Solutions, Suspension and Colloids | Plant Physiology

In this article we will discuss about Solution, Suspension and Colloids.

True Solution:

A true solution is made up of at least two components, the dispersed (the solute) and the dispersion (the solvent).

The solute does not settle down and remains evenly dispersed. Its particles are 1 nm or less in size and occur in molecular form. A spoonful of common salt or sucrose when stirred in a beaker full of water gets dissolved to produce a clear solution. Two components are obvious in this experiment; the common salt or sucrose (solute) and the water (solvent).

With constant stirring the solute molecules get evenly dispersed through the solvent in a uniform mixture of the two. In a dilute solution as described above, solute does not settle down but remains evenly dispersed. The solution may be dilute or concentrated depending upon the quantity of the solute added in the solvent.

At a given temperature and pressure, only a specific quantity of solute can be dissolved in a given solvent. The solution in such situations is said to be saturated. In our example, it may be stated that both common salt and sucrose form solutions but these are of different kinds. For instance, sucrose is a non-ionic substance whereas common salt is an ionic substance.

This means that while the former remains intact in water the latter ionizes in water. In other words, common salt (NaCl) breaks into Na₊ and CI sup ions in water and these are evenly distributed. As a result a true solution is formed.

Depending open the solvent whether it is liquid, solid or gas, following categories of solution may be formed.

Water is the most common solvent in the plant cell and its environment. It is universally available and has specific characteristic features.

It has highest boiling point and is highly polarized and behaves like dipoles. The heat of vaporization of H₂O is unusually high. Also, water has high surface tension and this accounts for its capillary action.

Water is significant to plant life in several ways. For example, it is a major constituent of protoplasm and it carries mineral nutrients from cell to cell.

It is a medium of metabolic reactions. Its hydrogen atom is incorporated in organic matter during photosynthesis, causes plant movements and is the metabolic end product of respiration. Water also causes cell and tissue turgidity during growth and participates in the cell elongation phase.

Three types of solutions occur in the cells:

(i) Solution of a gas in a liquid, carbon dioxide, nitrogen and oxygen are commonly found as gases in solution.

(ii) Solution of liquids in liquids fall into two classes; those in which liquids are freely miscible in all proportions e.g., ethyl alcohol in water and secondly those in which two liquids are sparingly soluble e.g. ether, chloroform in water.

Ether and water when shaken thoroughly in a flask and then allowed to stand form distinct layers they are immiscible.

The upper layer will be a dilute solution of water in ether and the bottom layer will be a dilute solution of ether in water.

The liquid-in-liquid solution is where components are freely miscible; the component in greatest amount is called the solvent.

(iii) Solution of a solid in a liquid is the common type of solution, e.g., solution of sucrose and common salt (NaCl) in water.

Sucrose molecule is non-ionic and, therefore, remains as such in water. Sodium chloride (NaCl), on the other hand, is an ionic compound and undergoes ionization in water.

That is, the sodium chloride molecule breaks down to form sodium and chloride ions. These molecules or ions are evenly distributed throughout the water, forming a stable homogenous mixture called a true solution.

Suspension:

In a suspension, the particles are greater than 1 µm in size and the particles do not separate into molecules but are aggregates of molecules which can be seen with naked eye. It is an unstable system. When allowed to stand, particles gradually settle down at the bottom of the container separated by a liquid layer. The most common example of suspension is sand in water.

Colloidal System:

The sizes of the dispersed particles as well as the properties of the system are midway between the suspension and the true solutions. The size of particles ranges from 0.001 µm to 0.1 µm in diameter and they remain dispersed throughout water in a stable manner, forming a two-phase system.

The liquid phase is called dispersion phase while the solid phase is called the dispersed phase. The colloidal particles are in the form of clusters of molecules. They can be seen under the electron microscope.

Expression of Concentration of a Solution:

Concentration of a solution is the relative proportion of the solute in relation to solvent.

It is expressed by any one of the following methods:

Percentage Solution (%) (w/v):

The volume or weight of the solute is calculated as per cent of the solution.

This is shown below:

10% sodium chloride = 10 g NaCl dissolved in water till a final volume of 100 ml

5% alcohol solution = 25 ml of alcohol diluted to a volume of 100 ml with water

Mass % of Sugar = (10 × 100)/(10 + 40) =20%

Molarity (M):

The molarity of the solution is expressed as the number of moles per litre of solution. It is not moles per litre of solvent.

Molar is abbreviated as M and does not stand for moles. Evidently it indicates concentration and not an amount. 0.1 or 0.5 M means 0.1 or 0.5 moles/litre.

The gram molecular weight of any substance contains 6.02 × 10²³ molecules of that substance.

Thus, equal volumes of all solutions of the same molarity will contain the same number of molecules.

