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Solid State: Definition of Solid, Properties, Classification, Practice Problems And FAQs

Solid State: Definition of Solid, Properties, Classification, Practice Problems And FAQs

It’s Sunday morning, and your mother has prepared some delicious cheese sandwiches. You are starving to have it but as usual lazy enough to go to the kitchen. Can the sandwich come to you on its own?

Well, it is obvious as it is a solid substance it can’t move on its own. Some external force is required for the locomotion of the particles of solids.

But have you noticed that the aroma of those delicious sandwiches has reached your nose very quickly? Why does that happen?


We know that gaseous particles are randomly oriented and widely distributed hence they can travel from one place to another easily through diffusion but the same is not true in the case of solids. Do you know why this happens? To know the answer to this question, we need to understand how different arrangements of particles result in such properties of substances. So, let's delve in deeper and try to understand what’s happening at the molecular level in solids. 

Table Of Content

Definition of Solid State

A solid is defined as that form of matter which possesses rigidity and hence possesses a definite shape and a definite volume. 

The three states of matter that is solid, liquid and gas have different arrangements of particles. But we know that these states of matter are interconvertible.

Let’s understand what are the factors which decide whether a compound will exist as solid, liquid or gas?

Why do some substances exist as solid?

Whether a substance will exist as a solid, liquid or gas depends upon the net effect of the two opposing forces:

  • Interparticle forces
  • Thermal energy

Interparticle forces

Interparticle forces are the forces of attraction and repulsion between the interacting particles (atoms, molecules or ions). These interparticle forces are known as the van der Waals forces.

Thermal energy

Thermal energy arises from temperature and the motion of particles, and kinetic energy originates from thermal energy. It is proportional to the substance's temperature.

At low temperatures, the thermal energy is very low. At the same time, the intermolecular forces are so high that the particles come closer to each other, so it can be said that the substance is in solid-state as interparticle forces are stronger. 

Now if we increase the temperature, thermal energy will be high and particles will start moving due to kinetic energy. The intermolecular forces will try to bring them closer but once the thermal energy is greater than the interparticle forces, particles won’t remain in their fixed positions and they start moving around and a substance is converted into liquid or a gas.

Properties of solids

  • A solid-state possesses rigidity, particles are densely packed and hence possess a definite shape and a definite volume.
  • Strong interparticle interactions between constituent particles. 
  • In contrast to gases and liquids, where fluidity is defined by the relative free mobility of their particles. Particles in solids are not free to move but can oscillate about their fixed locations due to significant inter-particle interactions.
  • Least inter-particle distance in solids as compared to liquids and gases. 
  • Solids have low diffusibility.
  • Solids have negligible compressibility.

Classification of solids

Solids can be classified in many ways but to generalise our study we have taken two important factors for consideration for classification. 

  • Classification based on the arrangement of constituent particles: 
  • Crystalline Solids
  • Amorphous Solids
  • Classification based on the nature of inter-particle forces among the constituting particles:


Practice Problems

Q 1. Which of the following does not represent a state of matter?

a. Solid
b. Liquid
c. Metals
d. Gas

Answer: (C)
States of matter is generally classified into 5 categories that are solid, liquid, gas, plasma and Bose-Einstein condensate. But on the basis of inter-particle distance and geometry of the substance, it is classified into 3 categories that are solid, liquid and gas. So, metals are not represented as a state of matter. 

Q 2. Among the following properties of solid, which one is not relevant?

a. Rigidity
b. Strong force of attraction between the constituent particles
c. High compressibility
d. Vibrational motion of particles.

Answer: (C)
Solids are rigid due to the strong force of attraction between particles and the particles don’t move as the positions of particles are fixed, they can only vibrate along with their mean positions. Compressibility is the property of being compressed by pressure into a smaller space. The change in bulk is caused by the pressure bringing the particles closer together, and this feature is caused by porosity. Due to the least inter-atomic space between the particles of solids, compressibility is almost negligible. 

Q 3. Forces of attraction between the constituent particles is highest in case of:

a. Solid
b. Liquid
c. Gas
d. None of the above

Answer: (A)
Gaseous particles are randomly distributed and have huge inter-particle spaces. The force of attraction between these particles is low as compared to solids and liquids. Liquids, on the other hand, have a mild force of attraction between the particles which helps them to flow whereas in the case of solids, the force of attraction is maximum hence particles acquire a fixed position.

Q 4. Classification of solids is done on the basis of:

a. colour of the substance.
b. the shape of the substance.
c. the arrangement of particles.
d. the texture of the substance.

Answer: (C) 
Solids are classified into crystalline and amorphous based on the nature of order or arrangement of the particles. 

Frequently asked questions-FAQs

Q 1. What exactly happens when we convert a liquid into a solid?
A liquid state can be converted to its solid-state. Freezing is the term for this conversion process. The most obvious example of liquids converting to solids can be found in our refrigerators, where water turns into ice. As the molecules in the solid-state stop moving, the process of liquid to solid conversion releases energy. During the conversion of liquid state into solid-state, some of the kinetic energy of molecules is converted into potential energy, while the rest is released as heat.

Q 2. Both solids and liquids have a definite volume but why liquid doesn’t have a definite shape?
A liquid has a defined volume but no distinct shape. The shape of a liquid changes as its container changes. The particles in a liquid are almost as closely packed as those in a solid. However, the particles in a liquid can freely move around. Freely moving particles allow a liquid to flow from one location to another. As a result, a liquid is a fluid or a substance that flows whereas in the case of solids the particles have a fixed position and only vibrate about their mean position. Hence, solids have a definite shape and volume.

Q 3. What is the role of kinetic energy in the movement of particles?
Kinetic energy is the energy of motion, which can be observed in the movement of an object, particle, or set of particles. Kinetic energy is used by any moving object: a person walking, a ball being thrown and a charged particle in an electric field are all examples of kinetic energy at work. The kinetic energy of the particles increases as the temperature rises, causing them to move faster.

Related Topics

Types of Solids

Crystalline solids

Crystal Defects 

Types of Unit Cell & Crystal Lattice

Crystal system and Bravais lattice 

Factors deciding the states of matter


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