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Kinetic Theory of Matter

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The energy of all particles changes based on the temperature of the sample of matter, which defines whether it is a solid, liquid, or gas. The energy of solid particles is the lowest, whereas the energy of gas particles is the highest. The average kinetic energy of the particles is measured by the temperature of a substance. When the energy of the particles is modified, the phase of the particles may change. There are spaces between matter particles. As a sample of matter travels from the solid to the liquid and gas phases, the average amount of vacant space between molecules grows. 

According to the kinetic theory of matter, all matter is made up of microscopic particles in random motion with space between them.

Four Phases of Matter

Solids, liquids, gases, and plasmas: these are the four phases of matter, which are simply the many forms that matter can take. It’s interesting to note that many compounds can exist in multiple phases. Consider water: it can exist as a solid (ice), a liquid (liquid water), or a gas (water vapour).

  • The amount of energy that separates these states is the difference. Solids contain the least energy, which helps to explain why their particles stick together so securely. Because liquids have more energy than solids, they will take the shape of their container up to the surface.
  • Liquids have more energy than gases. So much more, in fact, that their particles spread out to fill the container’s whole volume. Because gas particles have so much energy, they can’t stay motionless. They fly around in all directions, trying to put as much distance between themselves and the other gas particles as possible.
  • Plasmas are ionised gases that are rare on Earth in their native state. Neon signs and fluorescent light bulbs are examples of man-made objects. Plasma, on the other hand, is the most common phase of matter in the rest of the universe! Most stars, as well as the northern lights seen in the polar regions, are plasma.

Kinetic theory of matter

All substances have energy which depends upon the temperature the substance is placed in. It determines whether the matter is in a solid, liquid, or gaseous state. The temperature of that particular matter is a measurement of the kinetic energy of the particles. So, we can conclude the change in the state of a matter may occur due to the change in temperature. 

The energy possessed by a body due to its motion is known as kinetic energy. Kinetic theory of matter states that “Matter is made up of those substances or particles which are constantly moving.” The energy level of the particles depends upon the temperature possessed by the matter. This helps us to determine whether that matter is in a solid, liquid, or gas state. The kinetic theory of matter also gives us a description of the microscopic properties of atoms. 

The main purpose of this theory is to explain the existence of matter in various phases and they change from one state to another. Kinetic theory explains that:

  • At the molecular level, the interpretation of pressure and temperature can be explained.
  • It follows Avogadro’s theory and gas laws.
  • It describes the specific heat capacity of the different gases correctly.

The kinetic molecular theory of matter describes the microscopic properties of atoms (or molecules) and their interactions, which rise to macroscopic features that may be observed (such as pressure, volume, temperature). The idea can be used to explain why matter exists in different phases (solid, liquid, and gas), as well as how matter can shift from one phase to another. According to the kinetic molecular theory of matter,

  1. The particles that makeup matter are continually moving.
  2. Every particle has energy, however the amount of energy changes based on the temperature of the matter sample. The state of the substance is then determined by whether it is solid, liquid, or gaseous. Molecules in the solid-state have the least amount of energy, while gaseous particles have the most.
  3. The average kinetic energy of the particles is measured by the temperature of a substance.
  4. When the energy of the particles is modified, the phase of the particles may change.
  5. There are spaces between matter particles. As a sample of matter travels from the solid to the liquid and gas phases, the average amount of vacant space between molecules increases.
  6. Attractive forces exist between atoms/molecules, and when the particles draw closer together, these forces become stronger. Intermolecular forces are the term for these attractive forces.

Understanding behaviour of matter

Matter is a substance that occupies space and has volume. The matter is made up of atoms and molecules. The state of matter is determined on the basis of the arrangement of its molecules. Temperature, pressure, mass and volume are also the main factors that determine the state of matter. The state of matter may change if the temperature of the surrounding is increased. The particles of solid, liquid, and gas behave in different ways but give similar results. The particle is in random and constant motion in all the states at room temperature. 

With the rise in temperature and pressure, the kinetic energy and speed of the particles also increase. But, the force that is present between them doesn’t get affected or weaken. In, solid the particles are tightly packed, so they can be moved only with a small vibration. In liquid, The particles are placed a little far from each other in comparison to the solids. They require a high amount of vibration to do the movement. In gas, the particles are very far from each other so, we cannot see gases with our snake’s eyes. They need a high amount of particles to be moved. 

  • Mass: Mass can be defined as the weight of the matter. They are usually weighed in grams. When the particles of an atom are tightly packed the greater the mass they have. We all sometimes get confused to differentiate between weight and mass. So, we can know the difference by a simple example. First, take a lump of gold one on the moon and one on the earth, thus they will have the same mass but the weight will differ due to gravity.
  • Volume: The total space occupied by a substance is known as volume. They are measured in cubic meters. Depending on the physical states of the matter the ways are different to measure their volume
  • Density: The ratio of mass and volume is known as density. They are measured in pascal. If an egg is kept in normal water they sink but when the egg is kept in salty water it floats because the salty water has a higher density in comparison to the water.

 Illustration: Water

Let’s take the case of water. The water molecules in their solid-state (ice) have relatively little energy and cannot move away from each other. The molecules are arranged in a regular arrangement known as a lattice. The energy of the molecules in ice increases as it is heated. This means that certain water molecules are able to overcome the intermolecular interactions that keep them close together, allowing them to migrate further apart and produce liquid water. 

The molecules in liquid water have more freedom to move than they did in the solid lattice, which is why they can flow. The liquid water will become water vapour, which is a gas if the molecules are heated further. Gas particles contain more energy and, on average, are separated from one another by distances far greater than the size of the atoms/molecules. Given the enormous distances between the particles, the attraction forces between them are quite weak.

Processes to change the state of water are:

  • Evaporation: The process of changing a liquid state into a gas by providing heat is called evaporation.
  • Condensation: The process in which water changes from gas states to liquid by cooling them is known as condensation.
  • Melting: When the solid-state of water is changed into a liquid state also by the heating method is called melting.
  • Freezing: When the liquid state changes into a solid state it is called the freezing method.
  • Sublimation: When the solid-state of water directly changes into a gas state then it is known as sublimation.
  • Deposition: When the vapour of water directly changes into a solid state it is known as a deposition.

Sample Questions 

Question 1: Why does an egg sinks in salty water but floats in a normal one? Justify?


An egg sinks in salty water but floats in normal water because salty water has a higher density than an egg.

Question 2: Why was Dalton’s theory a success?


Dalton’s theory was a success because of the following points

  • Molecules are made up of atom, that is built up of molecules.
  • The atomic structure can be examined by an electron microscope.

Question 3: Why the energy levels of substances are different from each other? 


The energy level of matter is different from each other because it depends upon the temperature of the substances.

Question 4: What is the study of crystals called? Define crystal.


The study of crystals is known as crystallography. A crystal is a solid substance which constituents are arranged in ordered microscopic structure and have sharp edges or ends.

Question 5: Write down the formula of the perfect gas equation and what does it describe?


The formula of the perfect gas equation is PV = μRT. This equation describes how gas behaves in a specific condition.

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Last Updated : 15 Feb, 2023
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