Primary Painting Technician

Author: Chief Editor: Cui Shirong
Publisher:
Publish Date: 2005-01-01
Features: Excerpt: Knowledge training requires mastering the definitions of molecules and atoms, the mechanisms of chemical reactions, and the variation patterns of elemental properties. It is well understood that the world is composed of matter. In daily life and work, we frequently interact with various substances, often encountering a wide range of questions. In this ever-changing world of matter, what are the components of these substances? What is their internal structure? What properties and variation patterns do they possess? These are all topics of study in chemistry. As a branch of chemical substances, coatings also adhere to the various laws of chemical substances, determining their characteristics and applications, and like coatings, they are composed of molecules. Below, we will discuss molecules and atoms.
Section Molecules and Atoms
After encountering various types of coatings, we may notice that some can be diluted with water, while others require specialized solvents. Some coatings have strong odors, while others have very faint odors. These differences are closely related to the composition and structure of the coatings.
### 1. Molecules
We often observe chemical phenomena, such as certain coatings drying quickly after being applied to surfaces, while others take days or even longer to dry. Mixing 100 mL of alcohol with 100 mL of water results in a total volume less than 200 mL, which seems unrelated and difficult to explain at first glance. However, once people recognized that all substances are composed of corresponding particles, these phenomena became easier to understand. For example, we can smell the odor of certain coatings because the particles of the coatings and solvents are dispersed into the air and come into contact with our olfactory cells, allowing us to detect the scent. Coatings dry in the air because certain substances within the coating react with oxygen in the air.
Why does the total volume of 100 mL of alcohol and 100 mL of water mixed together become less than 200 mL? We can imagine that the particles of alcohol and water have gaps between them. When these particles mix, some occupy these spaces. The particles of coatings, solvents, water, and alcohol mentioned above are what chemists call molecules.
A molecule is a type of particle that constitutes matter. When a substance composed of molecules undergoes a physical change, the molecules themselves do not change. For example, when water turns into steam, the water molecules themselves remain unchanged, and their chemical properties do not change either. When salt dissolves in water, neither the salt molecules nor the water molecules change, and their chemical properties remain the same.
When a substance composed of molecules undergoes a chemical change, its molecules undergo chemical changes and transform into molecules of other substances. For instance, when sulfur or carbon burns in oxygen, the molecules of sulfur, carbon, and oxygen all undergo chemical changes, forming sulfur dioxide or carbon dioxide molecules with different chemical properties.
Therefore, a molecule is a tiny particle that maintains the chemical properties of a substance. Molecules of the same substance have the same properties, while molecules of different substances have different properties.
Molecules are extremely small. For example, a drop of water (assuming 20 drops equal 1 mL) contains approximately 1.67×1021 water molecules. If one billion people counted the water molecules in a drop of water, each person counting 100 molecules per minute without stopping day and night, it would take over 30,000 years to complete the task. If we compare the size of a water molecule to a ping-pong ball, it is like comparing the size of a ping-pong ball to the size of the Earth.
Molecules are so small that they are invisible to the naked eye. Today, scientists use electron microscopes to magnify molecules by hundreds of thousands of times and capture images of them. Figure 1—1 shows a photograph of a benzene molecule (reproduced).
The mass of a molecule is extremely small. For example, the mass of a water molecule is approximately 3×10?2? kg. Molecules are not static; they are always in constant motion. For instance, when a cup of water and a piece of sugar are placed on a table, they appear to be stationary, but in reality, the water molecules and sugar molecules are constantly moving. This is why sugar dissolves in water.
There is a certain distance between molecules. The thermal expansion and contraction of most objects occur because the distance between molecules increases when heated and decreases when cooled. The distance between molecules in gaseous substances is large, while the distances between molecules in liquid and solid substances are small.