NCERT 7th Class (CBSE) Science: Chemical and Chemical Changes

Question: What is a chemical symbol? What are the features of the modern method of representing elements?

Answer:  The representation of an element using the short form or abbreviation is known as chemical symbol. Features of the modern method are as follows:

1. For some elements, the first letter of their names is used as the symbol, which is written in capital letter. For example, C for Carbon, O for Oxygen and N for Nitrogen.
2. If two or more elements have their English names starting with the same letter, then a two-letter symbol is used. For example, C for Carbon, Ca for Calcium, Cl for Chlorine and Co for Cobalt.
3. For some elements, the symbols are derived from their Latin names. For example, the Latin name of gold is aurum and its symbol is Au.

Question: Ripening of a fruit is a chemical change why?

Answer: Ripening of fruit is a chemical change because after ripening, a new product with different properties is formed.

Question: What kind of change is rusting?

Answer: Chemical change.

Question: What is the colour of copper sulphate solution?

Answer: Blue colour.

Question: What are the essential conditions for rusting?

Answer: The essential conditions for rusting are – Presence of moisture and air.

Question: What is the final colour of the solution when iron nails are dipped in the copper sulphate solution?

Answer: Green colour.

Question: What kind of change takes place when iron reacts with copper sulphate?

Answer: Chemical change.

Question: Name the gas released when baking soda is mixed with vinegar.

Answer: Carbon dioxide.

Question: What is galvanisation?

Answer: The process of depositing a layer of zinc on iron is called galvanisation.

Question: What do you understand by crystallisation?

Answer: The process of getting crystals of pure substances from their solutions is called crystallisation.

Question: Name the two metals involved in galvanisation.

Answer: Zinc and iron.

Question: What kind of change is glowing of a electric bulb?

Answer: The glowing of electric bulb is a physical change.