This number is also called Avogadro’s number and is useful in expressing the concentration of solutions. 0.5 M or 0.1 M solution is obtained by diluting 1 M solution twice or ten times.

The combined total volume of the solute and solvent is one L.

To prepare 0.1 M NaOH, proceed as follows:

Mol wt. of NaOH = 40

Desired molarity of solution =0.1 M

.^.. −g of NaOH^-L of solution = 40 × 0.1 = 4 g

−Dissolve 4 g of NaOH in water and make final volume to 1 L.

1 M solution = 1 mole of substance^-L of solution

= 1 mmole/mL of solution

= 1 µmol/µL of solution

1 mM solution = 1 mmole/L of solution

Molar Solution:

If a gram molecular weight of a substance (mole) is dissolved in sufficient water to make exactly one litre of solution at 20° C, we refer to it as a molar (M) solution of that substance. Thus, the molar solution contains Avogadro’s number of molecules of the dissolved solute.

A molar solution of glucose contains 180.16 grams of glucose per litre of solution and Avogadro’s number of glucose molecules. A molar solution (1M) of sucrose contains 342.3 g of sucrose dissolved in a litre of solution. As in 1M glucose solution, the 1M sucrose solution contains Avogadro’s number of molecules of sucrose.

So to say, equal volumes of different solutions of the same molarity contain the same number of solute molecules but different number of solvent molecules. Dilution of a given volume of a molar solution with an equal volume of water results in a 0.5 M solution. Similarly, if a given volume of a molar solution is diluted with 9 volumes of water, then 0.1M solution is obtained.

Molal Solution (m):

When a gram molecular weight of a substance is completely dissolved in 1,000 g of water, a weight molar or a molal solution is obtained. The final volume of the solution, thus obtained, is generally greater than 1 litre. The volume by which the solution is more than one litre is called the solution volume of the solute.

Equal volumes of molal solutions, therefore, do not contain the same number of solute molecules since the volume is in excess of 1 litre and varies with the type of solute used. On the other hand, molal solutions of equal concentration have the same mole fractions of solute and solvent.

In physiological experiments it is convenient to prepare solutions on the basis of percentage by dissolving substances by weight or volume. A 10% solution of NaCl is obtained by dissolving 10 grams of solute in 90 ml of water. Similarly, a 20 % solution of ethyl alcohol is made by mixing 20 cc of alcohol in 80 cc of water.

Thus the amount of solute dissolved by weight or volume in solvent to make 100 g or 100 cc of solution gives percentage of the solution by weight or volume, respectively.

1. Here the amount of solvent is 1000 g.

2. Dissolve 1 mole of solute in 1000 ml of water (sp gravity =1).

3. Dissolve 106 g of Na₂ CO₃ in one kg of water = 1 m Na₂CO₃

Normal Solutions:

A normal solution of a substance is obtained by dissolving a gram equivalent weight of that substance in a litre of solution. If we dissolve 2 g equivalent weight in a litre of solution we get a 2N (two normal) solution.

The gram equivalent weight of an element is the weight in grams of the element that combines with or is otherwise equivalent to 1.008 g of hydrogen. It is also the weight of the compound that will interact with one equivalent weight of an element. The concentration of acid and alkali solutions is more easily expressed in terms of normality than molarity.

The gram equivalent weight of an acid or base is the quantity that will release or neutralize mole of an hydrogen ions. 1M solution of HCl is also a IN solution of the acid. However, 1M solution of H₂SO₄ would be 2N since it is capable of releasing 2 moles of hydrogen ions.

A 1M solution of NaOH is also IN since the moles of OH^– ions released in solution can neutralize 1 mole of hydrogen ion. But a 1M solution of Ba (OH)₂ is 2N since in solution the 2 moles of OH⁻ ions released are capable of neutralizing 2 moles of H⁺ ions.

A normal solution of an acid contains one mole or 1.008 grams of replaceable hydrogen per litre and a normal solution of a base contains one mole or 17.008 g of replaceable OH^– ions per lite.

Since 1.008 of replaceable H⁺ ions contain the same number of ions as 17.008 g of a replaceable OH^– ion, evidently equal normalities will exactly neutralize each other.

Dissolve 5.3 g Na₂ CO₃ in 1 L of solution = 0.1 N Na₂CO₃ (Eq wt of Na₂CO₃ = 53)

Parts per Million (ppm):

A gram of solute per million grams of solution or gram of solute per million ml of solution

or

NaCl (1 ppm) in water

1 ppm = 1 mg NaCl/L of solution

= 1 mg NaCl/1000 ml of solution.

Stock Solution:

Stock solution is prepared and used to prepare a desired volume of a solution of needed concentration of a substance:

N₁V₁ =N₂V₂

N₁ = consent, of solution intended to be prepared

N₁ = volume of solution to be prepared

N₂ = consent. of stock solution

N₂ = volume of stock solution.

