What are Solids?
A solid is a state of matter that has a fixed shape and a fixed volume.
The particles in a solid are packed closely together and held by strong forces of attraction. They cannot move freely from place to place, but they continuously vibrate about fixed positions.
Stone, wood, glass, metals, ice, plastic, and bricks are familiar examples of solids.
Particle Arrangement in Solids
The particles in a solid are very close together.
Strong attractive forces hold them in a stable arrangement. Because the particles have little freedom to move, a solid keeps its shape unless an external force changes it.
When a solid is heated, its particles gain energy and vibrate more strongly. When it is cooled, the particles lose energy and vibrate less.
Properties of Solids
Most solids have the following properties:
- fixed shape
- fixed volume
- closely packed particles
- strong forces of attraction between particles
- particles that vibrate about fixed positions
- high resistance to compression
- ability to retain their shape
- ability to expand slightly when heated
The specific properties of a solid depend on its material and internal structure.
Fixed Shape and Volume
A solid keeps its own shape instead of taking the shape of its container.
It also has a definite volume. Moving a solid from one container to another does not normally change its shape or volume.
A wooden block, for example, remains the same shape whether it is placed on a table, in a box, or on the floor.
Compressibility of Solids
Most solids are difficult to compress.
Their particles are already packed very closely together, leaving little empty space between them. A large force may deform or break a solid, but it usually produces only a very small reduction in volume.
This is why solids are widely used to construct buildings, bridges, machines, tools, and furniture.
Types of Solids
Solids may be broadly classified as:
- Crystalline solids
- Amorphous solids
Crystalline Solids
In a crystalline solid, the particles are arranged in a regular and repeating pattern.
This orderly arrangement forms a crystal structure. Crystalline solids generally have a definite melting point.
Examples:
- common salt
- sugar
- quartz
- diamond
- metals
- ice
The shape of a crystal often reflects the regular arrangement of its particles.
Amorphous Solids
In an amorphous solid, the particles do not have a regular long-range arrangement.
Amorphous solids do not usually melt at one exact temperature. Instead, they gradually soften over a range of temperatures.
Examples:
- glass
- rubber
- many plastics
- wax
Although their internal arrangement differs from that of crystalline solids, they behave as solids under ordinary conditions.
Comparing Crystalline and Amorphous Solids
Mechanical Properties of Solids
Different solids respond differently when forces act on them.
Some return to their original shape, while others bend permanently, stretch, dent, crack, or break. These behaviours are described by the following mechanical properties:
- Elasticity
- Plasticity
- Hardness
- Brittleness
- Malleability
- Ductility
- Strength
Elasticity
Elasticity is the ability of a material to return to its original shape and size after the deforming force is removed.
A spring and a rubber band show elasticity when they are stretched within suitable limits.
A material may not return completely to its original form if the applied force is too large.
Plasticity
Plasticity is the ability of a material to undergo permanent deformation without breaking.
Once the force is removed, a plastic material does not fully return to its original shape.
Clay and soft metals can be shaped because they show plastic behaviour.
Hardness
Hardness is the ability of a material to resist scratching, cutting, indentation, or wear.
Diamond is an extremely hard material. Talc, by comparison, is very soft.
Hard materials are useful in cutting tools, drills, and surfaces that must resist wear.
Brittleness
Brittleness is the tendency of a material to break or crack without undergoing much deformation.
Glass, chalk, and many ceramics are brittle. They may be hard, but they can break suddenly when struck.
Brittleness is not the same as softness.
Malleability
Malleability is the ability of a material to be hammered or rolled into thin sheets without breaking.
Gold, aluminium, copper, and iron are malleable metals.
Malleability allows metals to be made into foil, panels, containers, and sheets.
Ductility
Ductility is the ability of a material to be drawn into thin wires.
Copper and aluminium are ductile and are widely used to make electrical wires.
A material may be both malleable and ductile.
Strength
Strength is the ability of a material to withstand an applied force without breaking or permanently deforming.
Different types of strength may be important in different situations. A bridge material, for example, must withstand pulling, pushing, bending, and twisting forces.
Steel is widely used in construction because it combines strength with useful flexibility.
Deformation of Solids
Deformation is a change in the shape or size of a solid caused by an external force.
A solid may be:
- stretched
- compressed
- bent
- twisted
- sheared
The deformation may be temporary or permanent, depending on the material and the size of the force.
