Elements, Compounds and Mixtures

Be

Reason — The symbol of Beryllium is Be.
B is the symbol for Boron. Ba stands for Barium. Br represents Bromine.

B and Si

Reason — Na (Sodium) and Mg (Magnesium) are both metals. B (Boron) and Si (Silicon) are both metalloids. C (Carbon) and P (Phosphorus) are both nonmetals. He (Helium) and Ar (Argon) belong to the noble gas group.

They are heterogeneous.

Reason — Compounds are homogeneous, i.e. they have a uniform composition throughout. Compounds are made of elements combined chemically in a fixed ratio by mass, so they have definite formulae. Pure compounds have fixed melting and boiling points.

Phosphorus

Reason — Phosphorus has atomicity 4.
Iodine exists as I₂ and has atomicity = 2. Ozone exists as O₃ and has atomicity = 3. Sulphur exists as S₈ and has atomicity = 8.

a compound

Reason — A molecule is the smallest unit of a compound that retains all the chemical properties of that compound. Mixtures do not have fixed units like molecules. An ion is a charged particle.

Sulphur

Reason — Sulphur is soluble in carbon disulphide.
Iron is a metal and does not dissolve in carbon disulphide. Sodium chloride is an ionic compound and dissolves in water, not in carbon disulphide. Iron sulphide is a compound and does not dissolve in carbon disulphide.

(a) Sodium

(b) Zinc

(c) Graphite

(d) Mercury

(e) Tungsten

(f) Graphite

(g) Bromine

(h) Diamond

(i) Neon

(a) Gold and Silver

(b) Oxygen and Sulphur

(c) Boron and Silicon

(d) Helium and Neon

(a) An element is a pure substance which cannot be converted into anything simpler than itself by any physical or chemical process. It is made up of only one kind of atoms.

(b) A compound is a pure substance composed of two or more elements combined chemically in a definite proportion by mass.

Iron sulphide is a compound because of the following reasons:

1. Iron sulphide is a compound formed when iron and sulphur combine chemically, on heating, in 7 : 4 ratio by mass.
2. Iron is a grey black metal and Sulphur is a yellow amorphous non-metallic solid whereas iron sulphide is a black solid.
3. Iron is attracted by a magnet and Sulphur is soluble in carbon disulphide but iron sulphide is neither attracted by a magnet nor it is soluble in carbon disulphide.

From the above, we can say that properties of iron sulphide are entirely different from its constituent elements i.e., iron and sulphur. Hence, iron sulphide is a compound.

(a) Difference between Pure and impure substances:

(b) Difference between Homogeneous and heterogeneous substances:

The properties of sodium chloride are completely different from those of sodium and chlorine. Sodium is a soft, highly reactive metal. Chlorine is a poisonous greenish yellow gas with choking smell. But sodium chloride is a white crystalline solid which is non-poisonous and useful. It is added to our food as a mineral and also to add taste to it.

Four differences between elements and compounds are:

sublimation

Reason — The mixture is first subjected to sublimation. Ammonium chloride sublimes, leaving behind sand. The vapours of ammonium chloride solidify on cooling. Hence, sand and ammonium chloride are separated.

alcohol

Reason — A solvent for iodine is alcohol. In the solvent extraction method, iodine is dissolved in alcohol to separate it from the components of a mixture.

Ammonia

Reason — Ammonia (NH₃) is highly soluble in water.

fractional crystallisation

Reason — Fractional crystallisation is the process of separating two or more solids based on the difference in their solubilities in the same solvent. In a mixture of sodium chloride and potassium nitrate, potassium nitrate crystallises out first when the hot supersaturated solution is cooled, leaving sodium chloride in the solution. This process is repeated to separate the two.

fractional distillation

Reason — Fractional distillation is used to separate the components of crude petroleum. Since crude oil consists of different hydrocarbons with varying boiling points, fractional distillation helps separate them into fractions such as petrol, diesel, kerosene, and other petroleum products.

Both A and R are true and R is the correct explanation of A.

Explanation — Since elements and compounds are made up of only one kind of atoms and molecules respectively, they are pure substances.

Both A and R are true but R is not the correct explanation of A.

