Are the molecules of all substances in motion?
A substance is nothing but a pure form of matter. It can’t be separated into any other kinds of matter by physical process. Every substance consists of very tiny particles, called molecules. A molecule is the smallest part of matter that can exist independently and shows all the properties of that substance.
What are molecules?
Every substance consists of very tiny particles, called molecules. A molecule is the smallest part of matter that can exist independently and shows all the properties of that substance. Molecules of different substances have different sizes and shapes.
Some molecules are only a few billionths of centimeters in size, but some other molecules are a hundred times larger. A water molecule is about 0.27 of a nanometer across. DNA is about 2 nanometres across and can be stretched out to meters in length.
A tiny substance contains a large number of molecules and there is empty space in between them. The molecules of all substances are in constant motion, called “thermal motion” because it is caused and controlled by temperature. As the temperature increases, the motion of the molecule also increases.
The hotter the substance, the faster the motion of the molecules. The molecules of the gas move faster than the molecules of liquids and solids in comparison. The heat which causes the molecules to move is nothing but the kinetic energy of the molecules.
Particles of the earth move mostly because of heat and temperature. Due to the conservation of energy as the law of nature, if one particle loses energy, another gains the same. Even if one substance is cooled down to absolute zero, then its particles would still remain in motion. This is called zero-point energy, the lowest energy that a quantum can have. This is why molecules of ice are also in motion.
It is surprising to note that in spite of molecules of all substances being in motion, substances do not appear unstable. But why? Because of the existence of forces of attraction between the molecules in motion, which keeps them together.
When molecules of solid substances are heated, their speed increases, and the force of attraction between the molecules, decreases. If heating is continued, the movement of the molecules would rather be faster, and the solid would change into a liquid state and finally into a gaseous state. Molecules of gaseous state exhibit random motion, for which gases take the shape of a container and spread quickly in space.
The random motion of the molecules in the gaseous state is due to high kinetic energy in molecules and weak inter-molecular interactions between them. Scottish Botanist Robert Brown, in the year 1828, first observed that tiny particles in suspension were in constant motion, at a time when many scientists even did not believe in the existence of the molecules.
Molecular motions are measured by different types of spectroscopy methods like NMR spectroscopy, UV spectroscopy, mass spectroscopy, including Infrared Spectrums.
Types of molecular motion:
Molecular motion can be classified into three types as follows,
In translational motion, molecules can move from one place to another in the same or different direction remaining at the same axis. Translational motion arises as the solids melt into liquid or sublime into gases.
In rotational motion, molecules can rotate in and around the axis. The rotation of the earth and the wheels of moving vehicles are examples of rotational motion.
Molecules can vibrate at their mean position in vibrational motion. These motions are very common in the solid state.
Liquid molecules can vibrate, rotate and translate depending on the heat and temperature, but the forces that hold the liquid particles close to each other are greater than the forces that cause the motion that forces the particles away from each other.
It is definitely an amazing fact that molecules of all substances are always in motion but we could not feel the same, only scientific methods can prove the facts.