Is Matter Around Us Pure

A pure substance is a type of substance which contains a single type of constituent particles which are the same in their chemical nature.

Elements (Iron, copper etc) and compounds (water, carbon dioxide, etc) are examples of pure substances.

Crystallisation is a process that separates a pure solid in the form of its crystals from a solution. This process is used to obtain pure crystals of a substance from an impure mixture sample.

Example: Separation of pure crystals of copper sulphate from an impure mixture of copper sulphate solution is done using crystallisation.

(a) The solute is the component which gets dissolved in a solvent to form a solution. It is present in a smaller amount when compared to the solvent.

(b) The solvent is the component which dissolves a solute to form a solution. The solvent is present in larger amount when compared to the solute.

Example:

Alloy consists of metals mixed in a particular proportion. They exhibit the properties of the constituents. No new chemical properties are shown once the metals are mixed. Hence an alloy is considered as a mixture.

Example, steel is an alloy of iron and carbon as it is a mixture of these two elements where the larger proportion is of metal (iron) and smaller proportion of non-metal (carbon)

A solution is a homogenous mixture of 2 or more substances. The substance present in the larger amount (solvent) dissolves the substance present in the smaller amount (solute).
Example,

The constituent particles of air are evenly distributed. This means that a sample of air taken from any place would contain the constituent particles in the same proportion/composition. Air is constituted by more than one kind of substance and shows the properties of its constituent particles. Therefore, air is considered a mixture.

Solvent dissolves solute particles by overcoming the intermolecular forces between the solute (or one can call it the solute-solute interaction). So, to dissolve a solute, the solute-solute interaction must be broken by the solvent particles.

When the temperature increases, the kinetic energy of the solvent particles increases, causing to break the solute-solute interactions more effectively. This allows the addition of more solute particles, as there is space to accommodate more solute particles. So the solubility increases with increase in temperature. Lowering the temperature does the opposite. It decreases the solubility of the solute.

The phenomenon where the path of the light beam is visible as a result of scattering of light by colloidal particles is called Tyndall effect. The size of the particle should be such that it should scatter light. Particles which are very small (in the case of true solutions) or very large (in the case of suspensions) do not scatter light. So, the Tyndall effect is not observed in true solutions or suspensions.

This is the general case. In certain special cases, if the particle size of suspension is small enough to scatter light, then suspensions also show Tyndall effect.

Cream and milk differ in their density. Cream is denser than milk. When milk is churned (shaking or spinning of container containing milk by applying a rotating force) the denser particles (cream) tend to remain at the bottom, whereas the lighter particles (milk) stay at the top, along with the direction of spin. This process is called centrifugation.

Chromatography is the process of separation of solutes that dissolve in the same solvent.

Ink is a mixture of 2 or more colours (dye) dissolved in water. Different colours present in ink have different solubility in water. The component which is more soluble rises faster in the strip of filter paper. The component which is less soluble travels less distance in the filter paper. This is why different colours present in the ink rise to different levels during paper chromatography.

(a) A solution in which at a given temperature, no more solute can be dissolved by the solvent is called a saturated solution. The concentration of such solutions is the maximum at a given temperature.

To illustrate this, take a glass or a beaker and add some common salt to it. Fill the beaker with water and stir it well. One now gets a salt solution. Continue adding salt slowly to the beaker along with stirring. At some point of time, the solution reaches a state where even though salt is added and stirred, some of the salt stays undissolved. Then one can say that the solution has reached its saturation level at that temperature.

(b) A solution in which at a given temperature, more solute can be dissolved by the solvent is called an unsaturated solution. Unsaturated solutions can be made saturated by adding more solute to it. In the activity mentioned in (a), the solution obtained in the beginning is an unsaturated solution.

(c) The suspension is a heterogeneous mixture of 2 or more components. The particles forming the suspension are visible to the naked eye. In this mixture, the solute particles do not dissolve in the solvent, instead, they remain suspended in the solvent medium. When left undisturbed, the solute particles tend to settle down at the bottom of the container.

Eg: Mixture of sand and water forms a suspension.

A mixture of 2 miscible liquids whose boiling points differ by more than 50°C can be separated using a technique called distillation. The liquids must not decompose upon boiling.

The mixture of liquids is taken in a round bottom flask and is heated. The liquid component with a lower boiling point starts to escape from the mixture and the gas is passed through a condenser. The cold water helps to condense the gas into its liquid form and is collected in a beaker.

