subtance mixture homogenues heterogenues it has fixied boiling point its smell complitly some portion seems to be not melting
Answers
Answer:
For a layman, pure substances are pure honey, pure milk, pure cheese, pure water, etc. However, to a chemist none of the above mentioned substances are pure. For example, pure milk is made of a number of substances like proteins, carbohydrates, mineral salts, vitamins, water etc., present in variable amounts in the milk of different animals of same breed. Thus, milk can be called a mixture in which amount of various substances are not present in same fixed ratio. Before we answer the question:
What is a pure substance, let us see the broad classification of matter.
Classification of matter
2.2 PURE SUBSTANCES
To answer this question, let us see, with what, gold and water are made of. If you consider gold, it is made up of only one type of particles called gold atoms. Water is also made up of only one type of particles called water molecules. Such substances are called pure substances. Thus, a homogeneous material which contains particles of only one kind and has a definite set of properties is called pure substance.
Iron, silver, oxygen, sulphur are pure substances, because each one has only one kind of particles.
However, if a substance is composed of two or more different kinds of particles combined together in fixed proportion by weight, then the substance is also regarded as a pure substance.
Sodium chloride is a pure substance because it has a fixed number of sodium and chlorine particles combined together in fixed proportion by weight.
Similarly, magnesium oxide and carbon dioxide are pure substances.
Note: It does not imply that all homogeneous substances are pure. For example, common salt solution in water is a homogeneous solution, yet it cannot be called a pure substance, as it is made of two different substances, e.g. salt and water.
Characteristics of a pure substance
i) A pure substance is homogeneous in nature.
ii) A pure substance has definite set of properties. These properties are different from the properties of other substances.
iii) The composition of a pure substance cannot be altered by any physical means.
2.3 ELEMENT
To understand it, let us consider silver. What happens if you break it into tiny pieces? Do you get any new substances?
You get tinier particles of silver but you will not end up with gold or copper. Thus, silver remains as silver. Such substances are called element.
Thus, an element is a pure substance that cannot be broken into two or more simpler substances by any known physical or chemical means.
An element is made of only one kind of atoms. Chemists have discovered 115 elements so far. Amongst 115 elements, 82 are normal elements and 33 are radioactive elements.
Normal elements are those which do not give harmful radiation. Radioactive elements are those which give harmful radiation.
Thus, we can say, Eighty-two elements are non-radioactive and Thirty-three elements are radioactive elements.
Characteristics of Elements
i) An element is a pure homogeneous substance, made up of only one kind of atoms.
ii) Except during nuclear reactions, an element cannot be broken into two or more smaller parts.
iii) An atom is the smallest unit of an element. It shows all the pr