Comparison between physical and chemical changes - class-VII

Melting of wax is a physical change. In a burning candle there are both physical and chemical changes. The burning of the solid wax to form liquid wax and the evaporation of liquid wax to form wax vapour are physical changes. The burning of wax vapour is a physical change.

All of the following represents a physical change.
Stretched rubber band.
Melting of ice cream.
In winter, hair oil freezes.

When a metal rod is heated, it expands. During expansion, the length of metal rod increases.

All of the following represents a chemical change.

A physical change affects only the physical properties (like state or shape) of matter, while the chemical composition remains unchanged.

No new substances are formed in all these three changes. So these are physical change. 

Zinc oxide is a white powder, when heated strongly (above $800^0 C$) it turns yellow, but again becomes white again when cold. The hot zinc oxide absorbs some of the blue wavelengths of light making the reflected light more yellow. The loss of oxygen makes it a reversible chemical change.

Iron is magnetic at room temperature, but at 770 degrees Celsius iron loses its magnetism. However this change is temporary, hence it is an example of physical change.

and the change in color is a physical change because it is easily reversible without affecting the chemical nature of it.

In physical change properties of molecules remain the same even after the change. Physical properties like colour, density, volume, mass, boiling point, melting point only have an effect on their physical nature, the chemical nature of the substance remains the same. Physical properties are used to observe and describe matter.

Physical change does not lead to formation of new substances but only chemical changes can produce new substances.

Changes are of two kinds: Physical and chemical changes.
A physical change in a substance does not change what the substance is.
Where as in a chemical change, where there is a chemical reaction, anew substance is formed and energy is either given off or absorbed.

Physical changes require less energy than chemical energies, as they does not involve in making and breaking of bonds as in the case of chemical bonds, which require a lot of energy.

Dissolving sugar in water is a physical change because sugar molecules are dispersed within the water, but the individual sugar molecules are unchanged.

Since the law of conservation of mass is valid under all circumstances, hence, mass always remains the same, whether a substance undergoes physical change or chemical change. 

A plant collecting sunlight and turning it into food is a photosynthesis reaction which is an example of a photochemical reaction. This is a chemical change. 

A change in temperature is characteristic of a chemical change. When the reactants are mixed, the temperature change caused by the reaction is an indicator of a chemical change. 

Solid ammonium chloride vapourizes to form gaseous ammonium chloride. This is a physical change.
Ammonium chloride is partially dissociated. This a chemical change.
The products ammonia and chlorine recombine to form ammonium chloride. This is also a chemical change.

On burning candle, wax melts which is a physical change. Also, the wax near the flame burns and gives $CO _2$, soot, light and heat which is a chemical change. Hence, burning of candle is a physical as well as chemical change.

Physical change does not involve formation of new products. As cutting of trees do not produce anything it is said to be a physical change.

The difference between a physical reaction and a chemical reaction is composition. In a chemical reaction, there is a change in the composition of the substances. In a physical change, there is a difference in the appearance, smell, or simple display of a sample of matter without a change in composition. 

The texture of a substance can differ with a physical change. For example, if a piece of wood was sanded, waxed, and polished, it would have a very different texture than it initially had like a rough piece of wood. Although, we cannot see temperature change, unless if a change of state is occurring, so it is a physical change. 

All of the following involve a chemical change at a particular temperature except the solidification of vegetable oil at low temperatures.
It involves physical change. The physical state changes from liquid to solid.

$(I)$  ${ H } _{ 2 }{ O } _{ \left( l \right)  }\longrightarrow { H } _{ 2 }{ O } _{ \left( g \right)  }$   physical change (reversible)

Formation of gas not a physical change because some reactions has taken place which leads to the formation of gas hence, it is a chemical change.

$Ca{ (OH) } _{ 2 }+C{ O } _{ 2 }\rightarrow CaC{ O } _{ 3 }+{ H } _{ 2 }O$

$(A)$ Melting of ice is reversible, $\therefore $  physical change also no change in chemical composition.

1. If pressure is applied, according to clausices-ceapeyron equation, the melting point will increase for liquids and decrease for water.
   2. Any external changes that try to take it out of equilibrium will came the system to adjust in a way to counter act that change.

Covalent bonds have higher bond energy (ie, they are strong) compared to Hydrogen bondings.