Learn English with American Primary School Textbooks

Chapter 13 Science General Knowledge (1)
01 Physical Changes Physical Changes
Matter can be found as a solid, liquid, or gas. A solid has a fixed size and shape. The solid may be big like a brick. A solid may be powdery like sand or sugar. A solid may change its shape but stay a solid. A block of ice may be smashed into smaller pieces. The smaller pieces are still solid. Each solid piece has a fixed size and shape. If the ice is heated and melts into water, the shape changes. When the solid melts, a liquid forms. A liquid has a fixed size, but no fixed shape. Solids changing to liquids, change shape, but the size are fixed. Liquids take the shape of the container.
Some matter is not a solid or a liquid. It is a gas. A gas does not have a fixed size or shape. Gases are sometimes hard to see. Many gases are invisible.
What kind of matter is perfume? Is it solid, liquid, or gas? When you smell things you smell gases. Think about the food your mother cooks. You can smell the dinner. Can you see the “smells”?
Air is a gas. Some gases have no odor. These gases are hard to find. Gases with no odor or color are invisible.
When liquids disappear they become gases. When a water puddle dries up, the water has turned into a gas. You smell the perfume because the liquid has become a gas. Gases come from liquids. When you cool a gas, it becomes a liquid again .
As ice melts, a physical change takes place. A liquid changing to a solid is another physical change. A puddle of water drying up is a physical change. Liquids' changing to gases is a physical change. A physical change may change the shape or size of something. That something is still the same thing after the change. The ice cube that melts is still water. The water that dries up is still water. Liquid water that freezes is still water. A physical change never changes something into something new .
There are other kinds of physical changes. Suppose you break a brick. Is this a physical change? Yes. The broken brick is still a brick. The broken brick did not change into something else. Suppose you smash a sugar cube. Is this a physical change? Yes. The smashed cube is still sugar.
Vocabulary
solid ["s?lid] adj. solid, solid, strong, reliable, pure;

n. solid
liquid ["likwid] n. liquid;

adj. Liquid, clear, bright, (sound) smooth, flowing
brick [brik] n. brick, brick;
vt. to build with bricks, to block with bricks

Powdery ["paud? ri] adj. powdery

smash [sm? ? ] n. Broken (sound), slam, slam, big success;

vt. & vi. break;

vt.
Melt [melt] vt. & vi. (make) melt (melt, melt), dissolve, (make) dissipate, disappear

container [k? n"tein?] n. container, container, container

invisible [in"viz? bl] adj. invisible, hidden
Perfume ["p?: fju: m] n. Aroma, fragrance, perfume;
vt. to fill with fragrance, to spray perfume on

odor ["? ud?] n. smell, reputation

Puddle ["p? dl] n. puddle, (especially on the road) rain puddle
dry up (make...) dry up, dry up, use up, exhaust, <English> dry dishes, <non-positive> shut up

freeze [fri:z] vt. & vi. (make) freeze;
n. cold period, freezing, freezing

Practice
1. Solids have a.

A． fixed size and fixed shape
B． fixed size and changing shape
C． changing shape and changing size
2. Liquid have a.

A． fixed size and fixed shape
B． fixed size and changing shape
C． changing shape and changing size
3. Gases have a.

A． fixed size and fixed shape
B． fixed size and changing shape
C． changing shape and changing size
4. Solids will if they get hot enough.

A． melt B． fall
5. Heat changes solids to ．

A． ice B． liquids
6. Liquids change to when they get hot.

A． gases B． the water
7. Liquids change to when they get cold.

A． solids B． gas
Translation
Objects can exist as solids, liquids, or gases.Solids have a definite size and shape, and they can be as large as a brick or as a powder like sand and sugar.A solid may change its shape but remain solid.A large block of ice can break into many smaller pieces that remain solid, each with its own size and shape.If ice is heated and melted into water, its shape changes.When the solid melts, it turns into a liquid form.Liquids have a definite size but no definite shape.Solids turn into liquids, changing shape, but size remains fixed.Liquids gain shape through containers.

Some objects are neither solid nor liquid, it is gas.Gases do not have a fixed size and shape, and are sometimes difficult to see. Many gases are invisible.

What kind of objects are aromatic?Is it a solid, liquid or gas?When you smell an odor, you smell gas.Think about the food your mom cooks and you can smell the dinner.Can you see the "taste"?

Air is a gas.Some gases are odorless, making them difficult to detect.Those colorless, odorless gases are invisible.

When the liquid disappears, they become a gas.When the water in a puddle dries up, the water has turned into gas.You can smell the aroma also because the liquid turns into a gas.A gas comes from a liquid, and when you cool the gas, it turns back into a liquid.

When ice melts, a physical change occurs.Liquid to solid is another physical change.A puddle of water drying up is a physical change, and a liquid turning into a gas is a physical change.Physical change can change the shape and size of a thing, but the thing remains the same thing after the change.An ice cube is still water when it melts, water is still water when it dries up, and liquid water is still water when it freezes.Physical change never changes one thing into another new thing.

There are other types of physical changes as well.If you break a brick, is that a physical change?Yes.A broken brick is still a brick, it has not turned into something else.If you were to crush a sugar cube, is that a physical change?Yes.Crushed sugar cubes are still sugar.

Exercise
1.There are solids.

A．Definite size and shape b.definite size, variable shape
C．Can vary in size and shape

2.There are liquids.

A．Definite size and shape b.definite size, variable shape
C．Can vary in size and shape

3.There is gas.

A．Definite size and shape b.definite size, variable shape
C．Can vary in size and shape

4.If given enough heat, the solid will.

A．melt b.decline

5.Heating will change the solid to .

A．ice B．liquid

6.When enough heat is applied, the liquid will become _____.

A．gas b.water
7.When a liquid cools, it becomes _____.

A．solid b.gas

1．A　2．B　3．C　4．A　5．B　6．A　7．A
02 Chemical Changes
How small can matter be? You can break a rock into many pieces, you can grind a lump of sugar into powder. But what is the smallest thing you can make? Scientists call the smallest bits of matter atoms and molecules. Millions of molecules make up the tiniest speck of dust. Atoms and molecules cannot be seen even with the most powerful microscopes. All matter is made of tiny atoms and molecules.
You have heated matter. You have cooled matter. You have smashed matter. You can change the size and shape of matter. All these changes are physical changes. Physical changes do not change one substance into another. molded. Sugar is still sugar even if put on your cereal, the sugar did not become salt. Physical changes only change the shape and size of a substance.
Not all changes are physical changes. Sometimes when matter changes, new matter is made. The new matter does not look like the old matter. The new matter does not act like the old matter. This kind of change is a chemical change
Many chemical changes are useful. The food you eat is changed chemically. When you digest your food, your body changes it chemically. The food you eat is no longer food when you digest it. Your body needs to change the food to use it. The digested food gives your body energy. A growing plant shows a chemical change. The food and water from the soil and air is changed to plant cells. These chemical changes are all useful chemical changes. New matter is formed from the old.
Chemical change may take place when two kinds of matter are mixed. Baking a cake is a chemical change. The different kinds of matter mixed to make the cake change as the cake bakes. The old matter joins together in a new way to make new matter .
Other chemical changes may not be useful. When food is kept too long it spoils. A chemical change takes place when food spoils. A fire is a chemical change. A forest fire is not a useful chemical change. A fire to warm your house is a useful chemical change. Burning can be useful or not useful. Burning is a chemical change that joins some matter and oxygen together.
(End of this chapter)