All objects and phenomena surrounding us, existing and occurring independently of our will, are called matter. Every experimentally detectable form is a part of it. Existing objects constitute physical bodies, which in turn are called substances. It is a type of matter with a constant chemical composition that a given body consists of. For example, when we talk about a silver necklace, we mean the physical body in the form of a necklace and the substance from which it is made – silver. Other substances are, for example, water, wood, polyethylene, sugar and air.
Distinguishing substances
Substances may differ from each other, and their strictly defined set of features allows for their recognition and appropriate use. These properties are also called the properties of a particular substance. For example, you can consider using a knife made of several substances – plastic, wood, and metal, to cut an apple. It is easy to see that each of the listed substances has various, characteristic properties. For cutting, the best choice will be a knife made of a good quality metal alloy that allows for proper sharpening. However, other substances also have their uses in the production of knives – cheap plastic is used for disposable cutlery, and wood is well suited for knives intended for spreading something on a slice of bread.
Substance characteristics
We describe the properties of a substance, taking into account two main relationships – considering physical phenomena and chemical reactions. Properties that can be examined by the senses (with the use of smell or sight), as well as with appropriate instruments (such as a density meter), are called physical properties. These are, among others, the state of aggregation, colour, hardness, density and changes in shape under the influence of appropriate forces (brittleness, elasticity, malleability). It can be a bit more complicated to determine the chemical properties of the substances of our interest. These are all properties that can only be described on the basis of their reaction to others, i.e., for example, toxicity, flammability, and reactivity. We define them on the basis of chemical changes that occur between a given substance and other factors, often with a change in structure.
Reactivity
Reactivity is a broad term that in practice describes the number of reactions that a given substance undergoes and the efficiency it receives under normal conditions. This means that if the considered reaction proceeds with high efficiency and requires only a slight change in temperature and pressure conditions, it is a chemical property of the substance. However, any reactions that occur only under the influence of several hundred degrees or atmospheres and with low yields are rejected. This property is suggested to us by the periodic table of elements, because as the atomic number increases, in the case of metals, the reactivity increases within the group and decreases within the period, while in the case of non-metals, it decreases within the group and increases within the period. The least reactive substances in the periodic table are the noble gases, because having the most stable electronic states (doublet and octet), they react reluctantly with other substances.
Reactions with oxygen
Combustion, or rather the ability to react with oxygen, allows us to determine another chemical property of a substance – flammability. If a material undergoes this exothermic reaction with the release of heat and light, it may be combustible or flammable. Experimental determination of flammability is based on the determination of the OI factor, i.e. the oxygen index, which expresses the percentage of oxygen in the mixture with nitrogen necessary to sustain the burning of the material, the temperature of which at the time of ignition is 20oC. If the factor is less than 21, the substance is considered flammable, in the range from 21 to 28 materials are considered flame retardant, and above 28, the substance is considered non-combustible. Flammables include gasoline and natural gas, flame retardants include polyester, and non-flammables include carbon dioxide, concrete, freons, and stone wool.
Harmful effect on organisms
Toxicity describes the ability of a substance to cause damage to a body when ingested or absorbed through the skin or inhalation. It can lead to a dysfunction of cells and organs, and consequently to poisoning of the body. Such effects are mainly investigated in in vitro toxicology experiments using cells or biomarkers, and in vivo in laboratory animals. For example, arsenic, i.e. diarsenic trioxide, is a very toxic substance.
Example – chemical properties of water
When considering water chemistry, parameters such as pH, oxidation-reduction conditions, acidity and alkalinity, mineralization, dry residue and hardness are examined. These are features based mainly on the reactivity of the substance and closely related to a specific type of water. Distilled water, which is a pure substance, will have different parameters, and rainwater with a slightly different composition will have different parameters. However, there are also such chemical properties as non-flammability, non-sustaining combustion, non-toxicity and good ability to dissolve other substances.
Example – chemical properties of metals
In the case of metals, it is easiest to find relationships by dividing them into s, p and d blocks, in accordance with the periodic table. Due to the electron configuration of the s-block metals, the transition of valence electrons does not require much energy. This makes them the most chemically active. In reaction with water, they form alkaline hydroxides, they undergo chemical transformations also in contact with acids. They have reducing properties, visible in reactions with water, oxygen, hydrogen or anaerobic acids, which increase with the increase in the atomic number in the group. Burning in a flame, they give a characteristic colour, for example barium dyes it yellow-green. The p-block metals have valence electrons located only in the last shell and are also reactive. The chemically most important element in this group is aluminium, which, despite its reactivity, when used in alloys, eliminates the corrosive effect of oxidizing acids, creating a passive layer. It is an amphoteric reducing agent and reacts with both acids and bases. The d-block metals, on the other hand, have valence electrons in the last and penultimate outer shells, which is why they mostly occur in various oxidation states and willingly donate electrons from the S shell. An iron atom can donate two or three electrons to form Fe2+ or Fe3+ ions. It is a moderately reactive substance, reacting with non-metals such as sulphur or chlorine, with water vapour and oxygen under water vapour conditions. It also reacts with oxygen acids to form a passive layer. Group 12 zinc metals usually form bi-positive cations. Their reactivity decreases with increasing atomic mass. Among the metals, copper metals have the lowest ability to react chemically. They are weak reducing agents and their most typical reactions are with oxidizing acids. They are unable to displace hydrogen from anaerobic acids.