In the given mixture, at around 80°C, benzene gets converted into a gaseous state and is sent through the condenser to get benzene back in the liquid state.

Finally, diesel remains in the round bottom flask and benzene in the beaker.

Take a kettle or a teapot. Add water to it. Boil the water and add tea powder/tea leaves to it based on your choice. The more tea you add, the stronger the liquid becomes. Note that tea is the solute and water is the solvent. Also the tea powder insoluble in water, but the essence is absorbed by the liquid. Once the tea+mixture starts to boil, dissolve sugar (another solute) in it. You may add sugar as long it is soluble in it for maximum sweetness. To this solution, add milk if you need milk tea. Once it is boiled, remove it from the heat source and filter the solution for tea powder. You can see that the residue you get is tea powder, and the filtrate (the liquid after filtration) is black/milk tea.

The concentration of a solution is the amount of solute present in a given amount of solution or solvent. This can be mathematically written as:

The concentration of solution =

Or

The concentration of solution =

Mainly one expresses concentration in 2 ways:

(a) Mass by mass percentage:

(b) Mass by volume percentage:

Eg:

Consider 30g of sugar (solute) dissolved in 150g of water (solvent).

Then the mass of solute = 30g

Mass of solvent = 150g

Mass of solution = Mass of solute + mass of solvent

= 30+150

= 180g

Therefore the concentration of solute expressed using mass by mass percentage is:

Mass by mass percentage of solution = (Mass of solute/Mass of solution) x 100

= 16.6%

(a) A pure substance is a type of matter which is formed by a single type of particles. The constituent particles are identical in their chemical nature. The constituent particles of a particular pure substance will always be found in a fixed composition and cannot be separated using physical methods.

Elements and compounds are examples of pure substances.

(b) Mixtures are a type of matter formed by more than one pure substance. The constituent particles just mix together in different proportions to give different mixtures. The properties exhibited by the mixture are the same as the properties exhibited by the individual constituent particles. These constituents can be fairly separated using physical techniques.

Sugar solution, milk, air are examples of mixtures.

Note: The sub-division of matter is shown as:

Fractional distillation is the process used to separate a mixture of miscible liquids whose boiling points differ by less than 25°C. The experimental setup is similar to that of distillation except for a fractionating column which is filled with glass beads.

In this method, the mixture is heated using a burner, and both the liquids most likely enter the vapour state. The vapours hit the glass beads in the column and tend to condense. Out of the two vapours, the vapour component which is less volatile cools faster than the other and returns to liquid state. Gradually, the more volatile component reaches the top of the column and is passed through the condenser as shown in the figure and gets liquefied.

The less volatile component remains in the round bottom flask.

The process is repeatedly done to completely separate the two liquids.

Separation of 2 immiscible liquids is done using a separating funnel. The above figure shows a separating funnel clamped to a stand. In this case the mixture to be separated contains oil and water.

Immiscible liquids do not mix well and have different densities. When the mixture is left undisturbed, the denser liquid comes to the bottom of the funnel, and the lighter liquid stays on top of it. Now by operating a stopcock carefully, one can transfer the denser liquid (here, water) to the conical flask. The stopcock is closed when water is completely transferred from the funnel. The separation is done multiple times to ensure complete separation of the two components. Oil remains in the funnel and water is in a conical flask.

Chromatography is a technique used to separate components which dissolve in the same solvent. Different components present in the ink have different solubility in the solvent. The component which is more soluble in the solvent travels more distance with the solvent. Likewise, the component with less solubility travels less distance with the solvent.

Paper chromatography is illustrated in the above figure. An ink spot is marked in a thin strip of filter paper. The bottom of the paper is immersed in a beaker containing water. As the water rises up, it takes along the different components present in the ink to different heights according to their solubility. As a result, colours (individual components of the ink) are seen along the height of the strip.

This way, the individual components are separated out using chromatography.

The salt present in sea water may contain impurities in dissolved form. Simple evaporation causes the water to evaporate, leaving the impurities with the salt. In crystallisation, pure solid is obtained in the form of crystals from a solution. So, crystallisation ensures the purity of the product obtained.

To separate the different gases from air, they are to be liquefied first to implement fractional distillation. The air is cooled and compressed to obtain liquid air. One can see that the boiling points (b.p.) of various constituents differ by less than 25°C.

For instance, the b.p. of oxygen is -183°C whereas the b.p. of nitrogen is -196°C. So simple distillation would convert both into gaseous state, and one would fail to separate them. Due to this reason, fractional distillation is used.

