Solutions and state diagrams

To understand, how steels get their strength, very useful basic understanding of two things: solution and status diagram. The state diagram is also called phase equilibrium diagram and phase diagram.

Liquid solution arises, when a substance dissolves in a liquid like salt in water. Solid solution is similar to liquid, as, eg, salt solution in ice. Substance, which dissolves in solution (salt in salt solution, carbon in steel) lose their identity and are out of sight, so they are absorbed by the solution. The solutions are homogeneous and have the same molecular or atomic structure at every point. Each solution is called a phase.


The idea of ​​a solution can be explained with a simple example. Put a teaspoon of table salt in a glass of water. First, all the salt will lie on the bottom of the glass.. However, after stirring the water well with a spoon, all the salt will disappear and only clear water will remain..

What happened to the salt? Table salt is composed of NaCl molecules. When the salt goes into solution, individual molecules are separated into their constituent atoms. These atoms are surrounded by water molecules in the form of charged atoms - ions.

Salt molecule disintegrates, following the reaction
NaCl - ›Na+ + Cl, where the symbol Na+ stands for positively charged sodium ion, and the symbol Cl – negatively charged chlorine ion. Hard salt is not visible, because chemical bonds, by which the atoms were combined into a molecule, ripped apart. The former solid salt structure has disappeared, and its constituent atoms were absorbed by liquid water and placed between water molecules.

Calcium chloride solution against icing

Usually, if a solution is formed by dissolving something in a liquid, then the freezing point of this liquid decreases. That is why, in the winter months, calcium chloride is used on roads and sidewalks to combat icing.. This salt dissolves in rain or melt water and lowers its freezing point.. Therefore, ice does not form even when the water temperature drops below zero..

This effect is shown graphically in the figure. 1. Water temperature is plotted vertically, and horizontally - the amount of salt dissolved in water. The amount of dissolved salt is indicated as a percentage of its mass to the total mass of the solution.. Usually two terms are used to describe a solute. The quantity 10 % calcium chloride is called concentration salt in solution or composition solution.

On the picture 1 can see, that after a certain salt concentration, the freezing point suddenly begins to rise rapidly. This concentration is called eutectic concentration, which will be discussed in the article on the phase diagram of steel - the phase diagram of iron-carbon.

xloristyy-natriy-xloristyy-kalciyPicture 1 – Lowering the freezing point of water when salts are dissolved

State diagram

Geometric order of water molecules (then, what is called molecular structure) is the same at every point of the water. Water in this sense is liquid phase. Likewise, the molecular structure of ice is also the same at all points, and ice is also phase - solid phase. However, the molecular structure of ice and water is very different from each other., one of them is liquid, the other is solid. Therefore, liquid water and ice are two different phases..

As it shown on the picture 1 the freezing point of water is dropping, when salt dissolves in it to form a solution. The molecular structure of this saline solution is almost the same as that of pure water., since salt ions are located between water molecules without violating their geometric order relative to each other. Therefore, clean water and brine, when salt dissolves in water, are the same phase.

The state diagram is a graphical representation of the state of the solution. It shows in the coordinates temperature-concentration which phases or mixture of phases can exist in a given solution. On the picture 2 part of the diagram of the state of a solution of calcium chloride in water is shown.

diagramm-sostoyaniya-xloristogo-kalciyaPicture 2 – Part of the diagram of the state of an aqueous solution of calcium chloride

Diagram of the state of an aqueous solution of calcium chloride

Same as in the picture 1 the temperature is measured vertically, and horizontally - the concentration by mass of calcium chloride. Freezing point line for liquid brine is in the same place, as in the picture 1, only now is the sharp rise in the freezing point above the eutectic concentration expanded to the maximum temperature of the diagram. Shaded area above the freezing point line, which is indicated by the inscription "Liquid", shows all possible coordinates in the axes "temperature" - "salt concentration" where the existence of a liquid phase is possible. Note, that there is a thin area at the very left edge of the chart, which is designated "Hard Ice". This solid ice has the same molecular structure, like pure ice, but contains a very small amount of dissolved salt. This is the same phase, like pure ice. Therefore, this thin area is the area of ​​existence of solid ice..

State diagram lever rule

Consider a saline solution with a concentration of calcium chloride 5 %, which is cooled to a temperature of -40 ° С (small circle in the picture diagram 2). This point of the phase diagram does not lie in the "liquid", nor in the "hard" areas of the diagram. This solution at -40 ° C cannot be completely solid, nor completely liquid. In the picture diagram 2 shows a horizontal line with arrows at both ends, designated by the letters A and B. The diagram tells us, that this 5% solution at a temperature of -40 ° C is a mixture of solid ice, having composition A (about 7 % salt) and liquids, with composition B (approximately 28,3 % salt). This solution, depending on the proportion of ice and liquid, it can be called ice "porridge", "Suga", "Slush" – a mixture of ice and brine.

The solution at any point in the shaded area of ​​the state diagram is a mixture of a liquid ice-salt solution. In this way, the phase diagram shows which phases or mixtures of two phases can exist for each point in the coordinates "temperature-concentration". Besides, for two-phase regions (shaded area) the chart also predicts the composition of each of the two phases at a given temperature. However, for single-phase solid phases or single-phase liquid phases, the diagram cannot say anything about the composition of the phase at any temperature.