Elastic and Permanent Deformation
Elastic deformation is temporary. The solid returns to its original shape after the force is removed.
Permanent deformation remains after the force is removed.
A metal ruler bends slightly and returns to its original form when a small force is applied. If it is bent too far, however, it may remain permanently deformed.
Thermal Expansion of Solids
Most solids expand when heated and contract when cooled.
Heating gives the particles more energy, causing them to vibrate more strongly. Their average separation increases slightly, so the solid expands.
The expansion is usually small but can become important in large structures.
Examples include:
- gaps left between railway tracks
- expansion joints in bridges
- overhead wires becoming more slack in hot weather
- metal lids becoming easier to open after warming
Melting of Solids
When a solid is heated sufficiently, it may change into a liquid.
At the melting point, the particles gain enough energy to overcome some of the forces holding them in fixed positions. They begin to move past one another, and the solid becomes a liquid.
Ice melting into water and wax melting near a flame are familiar examples.
Different substances have different melting points.
Density of Solids
Density describes how much mass is contained in a given volume.
The particles in many solids are packed closely together, so solids often have higher densities than liquids and gases. However, this is not always the case.
Ice is less dense than liquid water, which is why ice floats.
Density depends on the type of material and the arrangement of its particles.
Conductors and Insulators
Some solids allow heat or electric current to pass through them more easily than others.
Metals such as copper and aluminium are good conductors of heat and electricity.
Materials such as rubber, plastic, wood, and glass are generally poor electrical conductors and are used as insulators.
The choice of material depends on whether conduction or insulation is required.
Uses of Solids
Solids are used for different purposes according to their properties.
Examples include:
- steel in buildings and bridges because of its strength
- copper in electrical wiring because of its conductivity and ductility
- glass in windows because it is transparent
- rubber in tyres because it is flexible and elastic
- ceramics in heat-resistant objects
- plastic in lightweight containers and electrical insulation
- wood in furniture and construction
Understanding the properties of solids helps engineers and designers select suitable materials.
Solids in Everyday Life
Solids form much of the physical world around us.
Buildings, roads, vehicles, tools, books, furniture, electronic devices, and household objects are made from different solid materials.
Each material is chosen because it provides a useful combination of strength, hardness, flexibility, conductivity, durability, or appearance
Key Points
A solid has a fixed shape and fixed volume.
Its particles are packed closely together and vibrate about fixed positions.
Crystalline solids have an ordered particle arrangement, while amorphous solids lack long-range order.
Solids may be elastic, plastic, hard, brittle, malleable, ductile, or strong.
Most solids expand when heated and contract when cooled.
The properties of a solid determine how and where the material can be used.
Glossary of Key Terms
Recap of the Key Terms in Solids
- Amorphous Solid: A solid whose particles do not have a regular long-range arrangement.
- Brittleness: The tendency of a material to crack or break without undergoing much deformation.
- Compressibility: The ability of a material to decrease in volume when pressure is applied.
- Conductor: A material that allows heat or electric current to pass through it easily.
- Crystalline Solid: A solid whose particles are arranged in a regular, repeating pattern.
- Crystal Structure: The regular and ordered arrangement of particles in a crystalline solid.
- Deformation: A change in the shape or size of a solid caused by an external force.
- Density: The amount of mass contained in a given volume of a substance.
- Ductility: The ability of a material to be drawn into thin wires.
- Elastic Deformation: A temporary change in shape or size that disappears when the applied force is removed.
- Elasticity: The ability of a material to return to its original shape and size after a deforming force is removed.
- Fixed Shape: The property of a solid that allows it to retain its own shape.
- Fixed Volume: The property of a solid that allows it to occupy a definite amount of space.
- Force of Attraction: The force that holds the particles of a solid close together.
- Hardness: The ability of a material to resist scratching, cutting, indentation, or wear.
- Insulator: A material that does not allow heat or electric current to pass through it easily.
- Internal Structure: The arrangement and organisation of particles within a material.
- Malleability: The ability of a material to be hammered or rolled into thin sheets without breaking.
- Mechanical Properties: Characteristics that describe how a material behaves when forces act on it.
- Melting: The change of state from a solid to a liquid.
- Melting Point: The temperature at which a solid changes into a liquid.