In this case, both liquids get converted into vapour state and passes through the fractionating column. The less volatile component gets condensed and becomes a liquid again. The more volatile component suffers less condensation. As a result, the more volatile component tends to reach the top of the fractionating column. This vapour at the top is condensed and collected in a beaker.

For the separation to be effective, this process is done multiple times.

Two immiscible liquids are separated by the difference in their densities. When left undisturbed, the denser liquid tends to stay at the bottom of the separating funnel, and the lighter liquid stays at the top. Then the denser liquid is extracted out by carefully operating the stopcock.

In the case of miscible liquids, the densities of the constituent liquids are relatively the same and the mixture is homogenous. When left undisturbed, one will not obtain the liquids in layer as there is not much difference in their densities.

Tyndall effect is the phenomenon in which the path of the light beam becomes visible when a light beam enters a colloidal solution. The path becomes visible due to the scattering of light by the colloidal particles. Soap solution is an example of a colloidal solution and hence shows Tyndall effect. Salt solution, sugar solution and copper sulphate are examples of true solutions and do not show Tyndall effect.

A solution in which no more solute can be dissolved at a given temperature is called saturated solution, and the amount of solute present in a saturated solution at a given temperature is called solubility. Solubility is directly proportional to temperature. So, when temperature decreases (i.e., when the saturated solution is cooled), solubility also decreases. Therefore, the solution becomes supersaturated.

Water is a compound formed by the elements hydrogen and oxygen. Air, pond water, soda water are examples of mixtures.

Elements are pure substances which cannot be further broken down into simpler substances. Carbon dioxide gas, sulphur dioxide gas, nitrogen dioxide gas are examples of compounds and they can be further broken down into elements by chemical reactions.

Homogeneous mixtures are mixtures in which the constituent substances are mixed well and have a uniform composition throughout.

Options (a), (b) & (d) are homogeneous mixtures.

Starch Solution is heterogenous mixture. The starch solution is a suspension and not a solution.

Pure substances are those in which all the constituent particles present in them are the same in their chemical nature.

Elements and compounds are examples of pure substances. CaO, CO₂, NaCl are examples of compounds. Air, milk, brick and wood fall under impure substances.

Aqueous solution is a solution in which the solvent is water.

Sublimation is the process of conversion of a substance from solid state to gaseous state without entering the liquid state. When more pressure is applied, the gaseous particles would come closer and closer (because of compression) and tend to form liquids. The heat energy supplied under pressure will then force the solid to convert into liquid state, as now it cannot sublime due to the pressure applied over it.

Sublimation is the process of conversion of a substance from solid state to gaseous state without entering the liquid state. The substances which are sublimable are highly volatile in nature.

Chromatography is a technique used in the separation of a mixture into individual components. The different components dissolved by the same solvent travel with different speeds based on the solubility of the components and get separated.

Pure substances contain only one kind of particles and have the same composition throughout. Elements and compounds fall under pure substances, not mixtures. All elements including nickel are examples of nickel.

Corrosion is the process of degradation of a material as a result of chemical reactions with the environment. The material gets converted into its oxide, sulphide or another compound. Rusting of iron is an example of corrosion where iron is converted into iron oxide when exposed to air and moisture. Since there is a change in the material (formation of a new compound), corrosion is a chemical change.

Mixture of sulphur and carbon disulphide is an example of a true solution. True solutions are homogenous and do not Tyndall effect.

Tincture of iodine is made by dissolving iodine in alcohol. It is used as medicine.

Homogenous nature implies that the composition is uniform throughout. If you take different samples of the same substance, the components present in it will be in same composition. Ice is a pure substance whereas air is a mixture, but both are homogenous in nature as there is uniform composition of constituents.

True

Properties such as density, boiling point, melting point are physical properties and they can be observed or measured directly.

False

Both physical and chemical changes occur during burning of a candle. There is a change in the state of wax from solid state to liquid state and from liquid state to vapour state. Change in state of the matter is a physical change.

Burning involves the process of combustion. The candle is basically a carbon containing material which combines with oxygen to give out carbon dioxide. This process is a chemical change.

False

Metals are lustrous and are good conductors of heat and electricity.

True

Metals can be bet to form thin sheets (malleable), drawn to form thin wires (ductile). They produce a ringing sound when they are hit (sonorous) and have a shiny appearance (lustre).

True

Compounds are pure substances composed of 2 or more elements chemically combined with one another in a fixed proportion. Water, carbon dioxide are examples of compounds.