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The process of obtaining large crystals of pure substances from their solutions is called as Crystalisation. 

Manmade‘ change, useful change and a chemical change

Natural change.

non-periodic change.

1. Canned food is processed food. 

2. After certain period of time it may get spoilt due to some growth of micro-organism it may get intoxicated. 

3. If we consume such food we may get food poisoning, so we should check the expiry date.

Physical changes are those in which no new substance is formed. In sublimation, there is the direct conversion of solid into gas and vice versa. Thus the substance remains the same.

Photosynthesis requires sunlight which is not available at night time. Therefore the process stops at night.

Man-made changes are those that occur because of human activities e.g. changing the direction of flow of a river by building a dam.

1. Spoiling of milk. 

2. Air pollution.

Photosynthesis requires is sunlight.

The changes that are not useful or changes that harms living things or the environment are called harmful changes.

Man-made changes are result of human- activity. E.g. Burning of fossil fuel.

change in molecular composition.

A physical change is defined as a change in which no new substance is formed. A physical change is temporary because this change can be easily reversed by reversing the conditions. For example when ice is melted, it gets converted into water by absorbing heat. Water so formed can be easily converted to ice by cooling thus heat is released. Thus physical change is temporary.

Many changes such as baking bread, cooking food are useful to us, they are called useful changes.

Changes that occur naturally like ripening of a fruit, spoiling of milk are called natural changes.

1. True

2. True

3. False. 

Correct: Decomposition of leaves is a slow change.

4. False. 

Correct: Dissolving of solute in a solvent is a physical change and rusting of iron is a chemical change.

1. In a chemical change, molecules and atoms rearrange themselves to form new substances. This rearrangement involves some energy changes. It can either be absorption or evolution of energy.

2. Photosynthesis requires sunlight which is not available at night time. Therefore the process stops at night.

Physical changes are temporary.

1. Ripening of mango 

2. Curdling of milk.

Copper and brass articles are coated with tin. This is called tinning.

(b) Removing iron nails with help of magnet.

(d) Conversion of ice cubes into water.

Rising and setting of the sun at a given place is a periodic change.

(a) Ripening of fruits is a desirable change because this change is beneficial for the mankind and we desire to happen.

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

1. Baking bread 

2. Cooking food.

Rising and setting of the sun at a given place is a periodic change.

Iron articles are given a thin coat of zinc to prevent corrosion this is called galvanization.

1. Lighting in the sky. 

2. Earthquake.

1. Desirable: Changes that are useful to man are desirable changes, e.g. change of milk into curd.

2. Undesirable: Change that brings about destruction is an undesirable change, e.g. floods and epidemics are undesirable changes.

3. Periodic: Changes that are repeated at regular intervals of time are called periodic changes, e.g. change of day and night.

Melting of ice is a reversible change.

(a) (i) Physical change: A temporary change in which no new substance is formed, the composition of substance remains the same, though its state, shape and size may change.

(ii) Chemical change: A permanent change in which new substances are formed whose composition and properties are completely different from those of the original substances.

(b) Physical change:

(i) Change of water into its vapors.

(ii) Heating of iron rod.

Chemical change:

(i) Burning of wood.

(ii) Breathing.

When an iron article rusts, a reddish brown layer (of iron oxide) forms on it. A greenish layer is seen to form on a copper article. This is called corrosion of metals.

1. Ripening of a mango. 

2. Burning of wood

1. Reversible change: When a change in a substance can be reversed by changing the conditions, it is said to be a reversible change.

Example: Melting of ghee or wax.

2. Irreversible change: When a substance cannot be brought back to its original state after a change, it is said to be an irreversible change.

Example: Souring of milk

The process by which matter changes from its vapour or gaseous state into its liquid state by cooling is called condensation.

1. Melting of wax. 

2. Freezing water into ice.

The changes which we cannot reverse back to same thing are called Irreversible changes e.g. Burning of wood, paper.

The changes that can occur in a forward and reverse direction again and again are called reversible changes.

Changes that do not repeat themselves after a fixed interval of time are called non-periodic changes.

Some changes occur again and again after. a definite interval of time, such changes are called periodic changes.

A chemical change produces a new substance having new and different properties and which cannot be reversed by reversing the conditions.

We can dissolve the ash comes in the above reaction with water gives Magnesium hydroxide 

Magnesium Oxide + water ➝ Magnesium hydroxide 

MgO + H₂O ➝ Mg(OH)₂

For rusting, the presence of both is required. The coat of the paint prevents direct contact of iron with air and oxygen and this prevents it from rusting.

flame. It bums with brilliant white light and leaves powdery ash. This is chemical change. It is represented by an equation. 

Magnesium + Oxygen Magnesium Oxide

Mg + O₂ ➝ MgO.

Both air and moisture are required for rusting to take place. In coastal areas, the quantity of moisture present in air is more than that in deserts. In desert areas, the amount of moisture in air is even lower. Therefore, rusting of iron objects is faster in coastal areas than in deserts.

Whenever iron things kept in open places for sometime, it acquires a thin film of brownish substance. This is rust and this process is rusting. Chemically iron comes in contact with oxygen and water the reaction takes place forming iron oxide. This can be represented by an equation

Fe + O₂ + H₂O ➝ Fe₂O₃

Heat:

1. Burning of paper.

2. Cooking of food needs heat.

Light :

1. Formation of food by plants i.e. photosynthesis need light.

2. Hydrogen and chlorine react in presence of light

Electricity:

1. Water breaks into hydrogen and oxygen on passing electricity.

2. Sodium chloride solution breaks up into its components sodium and Chlorine on passing electric current through sodium chloride solution

1. water is boiled

On boiling water changes into steam (gas) physical change.

2. a piece of paper is burnt

On burning piece of paper produces carbon dioxide and ash is left behind. Is a chemical change.

3. some ice cubes are kept in a glass tumbler

Ice cubes (solid) turn into water (liquid) only state changes (physical change).

4. solid ammonium chloride is heated

Solid ammonium chloride on heating changes into vapors (change of state) is the physical change.

5. an iron nail is kept in tap water for a few days

We observe reddish-brown coating on the nail called rust (entirely new substance) is chemical change.