An electric current is a directed movement in a closed electric circuit of charged particles. Charged particles move under the action of an electromotive force to the pole, the sign of which is opposite to the sign of their charge. Why an electric current does not conduct a sugar solution, but a sodium chloride solution conducts, is described in the article.
What is necessary for the existence of electric current
Before considering the question of why an electric current does not conduct a sugar solution, and a sodium chloride solution conducts, we consider the conditions under which this current can exist.
Firstly, the main condition for the occurrence of electric current is the presence of the so-called electromotive force. This force, acting on charged particles, leads to their directed movement in a closed circuit. An electric battery, a generator, and some other devices can act as a source of electromotive force.
Secondly, for the existence of a closed current circuit, the presence of charged and free particles, that is, those that, under the influence of an electric field, could be directed to the positive or negative poles of the current source, is necessary.
For example, in metallic materials there is a large number of free valence electrons that are carriers of an electric charge. Solids that conduct electric current are called conductors.
In the case of solutions, charged groups of atoms (cations and anions) can act as charge carriers. Conducting electric current solutions are called electrolytes.
Electrical conductivity of water
From a chemical point of view, water is a H 2 O compound. A water molecule is electrically neutral, therefore it cannot participate in the transfer of electric charge, in other words, pure water is a poor conductor of electric current, however, the molecule itself is electrically polar, since the high electron density is concentrated in the region of the oxygen atom.
For water, electrical conductivity increases due to the presence of various ions in it. So, even pure distilled water has some conductivity due to the dissolution of carbon dioxide in it with the formation of free H + protons and negatively charged hydrocarbonate groups (HCO 3 ) - . Due to this process, the electrical conductivity of distilled water is 5.5 * 10 -6 S / m. To understand the significance of this figure, we note that the copper conductivity at 20 ° C is 5.96 * 10 7 S / m, which is 13 orders of magnitude greater than the conductivity of pure water!
What is sugar
From the point of view of chemistry, sugar is a disaccharide whose formula is C 12 H 22 O 11 . Sugar consists of a sucrose molecule and a fructose molecule. A sugar molecule is formed due to strong covalent bonds between carbon, oxygen and hydrogen atoms, which is an important point for understanding why sugar solution does not conduct electric current.
Speaking about the physical properties of sugar, it should be noted that it has high solubility in water. So, at 20 ° C in 100 g of water, 203.9 g of sugar can be dissolved. With increasing water temperature, this indicator also grows, reaching 478.2 g at 100 ° C. An aqueous solution of sugar is called syrup.
What is sodium chloride?
Sodium chloride or table salt is a substance whose chemical formula is NaCl. In nature, sodium chloride is present in the form of a halite mineral. In the solid state, NaCl is an ionic crystal formed by Cl - anions and Na + cations located at the sites of the crystal lattice. Each ion in the lattice is surrounded by six ions having the opposite sign and located at the vertices of the octahedron.
In sodium chloride, the crystal lattice is complex. It can be represented as two face-centered cubic lattices (one formed by Na + cations, and the other by Cl - anions) inserted into each other.
To understand the answer to the question of why the sugar solution does not conduct electric current, and the sodium chloride solution conducts, it is also important to know that sodium chloride is highly soluble in water.
What happens when sugar and sodium chloride dissolve in water
Knowing the necessary conditions for the occurrence of electric current, as well as the chemical composition and crystalline structure of sugar and sodium chloride, we go directly to the answer to the question of why the sodium chloride solution conducts and the sugar solution does not conduct electric current.
First, consider what happens to a NaCl crystal in water. The polarity of the H 2 O molecules leads to the fact that they surround the cations and anions of the NaCl crystal and simply “disassemble” it into parts. Dissolving in water, sodium chloride passes into free Na + and Cl - ions, which are able to participate in the formation of an electric current. Depending on the concentration of dissolved salt, the conductivity of water increases by several orders of magnitude.
Why doesn’t sugar conduct an electric current? Quite simply, the polar water molecules also destroy the bonds between the disaccharide molecules in the crystal (these bonds are of van der Waals nature), as a result, C 12 H 22 O 11 molecules that are electrically neutral, which are surrounded by water, appear in the solution, that is, they are not able to participate in maintaining the electric current in this solution.