Revision: Term - 2 >> Changes Around Us Science SSLC (English Medium) Class 6 Tamil Nadu Board of Secondary Education

Changes which take place over a long period of time (hours/days/months/years) are known as slow changes.

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

Chemical change:
(i) Burning of wood.
(ii) Breathing.

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.

Example of physical change:

1. Change of water into its vapours.
2. Heating of iron rod.

The triple point of water is that point where water in a solid, liquid and gas state co-exists in equilibrium and this occurs only at a unique temperature and a pressure.

The process of change from one state to another at a constant temperature is called the change of phase.

OR

A transition from one state of matter (solid, liquid, or gas) to another is called change of state.

Sublimation is the process in which a solid changes directly into a gas on heating, without passing through the liquid state.

It can be defined as the fixed temperature at which a solid start changing to its liquid state is Melting Point

Heat energy is absorbed by a solid during melting and an equal amount of heat energy ‘ is liberated by the liquid during freezing, without any temperature change. It is called the latent heat of fusion.

Melting or fusion: The process in which a solid is converted to a liquid by absorbing heat is called melting or fusion.

The physical process that results in the transition of a substance from the solid-state to the liquid state on heating at a fixed temperature is called Melting.

The particular temperature at which vaporisation occurs is called the boiling point of liquid.

The change from vapour to liquid phase on liberation of heat at a constant temperature is called condensation (or liquefaction) and the particular temperature at which the condensation occurs is called the condensation point of vapour.

The change from liquid to gas (or vapour) phase on absorption of heat at a constant temperature, is called vaporisation.

The change of a solid substance directly into a gas or vapour without first changing into a liquid is called sublimation.

OR

The change from solid state to vapour state without passing through the liquid state is called sublimation, and the substance is said to sublime.

The change of state from solid to gas directly is called sublimation.

A homogenous mixture of a solute in a solvent is called a solution.

A solvent is a medium in which a solute dissolves. It is in large quantity in a solution. Water is the most common solvent. The other solvents are alcohol, carbon tetrachloride, etc.

Example: Water

The substance which dissolves or disappears in the solvent, i.e., liquid, to form a solution is called a solute. E.g., sodium chloride.

A solute is a substance that dissolves in a medium, which can be water or any other substance. A solute is in a smaller quantity in a solution. e.g., sodium chloride.

• A change of state occurs when heat exchange causes a substance to transition between solid, liquid, and gas phases.
• Temperature remains constant during a phase change because heat energy changes molecular arrangement (potential energy), not molecular speed (kinetic energy).
• The heating curve has flat plateaus at the melting point (0 °C) and boiling point (100 °C) for water, with rising slopes in between.