Thus-

Stock of Tris = 75 mM

i. For preparing 150 ml of solution having 10 mM Tris

V₂ = (N₁V₁)/N₂ = (10 × 150)/75 = 20ml

ii. Take 20 ml of 75 mM stock solution of Tr is. Make final volume to 150 ml (Add H₂O)

iii. So 10 mM Tris is secured.

Solution Strength:

There are several ways in which solution strength can be expressed and these are given below:

1. Weight per unit weight:

When 20 g of sugar is dissolved in 80 ml of water, it is expressed as 20% (w/w) sugar in water or 20 g of sugar per 100 ml of solution.

2. Weight per unit volume:

When 20 g of sugar is dissolved to make 100 ml of solution, it is expressed as 20% (w/v) sugar in water. It is nearly equal to 10% w/w solution.

3. Volume per unit volume:

This is usually employed for solutions of liquid in liquid. Thus 80% (v/v) alcohol in water indicates 80 ml of alcohol in 100 ml of solution. Since the two liquids differ in specific gravity, this would differ from 10 ml of alcohol mixed with 20 ml of water.

Acids, Bases and Salts:

An acid is defined as a substance which is capable of giving hydrogen ions (H⁺) when dissolved in water.

The ‘strength’ of an acid depends upon its degree of ionization. The greater the proportion of H ions an acid produces in a solution at a given concentration, the “stronger” it is e.g. acids like HCl, H₂SO₂ and HNO₄ ionize almost completely at ordinary concentrations and are called strong acids while organic acids such as acetic acid (CH₃COOH) and oxalic acid (C₂H₂O₄) ionize to a very small extent at ordinary dilutions and are known as weak acids. Acids have a sour taste and can neutralize a base.

Electrolytes and non-electrolytes:

In recent years acid and base are defined on the basis of exchange of proton and electron.

Thus acid is a substance which can donate a proton or accept an electron pair. On the other hand, a base is a substance which can accept a proton or donate an electron pair.

Colloids:

A spoon of clay soil when added to a glass of water and shaken thoroughly produces a liquid.

On allowing this mixture to stand, it begins to clear, the larger soil particles being the first to settle followed by the smaller ones.

However, after a greater length of time it becomes nearly clear but some of it remains indefinitely in suspension.

The stable heterogeneous mixture thus produced is called a colloidal suspension composed of very minute soil particles called clay micelles suspended in water.

The suspended phase in called the dispersed phase whereas the medium in which the dispersion occurs is called the dispersion medium.

Colloidal systems have two components-a continuous phase of dispersion medium as a liquid, and a discontinuous phase or dispersed phase comprising distinct particles.

These particles are in the form of molecule aggregates which remain suspended throughout the liquid like the true solution.

The size of particles range from 5-200 nm in diameter, somewhat between the size of particles found in unstable suspensions and that of true solutions. The colloidal particles may be detected with the electron microscope.

The dispersion medium may also be a gas or a solid. For example, smoke is composed of solid particles dispersed in a gas. Milk is a liquid, dispersed in the liquid. Two systems typical of protoplast are emulsions, sols and gels.

Properties of Colloidal Dispersions:

(i) Tyndall Effect:

If a strong beam of light is passed through a colloidal suspension and observed at right angles through an ultra-microscope, its path is easily observed.

This is due to the scattering of light by minute particles which appear as bright spots. The size and form of the particles cannot be seen. This phenomenon is known as the Tyndall effect.

(ii) Brownian Movement:

The colloidal particles show Brownian movement first described by Scottish botanist Robert Brown in 1827.

This is an erratic and continuous movement caused by unequal bombardment of the colloidal particles by moelcules of the dispersion medium.

(iii) Filtration:

The colloidal particles are incapable of passing through parchment membrane or collodian membrane because the size of pores in them is smaller than those of the particles.

So they can be easily separated from the dispersion medium. The components of true solution cannot be separated in this manner.

(iv) Stability:

When undisturbed, pure colloidal solutions are highly stable and remain so for several years.

(v) Heterogenous Nature:

The colloidal solutions are heterogenous in nature and consist of minute solid particles suspended in the liquid medium.

(vi) Adsorption:

Molecules or ions tend to adhere to the interface of colloidal systems, and the process is called adsorption.

It is a surface phenomenon and the capacity for adsorption, therefore, is determined by the amount of surface exposed and also the chemical nature of the constituents in question. The process of sub- division of substances increases its surface area.

Therefore, the adsorptive capacity of a colloidal system is very high for a given weight of colloidal particles.

Most of the important functions of colloidal system found in the living cell are dependent upon their immense adsorptive capacity.

This enables the protoplasm to carry on a large number of complex chemical reactions at ordinary temperature which otherwise would need high temperatures in the laboratory.

