الأسبوع 1 · الوحدة الأولى «المادة والطاقة» (افتتاحية) والدرس الأول «حالات المادة»: المواد الصلبة والسائلة والغازية وخواصها ونموذج الجسيمات والبلازما · 3 دروس
1
Three states, three behaviours
Science Prep 2 Term 1, Unit 1: pp. 1–9
GoalCompare solids, liquids and gases by shape, volume, flow and compressing, and say what a fluid is.
New words
fluid · a liquid or a gas, because both can flowviscosity · how slowly a liquid flows; honey is more viscous than watercompress · to be squeezed into a smaller volume
Explanation
Matter has mass and takes up space, and it usually appears in three states. A solid keeps its own shape and its own volume: a stone stays a stone in any bag. A liquid takes the shape of its container but keeps its volume: 200 mL of water is still 200 mL in a cup or a bowl. A gas takes both the shape and the volume of its container: air fills a whole balloon, however big it is. Liquids and gases can flow, so we call them fluids. Fluids flow at different speeds: honey is more viscous than water, so it pours more slowly. Gases are easy to compress, liquids and solids are almost impossible to compress. Careful: sand and flour seem to pour, but each grain is a tiny solid, so a heap of sand is not a liquid.
State
Shape
Volume
Flows?
Easy to compress?
solid
fixed
fixed
no
no
liquid
takes the container’s
fixed
yes
no
gas
takes the container’s
takes the container’s
yes
yes
Worked examples
A bottle holds 500 mL of juice. Pour it into a jug: the shape changes but the volume is still 500 mL. So juice is a liquid. Now let out the air of a balloon into a room: the air spreads through the whole room, so air is a gas.
Honey, water and oil are poured at the same moment from three equal cups. Water reaches the bottom first and honey last. Honey is the most viscous, but all three are liquids and all three are fluids.
A sealed syringe of air is pressed and the plunger goes in far. The same syringe full of water barely moves. This shows that a gas is easy to compress and a liquid is not.
Class activity
Squeeze test, with a grown-up. Use a plastic syringe without a needle and a closed end. Fill one with air, one with water and one with fine sand, and seal each tip with a plug the grown-up checks first. Press each plunger and describe what you see. Then pour honey, water and sand from three equal spoons and decide which of the three really flows like a fluid.
Practice
1
Which state keeps its shape and its volume?
The solid state.
2
200 mL of milk is poured from a glass into a bowl. What changes and what stays the same?
The shape changes; the volume stays 200 mL, because milk is a liquid.
3
Why is a heap of sand not a liquid, although it pours?
Each grain is a small solid with its own shape; the heap only pours because the grains slide over each other.
4
Honey and water are both liquids. Which one is more viscous, and how can you tell?
