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  • State the common phases of matter.
  • Explain the physical characteristics of solids, liquids, and gases.
  • Describe the arrangement of atoms in solids, liquids, and gases.

Matter most commonly exists as a solid, liquid, or gas; these states are known as the three common phases of matter . Solids have a definite shape and a specific volume, liquids have a definite volume but their shape changes depending on the container in which they are held, and gases have neither a definite shape nor a specific volume as their molecules move to fill the container in which they are held. (See [link] .) Liquids and gases are considered to be fluids because they yield to shearing forces, whereas solids resist them. Note that the extent to which fluids yield to shearing forces (and hence flow easily and quickly) depends on a quantity called the viscosity which is discussed in detail in Viscosity and Laminar Flow; Poiseuille’s Law . We can understand the phases of matter and what constitutes a fluid by considering the forces between atoms that make up matter in the three phases.

This figure has three parts. Part a shows a solid, and the atoms in the solid are shown as small red spheres held together in a grid. Part b shows a liquid in a short cylindrical container, and the atoms in the liquid are represented by small red spheres that can move past one another. The movement of the atoms is represented by arrows. Part c shows a cylinder that is labeled to indicate that it contains oxygen gas. The atoms in the gas are represented by small red spheres that move around. Their motion is indicated by arrows./
(a) Atoms in a solid always have the same neighbors, held near home by forces represented here by springs. These atoms are essentially in contact with one another. A rock is an example of a solid. This rock retains its shape because of the forces holding its atoms together. (b) Atoms in a liquid are also in close contact but can slide over one another. Forces between them strongly resist attempts to push them closer together and also hold them in close contact. Water is an example of a liquid. Water can flow, but it also remains in an open container because of the forces between its atoms. (c) Atoms in a gas are separated by distances that are considerably larger than the size of the atoms themselves, and they move about freely. A gas must be held in a closed container to prevent it from moving out freely.

Atoms in solids are in close contact, with forces between them that allow the atoms to vibrate but not to change positions with neighboring atoms. (These forces can be thought of as springs that can be stretched or compressed, but not easily broken.) Thus a solid resists all types of stress. A solid cannot be easily deformed because the atoms that make up the solid are not able to move about freely. Solids also resist compression, because their atoms form part of a lattice structure in which the atoms are a relatively fixed distance apart. Under compression, the atoms would be forced into one another. Most of the examples we have studied so far have involved solid objects which deform very little when stressed.

Connections: submicroscopic explanation of solids and liquids

Atomic and molecular characteristics explain and underlie the macroscopic characteristics of solids and fluids. This submicroscopic explanation is one theme of this text and is highlighted in the Things Great and Small features in Conservation of Momentum . See, for example, microscopic description of collisions and momentum or microscopic description of pressure in a gas. This present section is devoted entirely to the submicroscopic explanation of solids and liquids.

In contrast, liquids deform easily when stressed and do not spring back to their original shape once the force is removed because the atoms are free to slide about and change neighbors—that is, they flow (so they are a type of fluid), with the molecules held together by their mutual attraction. When a liquid is placed in a container with no lid on, it remains in the container (providing the container has no holes below the surface of the liquid!). Because the atoms are closely packed, liquids, like solids, resist compression.

Atoms in gases are separated by distances that are large compared with the size of the atoms. The forces between gas atoms are therefore very weak, except when the atoms collide with one another. Gases thus not only flow (and are therefore considered to be fluids) but they are relatively easy to compress because there is much space and little force between atoms. When placed in an open container gases, unlike liquids, will escape. The major distinction is that gases are easily compressed, whereas liquids are not. We shall generally refer to both gases and liquids simply as fluids    , and make a distinction between them only when they behave differently.

Phet explorations: states of matter—basics

Heat, cool, and compress atoms and molecules and watch as they change between solid, liquid, and gas phases.

States of Matter: Basics

Section summary

  • A fluid is a state of matter that yields to sideways or shearing forces. Liquids and gases are both fluids. Fluid statics is the physics of stationary fluids.

Conceptual questions

What physical characteristic distinguishes a fluid from a solid?

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Which of the following substances are fluids at room temperature: air, mercury, water, glass?

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Why are gases easier to compress than liquids and solids?

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How do gases differ from liquids?

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Questions & Answers

What does mean ohms law imply
Victoria Reply
what is matter
folajin Reply
Anything that occupies space
Any thing that has weight and occupies space
Anything which we can feel by any of our 5 sense organs
what is a sulphate
any answers
the time rate of increase in velocity is called
Blessing Reply
What is uniform velocity
Greetings,users of that wonderful app.
Frank Reply
how to solve pressure?
Cruz Reply
how do we calculate weight and eara eg an elefant that weight 2000kg has four fits or legs search of surface eara is 0.1m2(1metre square) incontact with the ground=10m2(g =10m2)
can someone derive the formula a little bit deeper?
what is coplanar force?
what is accuracy and precision
Peace Reply
How does a current follow?
Vineeta Reply
which one dc or ac current.
how does a current following?
AC current
AC current follows due to changing electric field and magnetic field.
you guys are just saying follow is flow not follow please
ok bro thanks
but i wanted to understand him/her in his own language
but I think the statement is written in English not any other language
my mean that in which form he/she written this,will understand better in this form, i write.
ok thanks bro. my mistake
u are welcome
what is a semiconductor
Vineeta Reply
substances having lower forbidden gap between valence band and conduction band
what is a conductor?
replace lower by higher only
convert 56°c to kelvin
How does a current follow?
A semiconductor is any material whose conduction lies between that of a conductor and an insulator.
what is Atom? what is molecules? what is ions?
Abubakar Reply
What is a molecule
Samuel Reply
Is a unit of a compound that has two or more atoms either of the same or different atoms
A molecule is the smallest indivisible unit of a compound, Just like the atom is the smallest indivisible unit of an element.
what is a molecule?
what is a vector
smith Reply
A quantity that has both a magnitude AND a direction. E.g velocity, acceleration, force are all vector quantities. Hope this helps :)
what is the difference between velocity and relative velocity?
Velocity is the rate of change of displacement with time. Relative velocity on the other hand is the velocity observed by an observer with respect to a reference point.
what do u understand by Ultraviolet catastrophe?
A certain freely falling object, released from rest, requires 1.5seconds to travel the last 30metres before it hits the ground. (a) Find the velocity of the object when it is 30metres above the ground.
A vector is a quantity that has both magnitude and direction
the velocity Is 20m/s-2
derivation of electric potential
Rugunda Reply
V = Er = (kq/r^2)×r V = kq/r Where V: electric potential.
what is the difference between simple motion and simple harmonic motion ?
simple harmonic motion is a motion of tro and fro of simple pendulum and the likes while simple motion is a linear motion on a straight line.
a body acceleration uniform from rest a 6m/s -2 for 8sec and decelerate uniformly to rest in the next 5sec,the magnitude of the deceleration is ?
Patricia Reply
The wording not very clear kindly
the magnitude of deceleration =-9.8ms-2. first find the final velocity using the known acceleration and time. next use the calculated velocity to find the size of deceleration.
Firstly, calculate final velocity of the body and then the deceleration. The final ans is,-9.6ms-2
8x6= 48m/-2 use v=u + at 48÷5=9.6
can i define motion like this motion can be define as the continuous change of an object or position
Shuaib Reply
Any object in motion will come to rest after a time duration. Different objects may cover equal distance in different time duration. Therefore, motion is defined as a change in position depending on time.
Practice Key Terms 1

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