wall, soon covering it with tiny drops. The drier and windier the weather, and the more sunshine, the faster evaporation goes — so dry the washing on a bright, breezy day.
- Step 6: Record the changesThe three states of water change in a cycle, and a diagram makes it clear. Draw a cycle of ice, water and water vapour: ice melts into water when heated, water evaporates into vapour, vapour condenses into water when cooled, and water freezes back into ice. Label the four arrows melting, evaporation, condensation and freezing, and the connections will stay firmly in your mind. Hang the finished cycle diagram in your room; one look each day fixes the four changes even more firmly.
4Diagrams Diagrams
5Worked example Example
Example: Ching left a glass of iced lemon tea on the table. Twenty minutes later the outside of the glass was covered with tiny drops, and the ice cubes inside had become smaller. Explain both observations.
- The ice shrank: it took in heat from the air and the tea and melted from solid to liquid, mixing into the tea.
- Drops appeared outside: water vapour in the air touched the icy wall of the glass.
- The vapour condensed into liquid droplets, clinging one by one to the outside of the glass.
- Melting and condensation happened at the same time — two changes of state at once, while the matter itself stayed the same.
- Answer: The ice took in heat and melted, so it grew smaller and smaller; water vapour in the air met the icy glass wall and condensed into tiny drops. Both are changes of state — no matter was created or lost, only the form changed.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the features of solids, liquids and gases?
Q2When does water melt, and when does it freeze?
Q3Where do the drops on the outside of a chilled drink come from?
Q4What is the process when wet clothes dry?
Q5When a substance changes state, does the substance itself change?
8Recap Recap
- Matter has three states: solid, liquid and gas.
- Melting: solid to liquid with heat; freezing: liquid to solid on cooling.
- Evaporation: liquid to gas; condensation: gas to liquid on cooling.
- Drops on a cold drink and dawn dew are both examples of condensation.
- Changing state never destroys matter — only its form changes.