Chapter 6: Physical and Chemical Changes

Question 1:

Mention whether the following processes given below are chemical or physical changes:

(i) Photosynthesis

(ii) Dissolving sugar in water

(iii) The burning of coal

(iv) Melting of wax

(v) The beating of aluminum to make aluminum foil

(vi)  Digestion of food

Answers:

(i) Photosynthesis is Chemical change

(ii) Dissolving sugar in water is a physical change

(iii) The burning of coal is a chemical change

(iv) The melting of wax is a physical change

(v) Beating aluminum to make aluminum foil is a physical change

(vi) Digestion of food is a chemical change

 

Question 2:

State true of false for the following:

(i) When pieces of wood  are cut, is it known as a chemical change.

(ii) When manure is formed from leaves, Is it known as a physical change.

(iii) When iron pipes are coated with zinc, they do no rust easily.

(iv) The composition of rust and iron are the same.

(v) When steam is condensed, it is not known as a chemical change.

Answers:

(i) False

Correct statement: Cutting a log of wood into pieces is an irreversible physical change.

(ii) False

(iii) True

(iv) False

Correct statement: Iron and rust are two different chemical substances.

(v) True

 

Question 3:

Fill the blanks for the given statements:

(i) Lime water is turned milky because ____________ is formed when carbon dioxide is passed through it.

(ii) ______________ is the chemical name for baking soda.

(iii) __________ and __________ are the two methods in which help in the prevention of rusting of iron.

(iv) When ________ properties of a substances change, it leads to the changes known as physical changes.

 (v) When new substances are formed by the changes, they are called as _________ changes.

Answer:

(i) Lime water is turned milky because Calcium Carbonate is formed when carbon dioxide is passed through it.

(ii)Sodium hydrogen carbonate is the chemical name for baking soda.

(iii) Painting or greasing and galvanization are the two methods in which help in the prevention of rusting of iron.

(iv) When physical properties of a substances change, it leads to the changes known as physical changes.

(v) When new substances are formed by the changes they are called as Chemical changes.

 

Question 4:

What is the type of change when baking soda is mixed with lemon juice, forming bubbles with the evolution of a gas? Explain.

Answer:

The reaction that takes place when baking soda is mixed with lemon juice is given below:

Lemon juice (Citric Acid) + Baking soda – – – – – – – – – > CO2 [bubbles] (Sodium hydrogen carbonate) + Other substances (carbon dioxide)

The changes that take place in the reaction are chemical changes.

 

Question 5:

Both Chemical and physical changes take place when a candle is made to burn. Identify the changes. And mention another process in which both physical and chemical changes take place.

Answer:

When the candle is made to burn the changes that occur are:

(a) Physical change: The wax melts when the candle is made to burn and the melted wax is vaporized

(b) Chemical change: Carbon dioxide, light and heat are produced by burning of vapors of wax.

Another example in which both the chemical and physical changes take place is LPG. The LPG comes out of the cylinder which is the physical change. The chemical change that takes place is when the gas that comes out, is converted from liquid to gaseous state and this chemical change that has occurred is when gas is burned in air.

 

Question 6:

How can it be shown that the setting of curd is a chemical change?

Answer:

The setting of curd is said to be a chemical change because when the curd is formed, we cannot get the original substance (i.e. milk) back. And a new substance which has a different taste, chemical properties and smell are formed.

 

Question 7:

Explain why the cutting of wood and the burning of wood are said to be two different types of changes.

Answer:

The burning of wood is a chemical process because when wood is made to burn new substances are obtained such as Charcoal etc.

Wood + Oxygen – – – – – – > Charcoal + CO2 + heat + light

But when wood is cut into small pieces, it is a physical change because no new substance is formed, only the size of the wood is made smaller.

 

Question 9:

Explain how when an iron gate is painted and prevented from rusting.

Answer:

We all know that for rusting to take place there should be the presence of oxygen and moisture and painting helps in the prevention of the iron gate coming into contact with moisture and oxygen.

 

Question 10:

Explain why the rusting of iron objects is faster in the coastal areas when compared to the deserts.

Answers:

As the content of moisture present in the air is higher in the coastal areas when compared to the air in deserts, the process of rusting is faster in the coastal areas.

 

Question 11:

The gas that is used in kitchens is known as LPG. When the gas is in the cylinder, it is of liquid state but when it comes out it is in gaseous state (Change 1) and is burnt (Change 2). The following statements related to the changes that take place. State the correct statement. 

(a) Process -1 is a chemical change

(b) Process -2 is a chemical change

(c ) Both processes are chemical changes.

(d) Both processes are NOT chemical changes

Answer:

(b) Process 2 is a chemical change.

 

Question 12:

The anaerobic bacteria digest the waste of animals and produce a biogas ( change 1) The biogas is burnt as fuel (Change 2). The following statements are the changes that take place. Choose the correct statement.

(a) Process 1 is a chemical change

(b) Process 2 is a chemical change

(c ) both the processes are chemical changes

(d) Both the processes are not chemical changes.

Answer:

( c) Both the process are chemical changes.

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