Definition and distinction of interpretations
In chemical science, substances are defined as gaseous, liquid, or solid bodies characterized by two key features: a constant chemical composition and constant properties. At the same time, the external form of the object has no effect on its chemical nature.
The interpretation of this term depends on the discipline:
- In chemistry: these are structures with invariable composition and properties.
- In physics: these are any objects possessing rest mass (including microparticles: electrons, protons, and neutrons).
- In everyday life: the word is often used as a simple synonym for the term «material».
Substance, physical body, and material
A material is what a specific object is made of. It can be either a pure homogeneous substance or a mixture (composition). To separate a chemical substance from a physical body or a mixture, it is important to abstract away from form:
- Physical bodies: In nature, there are deposits of marble from which sculptures are carved. If a statue breaks, both the intact sculpture and the pile of fragments remain identical in their properties. Marble here is the substance, while the sculpture is the physical body.
- Heterogeneous mixtures: Ceramics (roof tiles, bricks) when viewed under a magnifying glass consist of grains of sand bound together by clay. This is a heterogeneous body, and therefore it is not a substance. However, sand and clay individually are substances.
- Homogeneous mixtures: Salt water or metal alloys look uniform, but their characteristics (the taste of water or the properties of the metal) depend on the concentration and ratio of the components. Solutions and alloys are not pure substances, but mixtures.
Physical properties
Physical parameters are inherent to the material itself and demonstrate the property of invariance (they do not depend on the shape of the vessel or the external appearance of the object). The most famous example is water:
- States of aggregation: can be solid, liquid, and gaseous.
- Organoleptic indicators: has no taste, odor, and in a thin layer is colorless.
- Physicochemical constants: acts as a solvent for many substances, has a density of about $1 \text{ g/cm}^3$.
Chemical phenomena and transformations
Chemistry is focused on processes that alter the substances themselves. Upon the interaction of reactants or a sharp change in environmental conditions, the initial substances disappear, and new ones appear in their place.
Examples of chemical phenomena in nature and everyday life:
- Oxidation and corrosion: the appearance of a green coating on bronze monuments.
- Biochemical decay: wood rotting.
- Food processes: milk souring, dough fermentation upon adding yeast (volume increase), and changes in the smell and color of meat during heat treatment. This also includes denaturation: a transparent, viscous egg white turns white and becomes elastic upon heating.
- Combustion: the transformation of paper or wood into smoke and ash.
- Construction: when mixed with cement, water disappears as a free substance, becoming part of durable concrete.
- Thermal decomposition: when heated on a stove, sugar melts, then darkens and releases gas bubbles, turning into a black porous char.
Laboratory demonstration: ion-exchange reaction
The process of transforming some compounds into others is clearly demonstrated by mixing 10% solutions of calcium chloride and sodium sulfate.
- Initially, two transparent liquids are mixed.
- A latent period occurs: for about 10–20 seconds, the solution does not visually change.
- The reaction begins: small solid particles appear and settle to the bottom.
- After a few minutes, an abundant, loose white precipitate forms.
Conclusion: Two new substances with completely different characteristics were formed from the two initial reactants. Common salt (sodium chloride) remained in solution, while gypsum (calcium sulfate) precipitated out, its crystals having a needle-like shape at 20x magnification.
Briefly: answers to topical questions
What levels of spatial structure and chemical bonds are disrupted during the thermal denaturation of a protein?
During the thermal denaturation of a protein, the secondary, tertiary, and quaternary structures are disrupted, while the primary structure is preserved. High temperatures cause the rupture of weak non-covalent bonds that stabilize the conformation of the molecule:
- Hydrogen bonds — between the atoms of the peptide backbone and amino acid radicals.
- Ionic bonds — between the functional groups of radicals.
- Hydrophobic interactions — between nonpolar regions.
Peptide bonds are not broken by thermal exposure.
Is a medical physiological saline solution a pure chemical substance or a mixture?
A medical physiological saline solution is not a pure chemical substance, but a homogeneous mixture of variable composition. In the chemical sense, only its individual components are pure substances:
- Water — acts as the solvent.
- Sodium chloride (NaCl) — the evenly distributed solute.
Since the properties of solutions depend on the concentration of their components, they are classified as mixtures rather than individual chemical substances.
Which biogenic macronutrients are part of the precipitating gypsum?
The precipitating gypsum (calcium sulfate) contains only oxygen among its biogenic macronutrients. The other chemical elements forming this substance belong to another group:
- Oxygen (O) — a macronutrient.
- Calcium (Ca) — a micronutrient.
- Sulfur (S) — a micronutrient.
Thus, despite the presence of sulfur and calcium in the formula of gypsum, according to the classification of elements by cellular content, the only macronutrient among them is oxygen.
Class and exam questions
Is salt water a substance?
No, salt water is a homogeneous mixture. Its properties are non-constant and depend on the concentration of dissolved salt, whereas a pure chemical substance always has a constant composition.
What is the difference between a physical body and a substance?
A substance is a material with constant properties that makes up an object. A physical body is the object itself, having a specific shape (for example, marble is a substance, while a statue made of it is a physical body).
What happens to a substance during a chemical phenomenon?
During a chemical phenomenon, a rearrangement of the mutual positioning and bonds of atoms takes place. The initial substances cease to exist, turning into new compounds with different properties.
What else to study on this topic
- Mechanisms of protein denaturation
- Metal oxidation and corrosion processes
- Biochemical breakdown and putrefaction
- Thermal decomposition of organic compounds
- Ion exchange reactions in aqueous solutions
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Other topics in the section "Chapter 1. Substances. Properties and Structure"
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