NCERT Solutions Class 6th Science Chapter – 6 Changes Around Us Notes

We can bring about a change in a substance by doing one or more of the following processes

• Heating.
• Applying force.
• Mixing it with something else.

Changes caused by heating – When an object is heated, it gets affected in one or many possible ways.

• Some objects get hot but do not change in any other way.
• Some objects get hot and also expand in size.
• Some objects get hot and begin to bum.
• Some objects get hot and change their state.

Changes by applying pressure – When we apply force to an object.

• We can change its shape and size.
• Air can be compressed.
• Metals can be hammered into thin sheets.
• Elastic can be stretched.
• Cotton can be spun into thin threads.

Characteristics of physical changes 

• No new substances are formed.
• Products are identical to the reactants.
• These changes are reversible.

Characteristics of chemical changes

• The properties of products are different from the properties of reactants.
• Most of the chemical changes are irreversible.
• These changes always result in energy changes.

Reversible Changes – Changes that be reversed are called reversible changes.

• What happens to ice cream if you do not finish it quickly? It melts. Can you change the molten ice cream back into a solid? Yes! Just keep it in the freezer. Molten ice cream can be changed back to its solid form. Thus, melting is a reversible change. Melting of butter and chocolate are also reversible changes.

• What about changes like condensation, freezing, and evaporation of materials? If you take out some ice cubes from the freezer and keep them outside, the ice cubes will absorb heat from the surroundings and melt. When this water (molten ice) is heated for some time, it starts boiling (liquid starts to evaporate) and steam escapes from the container.

• Now, if you hold a lid over the container, the steam will again liquify or condense into small droplets of water on coming in contact with the cold lid. This water can be cooled down further and then kept in the freezer to form ice again. Thus, the three physical states of water are reversible and can be changed from one state to another by heating or cooling.

Irreversible Changes 

• Changes that cannot be reversed are called irreversible changes.

• There are a large number of irreversible changes that take place around us. These result in a new material being produced, which may or may not be useful. Some examples of irreversible changes are given below.

• Ripening of fruits is an irreversible change because it is not possible to get back the raw fruits from ripened or mature ones.

• The Blooming of flowers is an irreversible change because flowers cannot change back into buds.

• Milk gets spoiled when not refrigerated, particularly in summer. This is called curdling or souring of milk, which is an irreversible change. Curdling of milk is also done by adding lemon juice to milk for making cottage cheese or paneer.

Cooking food – is an irreversible change because we cannot get back the ingredients in their original form after cooking them. For example, after a cake is baked using flour, egg, milk, chocolate, etc., we cannot get back the ingredients.

• The burning of a candle is often cited as an example of physical change because what we see immediately is the melting of wax that solidifies on cooling. However, when a candle burns, the wax is undergoing two changes at the same time: first, it melts, and then it burns. What burns is actually melted wax. The melted wax burns on the wick – the wick itself isn’t burning, it is just the wax on it.

Physical And Chemical Changes

• Changes in which no new substances are formed are called physical changes. For example, breaking glass, freezing of water, tearing of paper, etc.

• Changes in which new substances with different properties are formed are called chemical changes. Cooking of food, and burning of substances are chemical changes as entirely new substances are formed. The burning of a candle wax releases carbon dioxide and water vapour (new substances).

Expansion And Contraction Of Materials – Some materials expand on heating and some contract on cooling. Heating makes the particles (that form the material) expand or become loose.

• Cooling makes the particles (that form the material) contract or become tight.

• The amount of expansion differs in solids, liquids, Physical change
and gases. Gases expand the most while solids expand the least.

• Cooling does the opposite of heating. Cooling causes the material to contract. Solids contract the least while gases contract the most. Table 6.2 lists some examples of contraction.

Applications of Expansion and Contraction

• Expansion by heating can be used in several everyday activities.
The jammed metal lid of a jam jar can be opened by heating. The jar is inverted and just the lid is dipped in hot water. After some time, the lid can be opened easily as the lid gets slightly expanded.

• The fact that materials expand on heating is used in thermometers. In many thermometers, mercury is used. When the bulb of the thermometer comes in contact with a hot object, the mercury expands and its level rises in the glass tube, indicating the temperature.

• Why the electric lines are never hung tautly between the poles? Wires in the outside
environment are subjected to extreme weather conditions ranging from acute hot to cold temperatures. A taut wire on contraction in winters can snap.

Chemical change

• A change where new substances with different properties are formed is called a chemical change.
• Changes happen around us and also within us.
• Some changes are reversible, while some others are irreversible.
• Some changes are physical changes; some are chemical changes.
• Heating causes expansion in a material.
• Cooling causes contraction in a material.
• Gases expand the most and solids expand the least.
• Gases contract the most and solids contract the least.