Metals are substances that have a crystalline structure. When heated, they are able to melt, that is, go into a fluid state. Some of them have a low melting point: they can be melted by placing in a regular spoon and holding candles above the flame. It is lead and tin. Others can only be melted in special furnaces. Copper and iron have a high melting point . To lower it, additives are introduced into the metal. The resulting alloys (steel, bronze, cast iron, brass) have a melting point lower than the starting metal.
What does the melting point of metals depend on? All of them have certain characteristics - heat capacity and thermal conductivity of metals. Heat capacity is the ability to absorb heat when heated. Its numerical indicator is the specific heat. By it is meant the amount of energy that a unit mass of metal is able to absorb, heated at 1 ° C. The fuel consumption for heating a metal billet to the desired temperature depends on this indicator. The heat capacity of most metals is in the range of 300-400 J / (kg * K), metal alloys - 100-2000 J / (kg * K).
The thermal conductivity of metals is the transfer of heat from hotter to colder particles according to the Fourier law with their macroscopic immobility. It depends on the structure of the material, its chemical composition and type of interatomic bond. In metals, heat is transferred by electrons, in other solid materials by phonons. The thermal conductivity of metals is higher, the more perfect the crystal structure they have. The more metal has impurities, the more distorted the crystal lattice, and the lower the thermal conductivity. Doping introduces such distortions into the structure of metals and reduces thermal conductivity relative to the base metal.
All metals have good thermal conductivity, but some are higher than others. An example of such metals is gold, copper, silver. Lower thermal conductivity is found in tin, aluminum, and iron. The increased thermal conductivity of metals is an advantage or disadvantage, depending on the scope of their use. For example, it is necessary for metal utensils to quickly heat food. At the same time, the use of metals with high thermal conductivity for the manufacture of pens makes it difficult to use - pens heat up too quickly, and it is impossible to touch them. Therefore, heat-insulating materials are used here.
Another characteristic of a metal that affects its properties is thermal expansion. It looks like an increase in the volume of the metal when it is heated and a decrease when it is cooled. This phenomenon must be taken into account when manufacturing metal products. So, for example, the lids of pots are made overhead; dummies also have a gap between the lid and the body so that the lid does not jam when heated.
The coefficient of thermal expansion is calculated for each metal . It is determined by heating at 1 ° C a prototype having a length of 1 m. The largest coefficient is lead, zinc, tin. It is smaller in copper and silver. Even lower is iron and gold.
By chemical properties, metals are divided into several groups. There are active metals (such as potassium or sodium) that can instantly react with air or water. The six most active metals that make up the first group of the periodic table are called alkaline. They have a low melting point and are so soft that they can be cut with a knife. Combining with water, they form alkaline solutions, hence their name.
The second group consists of alkaline earth metals - calcium, magnesium, etc. They are part of many minerals, harder and refractory. Examples of metals of the following, third and fourth groups are lead and aluminum. These are fairly soft metals and are often used in alloys. Transition metals (iron, chromium, nickel, copper, gold, silver) are less active, more forged and are often used in industry in the form of alloys.
The position of each metal in a series of activity characterizes its ability to react. The more active the metal, the easier it takes oxygen. They are very difficult to isolate from the compounds, while low-activity types of metals can be found in pure form. The most active of them - potassium and sodium - are stored in kerosene, outside of it they are immediately oxidized. Of the metals used in industry, copper is the least active. It makes tanks and pipes for hot water, as well as electrical wires.