(vii) Electrical Properties:

Colloidal particles carry electric charge which may be positive or negative. For any one colloidal system the charge is the same on all the particles.

The charges are due to the adsorption of free ions in the dispersion medium. The preferential adsorption of positive ions by a colloidal particle will give it a positive charge and the reverse is true of negatively charged colloidal particles.

In case the dispersed phase has a positive charge, all the particles of a colloidal system will collect at the cathode and in case of negatively charged, they will collect at the anode. This phenomenon is called electrophoresis.

The similarity in the charge of the particles of a suspension is responsible for the stability of the colloidal suspensions.

The fact that units of like charge repel each other, prevent their aggregation and precipitation out of suspension.

(viii) Flocculation:

Destruction of the electric double layer enforces the dispersed particles of a colloidal suspension to collide, aggregate and precipitate out of suspension.

This is caused by the addition of the certain amount of electrolyte. This practice of flocculation by adding electrolytes is called salting out.

Valency of an ion is very significant in determining the extent to which precipitation is caused by it when added to a colloidal system.

For instance, the monovalent sodium ion is far less effective than the divalent barium ion or the trivalent aluminium ion.

Viscosity:

The viscosity of a colloidal solution is more than that of water. The collodial solution of albumin diffuses at a rate of about 1/7000 to that of sucrose solution.

Colour:

Collodial solutions are generally coloured and the colour is influenced by the size of the colloidal particles.

Protoplasm as a Colloidal System:

The protoplasm in a living cell does not occur as a true solution, even though several substances are dissolved in it.

Most of the particulate phase of the protoplasm is colloidal. Therefore, the protoplasm is generally regarded as a colloidal complex exhibiting several properties attributed to colloidal systems. The cell membrane and wall, centrosomes and chromosomes are all gel-like structures.

Much of the colloidal attributes of the protoplasm are due to their proteins which are large, complex molecules approaching colloidal dimension.

They occur throughout the ground substance of the protoplasm where they are associated with diverse metabolism e.g., respiration, digestions, secretion, biosynthesis, etc.

Enzymes provide large surface area dispersed in the protoplasm and this tact is of great significance to several enzyme-substrate reactions which support life itself. Obviously the colloidal system is an essential feature of living matter.

Emulsions:

When two immiscible liquids are vigorously shaken together an unstable emulsion is formed.

Droplets of one of the liquids will be dispersed throughout the other. These small droplets of disperse phase tend to coalesce, forming larger droplets until two liquids are again separated into two distinct layers. When an emulsifying agent is added an emulsion is made stable.

These substances reduce the surface tension of the liquids which reduce the tendency of the small droplets to combine or else they may form a protective layer around the droplets making it impossible for them to combine. Milk is an emulsion where butter fat is dispersed in water with casein as an emulsifying agent.

Common emulsions are of two types i.e. cream or butter type emulsions. In the former a small amount of the non-polar oil or liquid occurs as droplets in the continuous medium of water.

In the latter type a small quantity of a polar liquid e.g. water occurs as droplets in the continuous medium of oil. Membranes in living cells have some properties of a butter-type emulsion.

Hydrophilic and Hydrophobic Systems:

The different types of colloidal dispersions in water are divisible into hydrophilic and hydrophobic systems.

In the hydrophilic system, the dispersed phase and the water dispersion medium are attracted towards each other, while in the hydrophobic system the two phases repel each other.

When solids such as starch, gelatin or agar are added to hot water, some amount of water is adsorbed to form hydrophilic sols which are hydrated i.e. water molecules are absorbed to the surfaces of these particles.

Sols and Gels:

Some of the hydrophilic sols under certain conditions are able to form an extremely viscous solid ­like mass.

For instance, hot aqueous gelatin will set to form a jelly-like mass called a gel. When a sol is converted into a gel it is called gelation.

On the other hand, when a gel of gelatin is heated it will form a sol and the process is called solution.

Typically, a hydrophilic sol-gel system is reversible; its state depends upon temperature. However, a blood clot is an example of irreversible gel.

It is believed that gel structure comprises both solid and liquid phases and these are continuous.

The solid phase constitutes a mesh work of long interwoven threads of sub-microscopic dimensions, and the spaces within them are occupied by the liquid phase.

This structure is supported on several grounds e.g., diffusion of solutes, electrical conductivity and the velocity of chemical reactions.

Hydrophobic colloids are inorganic compounds and their preparation involves chemical reactions.

When a concentrated solution of FeCl₃ is added to hot water, a dark-red colloidal suspension of Fe(OH)₃ is formed. FeCl₃ ionizes and the hydrolysis of Fe³⁺ ions occurs to form Fe(OH)₃.

Aggregation of the Fe(OH)₃ molecules forms the colloidal particles of the dispersed phase.