- Particle: A tiny unit of matter, such as an atom or molecule.
- Permanent Deformation: A change in shape or size that remains after the applied force is removed.
- Plasticity: The ability of a material to undergo permanent deformation without breaking.
- Solid: A state of matter with a fixed shape and fixed volume.
- Strength: The ability of a material to withstand an applied force without breaking or permanently deforming.
- Thermal Expansion: The increase in the dimensions of a material when it is heated.
- Vibration: The repeated movement of a particle back and forth around a fixed position.
Questions and Answers
Recap the concepts you have learnt. Try to answer the questions. You can find the answer to any question by clicking on the icon.
What is a solid?
A solid is a state of matter with a fixed shape and a fixed volume.
How are the particles arranged in a solid?
The particles in a solid are packed closely together and held in a stable arrangement.
Do the particles in a solid move?
Yes. They vibrate about fixed positions but do not move freely from place to place.
Why does a solid retain its shape?
Strong forces of attraction hold its particles close together and restrict their movement.
Why are most solids difficult to compress?
Their particles are already packed very closely together, leaving very little empty space.
What happens to the particles of a solid when it is heated?
They gain energy and vibrate more strongly.
What happens to the particles of a solid when it is cooled?
They lose energy and vibrate less.
What are the two main types of solids?
The two main types are crystalline solids and amorphous solids.
What is a crystalline solid?
A crystalline solid has particles arranged in a regular and repeating pattern.
Give two examples of crystalline solids.
Common salt and sugar are examples of crystalline solids.
What is an amorphous solid?
An amorphous solid does not have a regular long-range particle arrangement.
Give two examples of amorphous solids.
Glass and rubber are examples of amorphous solids.
How do crystalline and amorphous solids differ in their melting behaviour?
Crystalline solids usually melt at a definite temperature, while amorphous solids soften over a range of temperatures.
What are mechanical properties?
Mechanical properties describe how a material behaves when forces act on it.
What is elasticity?
Elasticity is the ability of a material to return to its original shape and size after the force is removed.
What is plasticity?
Plasticity is the ability of a material to undergo permanent deformation without breaking.
What is hardness?
Hardness is the ability of a material to resist scratching, cutting, indentation, or wear.
What is brittleness?
Brittleness is the tendency of a material to crack or break without much deformation.
Is a brittle material always soft?
19.
No. A brittle material may be hard but can still break suddenly when struck.
What is malleability?
Malleability is the ability of a material to be hammered or rolled into thin sheets.
What is ductility?
Ductility is the ability of a material to be drawn into thin wires.
Why is copper used for electrical wiring?
Copper is a good electrical conductor and is also ductile, so it can be drawn into wires.
What is strength?
Strength is the ability of a material to withstand an applied force without breaking or permanently deforming.
What is deformation?
Deformation is a change in the shape or size of a solid caused by an external force.
What is elastic deformation?
Elastic deformation is temporary. The solid returns to its original shape after the force is removed.
What is permanent deformation?
Permanent deformation remains even after the applied force is removed.
In what ways can a solid be deformed?
A solid may be stretched, compressed, bent, twisted, or sheared.
What is thermal expansion?
Thermal expansion is the increase in the dimensions of a solid when it is heated.
Why are gaps left between railway tracks?
The gaps allow the metal rails to expand in hot weather without bending or buckling.
Why do overhead wires become more slack in hot weather?
The metal wires expand when heated, increasing their length.
What happens when a solid reaches its melting point?
Its particles gain enough energy to move past one another, and the solid changes into a liquid.
What is density?
Density is the amount of mass contained in a given volume of a substance.
Why does ice float on water?
Ice floats because it is less dense than liquid water.
What is a conductor?
A conductor is a material that allows heat or electric current to pass through it easily.
Give two examples of good conductors.
Copper and aluminum are good conductors.
What is an insulator?
An insulator is a material that does not allow heat or electric current to pass through it easily.
Give two examples of electrical insulators.
Rubber and plastic are electrical insulators.
Why is steel widely used in buildings and bridges?
Steel is widely used because it is strong and can withstand large forces.
Why is glass used in windows?
Glass is used in windows because it is a solid that allows light to pass through it.
Why are different solids used for different purposes?
Different solids have different combinations of strength, hardness, flexibility, conductivity, durability, and appearance.