Elements, Compounds and Mixtures

(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.

(a) Gold and Silver

(b) Oxygen and Sulphur

(c) Boron and Silicon

(d) Helium and Neon

(a) Difference between Pure and impure substances:

(b) Difference between Homogeneous and heterogeneous substances:

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) Sodium

(b) Zinc

(c) Graphite

(d) Mercury

(e) Tungsten

(f) Graphite

(g) Bromine

(h) Diamond

(i) Neon

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.

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

Mixtures — Air, Milk, Fruit Juice

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

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

(c) Mixture of Acetone and Water is a miscible liquid.

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

(a) Salt from sea water is separated by evaporation method. Evaporation method is used to separate the components of a homogeneous solid-liquid mixture. The liquid is escaped in the form of vapour and the solid is left behind.

(b) Sublimation method is used to separate ammonium chloride from sand. This method is used when one of the component is sublime in nature. Ammonium chloride is sublime in nature so when it is heated, ammonium chloride directly changes into vapours and sand is left behind. The vapours of ammonium chloride on cooling changes directly into solid.

(c) 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.

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

(e) Water and alcohol can be separated by fractional distillation method using a fractionating column fitted over the distilling flask. 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.

(f) Sodium chloride and potassium nitrate can be separated by fractional crystallization. This method is used when the solubility of solid components of a mixture is different in the same solvent. Both salts are soluble in water but solubility of potassium nitrate is more than that of sodium chloride in water.

(g) Solvent extraction method is used to separate calcium carbonate and sodium chloride mixture. 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.

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.

It is necessary to separate the constituents of a mixture because

1. To remove unwanted and harmful substances.
2. To obtain pure and useful substances.

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.

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.

The two applications of chromatography are:

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

sublimation

Reason — Sublimation method is used when one of the component is sublime in nature. Ammonium chloride is sublime in nature so on heating, it directly changes into vapour leaving behind the sand.

B and Si

Reason — Boron(B) and Silicon(Si) are metalloids because they show some properties of metals and some properties of non-metals.

They are heterogeneous.

Reason — Compounds are always homogeneous because the composition and properties is same throughout.

Alcohol

Reason — Iodine gets dissolved in ethyl alcohol.

Ammonia

Reason — Ammonia is highly soluble in water.