6. Physical and Chemical Changes Science class 7 in English Medium ncert book solutions Additional Questions With Solutions

Additional Questions With Solutions

Ques. – What do you understand by changes?

Ans. – When any object or material transforms its colour, size and taste etc that is known as changes. Ex – curd from milk, chapatti from Flour, chair from wood.

Ques. – What colour of light gives when ribbon of magnesium is burnt in air? What is called the Powderly ash obtained from this?

Ans. – It burns with a brilliant white light.

Ques. – What kinds are changes?

Ans. – There are two kinds of changes.
(1) Physical changes
(2) Chemical changes

Ques. – What are physical changes?

Ans. - A change in which a substance undergoes a change in its physical properties is called a physical change. Ex – Making any object from wood, Making kite of paper etc.

Ques. – What are chemical changes?

Ans. – A change in which one or more new substances are formed is called a chemical change. A chemical change is also called a chemical reaction. Ex- rusting, curd from milk.

Ques. – Name the changes in which no new substance is formed?

Ans. – physical changes.

Ques. - If you leave a piece of iron in the open for some time, it acquires a film of brownish substance.

(i) What is called this substance?

(ii) What is called this process?

Ans. –

(i) This substance is called rust.

(ii) The process is called rusting

Ques. – What new substance is formed when the ash of magnesium oxide is added in water?

Ans. – Magnesium hydroxide.

Ques. – Why does the blue colour of copper sulphate solution turn into green when an iron nail kept in?

Ans. – it occurs due to form a new substance iron sulphate.

Ques. – Write the chemical equation of reaction between carbon dioxide and lime water.

Ans. – The reaction between carbon dioxide and lime water is as follows:
Carbon dioxide (CO2 ) + Lime water [Ca(OH)2 ] Calcium Carbonate (CaCO3 ) + Water (H2O)

Ques. – Write the complete equation for rusting?

Ans. – The complete equation for rusting is as follow;
Iron (Fe) + Oxygen (O2, from the air) + water (H2O) rust (iron oxide Fe2O3)

Ques. – How would you show that setting of curd is a chemical change?

Ans. – Chemical change is the change in which there forms a new substance. Setting of curd is a new substance which is non reversible.

Ques. – Write the differences between chemical changes and physical changes.

Ans. –

Ques. – When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

Ans. – it is a chemical change. When lemon juice mixed with baking soda, acid of lemon juice reacts with baking soda (base) give salt and hydrogen gas, this cause forming bubbles.

Ques. - Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Ans. – Burning of wood is chemical change in which forms new substances coal and carbon dioxide while cutting into small pieces is a physical change.

Ques. - Describe how crystals of copper sulphate are prepared?

Ans. – The crystals of copper sulphate are prepared by the process of crystallisation. In which there is added powder of copper sulphate in hot boiling water till highly concentrated. There obtains crystals after cooling the solution.