2021-22 Science CH- PHYSICAL AND CHEMICAL CHANGES (NOTEBOOK AND WORKBOOK EXERCISE)

 

CH- PHYSICAL AND CHEMICAL CHANGES(NOTEBOOK AND WORKBOOK EXERCISE)

 

NEW WORDS

1.galvanisation

2. rusting

3. antacid

4. anhydrous

5.reactivity

NEW TERMS

1.   anhydrous: a substance that is free from water.

2.   Reactivity: it is the tendency of a substance to undergo a chemical reaction, either by itself or with other materials

 

 

Answer the following questions in one word or a sentence.

a.   What is rust?

ANS. A reddish-brown layer of iron oxide forms on iron articles which are exposed to air and moisture for a period of time. This reddish-brown layer is known as rust. The process of formation of rust is known as rusting.

 

b.   State the conditions necessary for rusting of iron.

ANS. 1.AIR. 2.MOISTURE

 

c.   Give two examples of antacids.

ANS. Magnesium hydroxide (milk of magnesia), aluminium hydroxide, sodium carbonate and calcium carbonate are common bases used as antacids.

 

d.   Give one example of a chemical change in which a change in colour is observed.

ANS. Cut fruits and vegetables get blackened when they are left exposed to air.

The chemicals present in the fruits and vegetables undergo a chemical change as they react with the oxygen present in the air.

 

Answer the following questions in brief.

a. Distinguish between physical and chemical changes.

ANS.

Physical change	Chemical change
Only physical changes like odour, physical state, density, volume etc. changed and chemical properties remain unchanged	The chemical composition and chemical properties undergo a change
No new substance is formed in a physical change.	A new substance is formed in a physical change.
Very little or no energy in the form of heat, light or sound is usually absorbed or given out in a physical change	A chemical change is always accompanied by absorption or evolution of energy
It is a temporary change	It is a permanent change
The original form of substance can be regained by simple physical method	Original substance cannot be obtained by simple physical method
Most of the physical changes reversible.	Most of the chemical changes are irreversible.

 

 Define the following.

i. Chemical equations

ANS. Chemical changes are also called chemical reactions and are represented in the form of an equation known as a chemical equation

ii. Reactants

ANS. The original substances that take part in a chemical reaction and change into new substances are known as reactants

iii. Crystallisation

ANS. Large crystals of pure substances can be formed from their solutions. The process of formation of crystals from their solution is known as crystallisation.

 

  Give reasons.

 i. Boiling of water is a physical change whereas the boiling of an egg is a chemical change.

ANS. Boiling of water is a physical change as the water changes into water vapor and can be reversed. But boiling of an egg is a chemical change because it cannot be reversed.

 

 ii. Iron coated with paint or grease does not rust.

ANS. Paint or grease form a layer on the iron article and prevent direct contact with moist air and iron material.

 

Answer the following questions in detail.

a. Explain the process of neutralisation with an example.

ANS. A reaction in which an acid and a base react to form salt and water is called a neutralisation reaction.

b. What are physical and chemical properties?

ANS. A physical property is a characteristic of matter that is not associated with a change in its chemical composition. Examples of physical properties include physical state, size, shape, color, density, hardness, electrical conductivity, melting and boiling points.

 A chemical property of a substance is a property that is observed during a reaction in which the chemical composition or identity of the substance is changed. Combustibility, reactivity, flammability are few examples of chemical properties.

 

c. Explain the function of antacids.

ANS. Our stomach produces hydrochloric acid for the digestion of food. Sometimes the acids are produced in excess and they cause acidity. Doctors give antacids to treat acidity. Antacids are weak bases which neutralise the hydrochloric acid and the person gets relief from acidity.

Magnesium hydroxide (milk of magnesia), aluminium hydroxide, sodium carbonate and calcium carbonate are common bases used as antacids.

 

 Diagram based question.

 a. What colour change will you observe in the beaker shown in the diagram when an iron nail is placed in it?

ANS. 

The colour of the solution changes from blue to green.

b. What type of reaction takes place in the beaker when the iron nail is placed in it?

ANS. Displacement reaction takes place.

Iron + Copper sulphate --------}   Iron sulphate + Copper

 

HOTS (Higher Order Thinking Skills)

a. Do physical and chemical changes take place simultaneously? Justify your answer with an example.

ANS. Yes, in some cases the physical and the chemical changes can occur together.

 One such example is the burning of candle. The wax present in the candle changes to liquid state.

At the same time, the constituents carbon and hydrogen present in wax react with oxygen of air to form new substances.

b. What will you observe when a copper wire is placed in an iron sulphate solution? Why?

ANS. When copper wire is placed in iron sulphate solution there will be no change observed. This is because copper is less reactive than iron, therefore, unable to displace it from its solution.

MIND MAP

LMR (LAST MINUTE REVISION NOTES)

 

LINK FOR WORKBOOK EXERCISE

https://drive.google.com/file/d/1eMdsnAW6dk6PEVScv9OEGQUlBNb6iMdp/view?usp=sharing