Explanation — Assertion (A) defines what a mixture is—two or more substances (elements/compounds) physically combined in any proportion, hence "impure."
Reason (R) states a property of one type of mixture (homogeneous = uniform composition). It doesn’t explain why mixtures are impure or their "any proportion" nature, so it isn’t the explanation for Assertion (A).

A is true but R is false.

Explanation — Compounds are always homogeneous; their composition is uniform throughout.

Both A and R are true but R is not the correct explanation of A.

Explanation — Metalloids are elements showing some properties of metals and some properties of non-metals. They are hard solids. But metalloids being hard solids does not explain why they have mixed (metal-like and non-metal-like) properties.

A is false but R is true.

Explanation — Assertion (A) is false because chromatography is used to separate the components of a mixture when all the components are very similar in their properties, not different in their properties. Reason (R) is true because the components of ink can be separated by chromatography.

(a) Homogeneous

(b) Gravity separation method

(c) heterogeneous

(d) an element

(a) False
Correct Statement — The elements present in a compound cannot be obtained by simple physical methods.

(b) True

(c) True

(d) False
Correct Statement — A mixture has no definite chemical formula.

Compounds — Carbon dioxide, Iron sulphide, Water, Common Salt, sugar.

Mixtures — Air, Milk, Fruit Juice, blood, brass.

(a) Brass is an example of solid-solid homogeneous mixture.

(b) Sand and water is an example of solid-liquid heterogeneous mixture.

(c) Acetone and water are miscible liquids.

(d) Ammonia and water is a liquid-gas homogeneous mixture.

(a) Mixture — Mixtures can be defined as a kind of matter which is formed by mixing two or more pure substances (elements and compounds) in any proportion, such that they do not undergo any chemical change and retain their individual properties.

(b) Crystallisation — Crystallisation is the process of formation of solid crystals from a hot supersaturated solution by cooling.

(c) Distillation — Distillation is the process of converting a liquid into vapour by heating and the subsequent condensation of the vapour back into liquid.

Difference between a residue and a sediment:

Difference between a filtrate and a distillate:

The advantage of distillation over evaporation is that in distillation, both the solid and the liquid can be recovered. On heating the solution, the liquid evaporates, passes through the condenser and is collected as the distillate, while the solid is left behind in the distilling flask. In evaporation, only the solid is recovered and the liquid escapes as vapour.

(a) Chalk powder and water can be separated by filtration method. Filtration method is used for separating insoluble solids from liquids. On filtration, chalk powder gets deposited on filter paper whereas water passes through the filter paper and gets collected in the beaker.

(b) Iron from sulphur can be separated by magnetic separation method. This method is used when one of the components is magnetic in nature. Iron gets attracted towards magnet and can easily be separated from sulphur.

(c) Water and alcohol can be separated by fractional distillation method. Fractional distillation method is used to separate miscible mixtures in which the liquids have different boiling points. Boiling point of alcohol is 78°C and water is 100°C. Hence, this method is the most suitable for separating them. 95.5% pure alcohol is obtained by this method.

(d) Sodium chloride and potassium nitrate can be separated by fractional crystallisation. This method is used when the solubilities of solid components of a mixture are different in the same solvent. Both salts are soluble in water, but the solubility of potassium nitrate is more affected by temperature than that of sodium chloride. Hence, potassium nitrate crystallises out first on cooling the hot supersaturated solution.

(e) Solvent extraction method is used to separate a mixture of calcium carbonate and sodium chloride. This method is used when one component is soluble in a particular liquid while the other component is insoluble. Sodium chloride is soluble in water but calcium carbonate is insoluble which settles down.

Four characteristics of mixtures are:

1. A mixture consists of two or more pure substances that exist together without any chemical combination between them.
2. A mixture may be homogeneous or heterogeneous.
3. The components of mixtures vary in their proportions.
4. In mixtures, the components are loosely held together and therefore they retain their individual properties.

It is necessary to separate the components of a mixture for the following reasons:

1. To Obtain Useful Substances: Some mixtures contain valuable substances that can be separated and used for various purposes. For example, when crude petroleum is separated, we obtain useful substances like LPG, CNG, petrol, diesel, and kerosene oil, which are primarily used as fuels.

