Classification and Separation of Substances Class 6

Pure Substances

a) A pure substance is a type of matter that contains only one kind of particle and has a consistent structure.
b) It cannot be separated into simpler substances by physical means.
c) Pure substances can be either elements or compounds.
d) Examples of pure substances include pure gold (element) and pure water (compound).

Element

a) An element is a type of pure substance that consists of only one type of atom. An atom is the smallest unit of matter.
b) It is the simplest form of matter and cannot be broken down into simpler substances by chemical means.
c) Elements are represented by unique symbols, such as hydrogen (H), oxygen (O), and carbon (C).
d) Each element has specific properties, including atomic number, atomic mass, and chemical reactivity.
e) Elements can combine with other elements to form compounds, but they cannot be further decomposed into simpler substances.

Compounds

a) Compounds are composed of two or more elements bonded together. The elements combine in specific proportions to form a compound with unique properties.
b) Elements stick together in compounds by forming special bonds. These bonds hold the elements tightly together.
c) Compounds have their own special qualities that are different from the elements they're made of.
d) Compounds can be separated back into their original elements, but it requires special processes and sometimes energy.
e) Some common compounds are water, table salt, carbon dioxide, and sugar.

Mixture

A mixture is an impure substance that contains different kinds of elements or compounds in any ratio.
The individual components of a mixture maintain their own properties and can be physically separated.

There are two types of mixtures: Homogenous mixture and heterogeneous mixture

Homogenous Mixture

a) Homogeneous mixtures, also known as solutions, have a uniform composition throughout, meaning that the components are evenly distributed.
b) These mixtures have no visible boundaries making it difficult to differentiate the components with the naked eye.
c) Examples of homogeneous mixtures include air (a mixture of gases), sugar dissolved in water (forming a sugar solution), and alloys like brass (a mixture of copper and zinc).

Heterogeneous Mixture

a) Heterogeneous mixtures have an uneven or non-uniform composition throughout, meaning that the components are not uniformly distributed.
b) The particles of the different substances in a heterogeneous mixture are not evenly mixed together, resulting in distinct regions or phases within the mixture.
c) Heterogeneous mixtures can be visually identified as having visible boundaries, allowing for the differentiation of components.
d) Examples of heterogeneous mixtures include oil and water (forming separate layers), sand and water (where sand particles are suspended in water), and a mixture of rocks and soil.

Differences between Pure Substances and Mixture

Methods of Separation

a) Methods of separation refer to the techniques or processes used to separate the components of a mixture.
b) A mixture is a combination of two or more substances that are physically combined and can be separated through different methods.
c) These methods aim to isolate or extract individual components, such as solid particles, liquids, or gases, from the mixture.
d) The choice of the method of separation depends on the nature of the mixture and the properties of its components.

Separation of Solid-Solid Mixtures

Solid-solid separation refers to the process of separating two or more solid components that are physically combined in a mixture. It involves the separation of solid particles based on their different properties, such as size, shape, density, or magnetic properties.

Some common methods include:

Hand-picking

a) This method involves picking out unwanted materials from a mixture by hand, usually when they are in small quantities and can be easily distinguished based on their shape, size, or colour.
b) For example, removing pebbles and broken grains from rice.

Threshing

a) It is a process used to separate grains from their stalks.
b) This is done by beating or striking the mixture, causing the grains to separate from the stalks.

Winnowing

a) This method uses the blowing of air or wind to separate components based on their weight.
b) For example, separating husks from grains by allowing the lighter husks to be blown away while the heavier grains remain.

Sieving

a) When the components of a mixture have different sizes, sieving is used.
b) A sieve with uniform-sized holes is used, allowing smaller particles to pass through while larger particles remain on top.

Magnetic separation

a) This method is used to separate magnetic impurities, such as iron, nickel, or cobalt, from a mixture of magnetic and non-magnetic materials.
b) A magnet is used to attract and separate the magnetic particles from the rest of the mixture.

