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quicklime will show a chemical change.

When sugar is heated, its color changes into black.

1. a spoon of sugar is heated in a pan 

When a spoon of sugar is heated in a pan, black (charred sugar) (carbon) is seen. Is a chemical change. 

2. lighted matchstick is brought near the mouth of the test tube containing hydrogen gas. 

We observe that hydrogen bums at the mouth of the test tube with the blue flame and pop sound are heard. It is a chemical change. 

3. quicklime is dissolved in water. 

The following two observations will be observed 

(i) A hissing sound is observed. 

(ii) The mixture starts boiling and lime water is obtained. 

4. little amount of curd is added to a bowl containing warm milk and kept for five hours. 

When a little amount curd is added to a bowl containing warm milk and kept for five hours, a permanent change occured. The milk will change to curd. On boiling water changes into steam (gas) physical change.

True.

In a physical change, no net energy change is involved.

True 

Is melting of ice a reversible change.

False. 

Correct: Change of seasons is a periodic change.

When baking soda (Sodium hydrogen carbonate) is mixed with lemon juice (citric acid), bubbles are formed. The bubbles are formed due to the evolution of carbon dioxide gas. This is a chemical change involving the formation of new substances.

Lemon juice + Baking soda ➝ Carbon dioxide + Other substances.

ii. Process -Bis a chemical change.

Melting of solid wax is a physical change because no new substance is formed during this change.

1.chemical change.

2.physical change.

3.chemical change.

4.chemical change.

1. Physical changes

2.Chemical changes

3.Chemical changes

4.Physical changes

True

Dissolving of a solid in a liquid is a physical change.

Physical change

(a) Drying of wet clothes

(b) Manufacture of salt from seawater

(d) Boiling of water

(f) Melting of wax

(h) Formation of clouds

(i) Making of a sugar solution

(j) Glowing of an electric bulb.

Chemical change

(c) Butter getting rancid

(e) Burning of paper

(g) Burning of coal

(k) Curdling of milk

1. True

2. True

3. True

(a) The process of a liquid changing into a solid is called freezing.

(b) A change, which alters the composition of a substances, is known as a chemical change.

(c) There is no change in the composition of the substance during a physical change.

(d) The reaction in which energy is evolved is called exothermic reaction.

The differences are Physical and Chemical Changes:

Physical change

1. In a physical change, no new substance is formed and the chemical composition of substance remains the same. There are changes only in physical properties and state.

2. Temporary change which can be reversed by simple physical methods.

3. Weight of original substance doesn’t change

4. Energy like heat, light etc. may or may not be absorbed or released

Chemical change

1. In a chemical change, new substance with an entirely different chemical composition and properties is formed.

2. Permanent change and irreversible

3. Weight of original substances may increase or decrease

4. Energy like heat, light etc. are given out or absorbed.

The various conditions that favor the process of evaporation are discussed as under : 

1. Concentration of water vapor in air: (humidity level): If the water vapor concentration is high in air the evaporation process gets slowed down. 

2. Pressure: Evaporation takes place faster when pressure is less and vice versa. 

3. Surface Area: If the surface area is large then evaporation takes place faster. 

4. Temperature: On increasing the temperature, the rate of evaporation increases. 

5. Rate of flow of air: If air is moving over the place all the time, then the concentration of water vapor decreases in the surroundings, thus increasing evaporation rate.

(a) Freezing of water to ice and evaporation of water is physical change because water can be brought back to its original (liquid) form by 

1. We can heat the ice to bring it back to water. 

2. We can cool down the vapors to bring it back to water. 

(b) When a candle is lighted, some of the solid wax first melts and turns into liquid, then it turns into vapors to produce a flame. New substances CO₂ and H₂O vapors are formed along with the evolution of light and heat energy. This shows a chemical change. When some of the molten wax drops to the floor, it again solidifies. Which shows a physical change. Thus the melting of candle wax is a physical change and the production of CO₂ and H₂O represents chemical change. 

(c) When a piece of paper is burnt a new substance ash is produced. Even when the burning is stopped, the ash cannot be changed back into paper. This shows that the formation of the ash from the paper is a permanent and irreversible change. 

(d) Because it does not change the chemical composition of cloth and the change is only in the state, size, shape, colour, texture or the smell of some or all of the substances that undergo physical change.

i. Periodic change

ii. Manmade, harmful change

iii. Manmade, harmful change

iv. Manmade (irreversible change)

v. Natural, chemical

vi. Natural, periodic

vii. Physical change

viii. Natural, slow, desirable.

ix. Irreversible change (fast).

i. Manmade change (useful), chemical

ii. Manmade, chemical change.

iii. Manmade (fast).

iv. Chemical change (harmful)

v. Chemical change

vi. chemical change (harmful)

vii. Chemical change (useful), irreversible

viii. Periodic change, natural change

ix. Periodic change

i. True

ii. True

iii. False, it is a physical change

A burning candle produces wax vapors which solidify and form wax again is a physical change. At the same time wax on burning produces water vapors and carbon dioxide which escape into the atmosphere which are new substances with new properties. It is a chemical change. 

Hence the burning of candle shows both physical and chemical changes.

i. chemical

ii. useful

iii. rusting

iv. iron

v. physical

vi. temporary

vii. physical change

viii. chemical, product.

Collect the melted wax and give the required shape when it is soft.

Heat the glass piece again at the link. When it becomes soft break it.

I will take into account the following facts.

1. Whether the shape, size or state of the substance is changed. 

2. Whether it is a temporary change. 

3. Whether it is a reversible change.

1. A change in which the substance can be brought back to its original state is called a reversible change.

2. Changes that occur in nature are called Natural changes.

3. An earthquake is a non-periodic change.

4. A new substance is formed in a chemical change.

5. Heat is released by a gas during the process of condensation.

I will take into account the following facts.

1. Whether a new substance is formed. 

2. Whether it is a permanent change. 

3. Whether it is a non-reversible change.

Burning is a fast process

In addition to new products, the following may accompany a chemical change : 

1. Heat, light or any other radiation may be given off or absorbed. 

2. Sound may be produced. 

3. A change in smell may take place or a new smell may be given off. 

4. A colour change may take place. 

5. A gas may be formed. 

Some examples for above statements:

1. Burning of any substance is always accompanied by the production of heat and light. 

2. Explosion of fireworks produce heat, light and sound. During this the unpleasant gases will produce and pollute the atmosphere. (It is not advisable, so it is avoided). 

3. The stale food gives bad adour (smell) 

4. The cut fruits and vegetables change its colour after some time.

1. Bud blooming into flower. 

2. Ice to water, Ice is in solid state, Water is in liquid state.

(i) Swinging of a pendulum

(ii) Rising and setting of the sun.

Natural and periodic change