2. To Remove Unwanted and Harmful Substances: Mixtures may contain impurities or harmful substances that can be harmful to health or degrade the quality of the product. For example, small stones and husk need to be removed from cereals like rice, wheat, and pulses before cooking because they are harmful.

3. To Obtain Pure Substances for Preparing Other Useful Substances: Sometimes, it is important to separate unwanted components to obtain a pure substance for specific purposes. For instance, drinking water contains air and mineral salts, which need to be removed to obtain pure water for laboratory use, medicine preparation, or for use in car batteries.

Hence, separating the components of a mixture ensures we obtain useful, pure substances and remove harmful or unnecessary impurities.

The following procedure can be followed for separating a mixture of sand, salt and ammonium chloride:

• The mixture is first subjected to sublimation. Ammonium chloride sublimes, leaving behind sand and salt. The vapours of ammonium chloride are collected and on cooling down, they solidify.
• The solvent extraction method is used to separate the mixture of sand and salt. Water is added to the mixture. Common salt dissolves while sand settles down. The salt solution is carefully decanted to separate it from sand.
• The solution is then evaporated to obtain salt from water.

(a) The process of separating different dissolved constituents of a mixture by their adsorption on an appropriate material is called chromatography. It is used to separate the components of a mixture when all the components are very similar in their properties. It is used to separate liquid-liquid mixtures.

(b) The advantages of chromatography are:

1. A very small quantity of the substance can be separated.
2. Components with very similar physical and chemical properties can be separated.
3. It identifies the different constituents of a mixture.
4. It also helps in quantitative estimation of the components of a mixture.

(c) The two applications of chromatography are:

1. It is used to purify many industrial products.
2. It is used to separate pigments from natural colours.

The mixture of kerosene oil and water is placed in a separating funnel, where they form two layers. Water, being heavier, forms the lower layer, and kerosene oil, being lighter, forms the upper layer. The stopper is opened to drain the water (the heavier liquid) into a vessel, and the process is stopped when the lower layer is completely removed, separating the two liquids.

(a) Solvent extraction — Solvent extraction is based on the difference in solubility of components in a solvent. A solvent is chosen in which one or more components of the mixture dissolve, while others do not. The soluble component is then separated from the insoluble ones.
Example: A mixture of sodium chloride and calcium carbonate can be separated by this method.

(b) Gravity separation — Gravity separation works on the principle of differences in the densities of components. Heavier particles settle at the bottom due to gravity, while lighter particles remain on top or are separated by other means.
Example: A mixture of sand and sawdust can be separated by this method.

(c) Fractional crystallisation — Fractional crystallisation relies on the difference in solubility of components in the same solvent. When a mixture of solids is dissolved in a solvent and the solution is heated, the components may have different solubilities. As the solution cools, the less soluble component crystallizes first, while the more soluble component remains dissolved. Repeating this process allows for the separation of the components.
Example: A mixture of common salt and potassium nitrate can be separated by this method.

(d) Fractional distillation — Fractional distillation is used to separate liquid-liquid homogeneous mixtures (miscible liquids) that have different boiling points. When the mixture is heated, the liquid with the lower boiling point vapourises first, and its vapour is condensed and collected in a separate receiver. The process is repeated, and the second liquid vapourises once the temperature rises to its boiling point.
Example: Water and alcohol are separated by this method.

(a) The teacher brought a heterogeneous solid-solid mixture because its components — iron filings, common salt and sulphur — are different substances and can be separated by physical methods.

(b) Iron filings (Component A) get attracted to the magnet and are separated because iron is a magnetic substance, while salt and sulphur are non-magnetic.

(c) Sulphur (Component C) remains insoluble in water and is collected on the filter paper. The substance left on the filter paper is called the residue.

(d) Yes, the component that dissolved in water can be recovered from its solution. The dissolved substance is common salt (component B). It can be obtained by evaporating the water. When water evaporates, common salt is left behind as solid crystals.

(e) Two examples of mixtures used in daily life are:

1. Salt and water
2. Tea