Sublimation

a) Sublimation is the process in which a solid directly converts into a gas or vapour without passing through the liquid state.
b) Some solids can sublime, leaving behind the other components.
c) This separation method is used when a solid component can sublime, escaping as a vapour, while the remaining solid components are left behind.

Separation of Insoluble Solid from Liquid

When dealing with a mixture of an insoluble solid and a liquid, several methods can be used for separation.

Filtration

a) Filtration involves passing the mixture through a filter medium, such as filter paper or porous material.
b) The solid particles are trapped by the filter, while the liquid passes through, resulting in the separation of the solid and liquid components.

Sedimentation and Decantation

a) Sedimentation and decantation are used when the solid component is denser than the liquid.
b) In sedimentation, the mixture is left undisturbed, allowing the heavier solid particles to settle down at the bottom.
c) Decantation involves carefully pouring off the clear liquid above the settled solid.
d) This process is often used for separating sand or other heavier solids from water.

Centrifugation

a) Centrifugation is a method that uses a centrifuge, a high-speed spinning device, to separate suspended solid particles from a liquid.
b) The centrifugal force generated by the spinning causes the heavier solid particles to settle at the bottom of the container, allowing the liquid to be separated.

Separation of Soluble Solid from Liquid

Evaporation

a) Evaporation is employed when the solid component is dissolved in the liquid. The mixture is heated, causing the liquid to vaporise and leave behind the solid particles.
b) This method is commonly used to separate dissolved substances like salt or sugar from water.

Crystallisation

a) Crystallisation is a method used to separate a dissolved solid from a liquid by forming crystals.
b) It involves the slow cooling of a hot saturated solution, which causes the dissolved solid particles to come together and form crystals. The crystals can then be separated from the remaining liquid.
c) Crystallisation is often used to obtain pure substances, such as salt or sugar, from their solutions.

Separation of Liquid Components from a Mixture (Miscible and Immiscible Liquids)

When dealing with a mixture of liquids, whether they are miscible (able to mix) or immiscible (unable to mix), different methods are used for separation.

Miscible Liquids

Fractional Distillation

a) Fractional distillation is a process used to separate a mixture of two or more miscible liquids with different boiling points. The boiling point is the temperature at which a liquid changes into a gas or vapour.
b) The mixture is heated, and the liquid with the lower boiling point vaporises first.
c) The vapours are then cooled and condensed back into liquid form, resulting in the separation of the components based on their different boiling points.
d) This method is commonly used in industries to separate different components from crude oil or to purify alcohol.

Immiscible Liquids

Decantation, using a separating funnel

a) This method is used to separate a mixture of two immiscible or insoluble liquids that have different densities.
b) The mixture is poured into a separating funnel, and due to the difference in density, the lighter liquid forms a distinct layer on top of the denser liquid.
c) By opening the stopcock of the funnel, the two liquids can be separated by allowing the denser liquid to be collected first, followed by the lighter liquid.
d) This method is often used to separate oil and water or other immiscible liquid mixtures.

Frequently Asked Questions

1. Why is a separating funnel used to separate immiscible liquids?

A separating funnel is used because immiscible liquids, like oil and water, do not mix and form distinct layers due to differences in density. The denser liquid can be drained from the bottom, leaving the less dense liquid behind.

2. Can substances be separated based on their physical properties?

Yes, substances can be separated based on physical properties like solubility, density, and magnetism, which determine their behaviour in specific separation methods.

3. How does evaporation differ from crystallisation in separating a dissolved solid from a liquid?

Evaporation involves heating a solution to remove the liquid, leaving the dissolved solid behind. Crystallisation is a more controlled process where the liquid is allowed to evaporate slowly, forming pure crystals of the solid.

4. Why can compounds only be separated by chemical means?

Compounds are made of elements that are chemically bonded together. Breaking these bonds requires a chemical reaction, meaning physical methods like filtration or evaporation cannot separate compounds into their elements.

5. What is the method of separating oil and water?

The process of separating oil and water is known as "decantation." In this procedure, the mixture is left undisturbed, enabling the oil to float on top due to its lower density. The clean water may then be gently drained, leaving the separated oil behind.