Physical changes - class-VI

Cutting a log to wood into piece does not change the composition of wood hence it is not a chemical change.

Physical properties $-$ Shape, state of the substance size etc.

Heating an iron rod causes it to glow red due to thermal radiation, but the iron remains iron. No new chemical substance is formed, so it is a physical change.

In a physical change, the substances are not altered chemically. No new products are formed. 
A physical change can affect the size, shape or color of a substance but does not affect its composition. The substances may be changed to another phase (i.e. gas, liquid, solid) or separated or combined so physical changes are temporary.

1. A physical change is temporary and is usually reversible.
   2) No new substances are formed during a physical change.
   3) There is no change in mass of the substance during a physical change.
   4) There is no net absorption or release of energy during a physical change.

When a solid is heated, the particles are given more energy and start to vibrate faster. At a certain temperature, the particles vibrate so much that their ordered structure breaks down. At this point the solid melts into liquid. This temperature change from solid to liquid is called  the melting point. Each solid has a set melting point at normal air pressure, but at higher altitudes the melting point lowers.

On decreasing temperature, a liquid turns in to solid, this process is called Freezing. Freezing or solidification is a phase transition state.
Most liquids freeze by crystallization. formation of crystalline solid from the uniform liquid. this is a first-order thermodynamic phase transition i.e as long as solid and liquid coexist the temperature of the whole system remains very nearly equal to the melting point due to slow removal of heat when in contact with the air, which is a poor heat conductor.

Sublimation is the transition of a substance directly from the solid to the gas phase without passing through the intermediate liquid phase. Sublimation is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point in its phase diagram. The reverse process of sublimation is desublimation or deposition.

During a physical change, physical properties such as colour, volume, size, shape and magnetic properties etc. of the substances may change.

A change in which only the physical properties of the substance get changed, when the chemical composition remain unchanged is called a physical change. Here we can obtain the sugar by boiling water.

In evaporation of water, only the physical state of water changes from liquid to gas, but not the chemical composition of water.

Magnetization of iron is a physical change and not a chemical change as there is no change of state, no change of temperature, no smell and no evolution of gas. It is because: - No Chemical process is involved. - After demagnetizing the iron it will lose its magnetic power.

Tearing of paper is a physical change because it remains same. Tearing of a paper is a physical change because when the paper is torn only the appearance of the paper is changed and no new substance is formed.

Cheese is made when an enzyme called rennet is added to milk. The rennet causes a chemical reaction where the milk solids curdle, separating themselves from the whey, or liquid portion of the milk

Change in only physical properties is known as physical change.

Melting of ice is a physical change in which substance changes its physical state but not properties.

Properties such as shape, size, colour and state of a substance are called its physical properties.A physical change is generally reversible. In such a change no new substance is formed.

Change in the state between solid, liquid and gas is a physical change because Properties such as shape, size, colour and state of a substance are called its physical properties which are reversible.

The process of obtaining salt by the evaporation of sea water is a process of crystallization which is a reversible process and a physical change, these crystals will convert again in salt solution (seawater) by dissolving them in water.

Solution:- (B) Melting of steel

Making an ice cube is a phase change from liquid to solid, which is a classic example of a physical change. The other options are incorrectly classified (e.g., folding paper is physical, breaking glass is physical).

False

When magnesium is burnt, it reacts with the heat and gives bright flames and this change cannot be reversed.
When iron heated strongly it turns red. When heating is stopped it cools and get backs to its original form. So this is a physical change.

When salt (solid crystal) is dissolved in water energy is required to break the intermolecular attraction between ions (Lattice energy). Hence the energy is collected from water which leads to a decrease in its temperature.

Adding sodium chloride to water is a physical change as the sodium chloride can be separated from water by evaporation.

Candle is made up of wax which melts on heating.When candle kept in a vessel and heated, the wax melts. When candle is allowed to cool, the wax solidifies. No new substance is formed. So, this is a physical change.

Physical changes in a substance do not form new substances, these reactions are reversible and the change is temporary.

Kerosene is more volatile than water and oil, meaning it evaporates more readily at room temperature due to weaker intermolecular forces between its molecules. While water does evaporate at room temperature (that's why puddles dry), kerosene has a higher vapor pressure and evaporation rate. Oil has the strongest intermolecular forces and evaporates very slowly. Ammonium chloride sublimates (solid to gas) rather than evaporates (liquid to gas).

Dew drops are formed due to condensation of water vapors. The air around us contains water vapors which we call moisture or humidity. Hot air contains more moisture as compared to cold air. During the night when the hot air comes into contact with some cold surface, water vapor present in it condenses on the cold surface in the form of droplets. These tiny drops of water are called dew drops.

Dissolution of carbon dioxide gas in water is both a physical and chemical change.
Carbon dioxide dissolves in water producing carbonic acid, also known as soda water.
$ CO _2 + H _2O\rightarrow H _2CO _3$
There will be an equilibrium constant that will dictate the relative concentration of dissolved gas to the corresponding acid that is formed.
Usually nonmetal oxides like$CO _2, SO _2, NO _2$  simply dissolve in water, depending on the partial pressure above the water, and Henrys law constant. This implies a physical change.

No new substances are formed. In a burning candle, there are both physical and chemical changes. The melting of the solid wax to form liquid wax and the evaporation of liquid wax to form wax vapour are physical changes. The burning of the wax vapour is a chemical change.

A physical change is any change in matter that involves the substance going from one physical state to another. 

The reference to a physical state involves solids, liquids, and gases. When a substance undergoes a physical change it does not lose its original properties because no new formation takes place when changing from one phase to another. The types of physical changes can vary. A substance can go from a solid to a liquid, a liquid to a gas, a gas to a liquid, a liquid to a solid, a solid to a gas, or a gas to a solid.

Energy flows always from a higher temperature to lower temperature. In this case there is no temperature  difference hence energy would not flow in any way.

Dissolving salt in water and then distillation
Process to separate sand and salt from the mixture :
Step 1: Add water to the mixture and stir until the salt dissolve.
Step 2: Turn your burner on and then keep the mixture over it.
Step 3: Pour the water mixture through a very fine strainer or cheese cloth filter . (Sand will remain in filter, salt and water will pass through it)
Step 4: Pour the water and salt mixture back into the burner . And allow the water to boil away and collect salt from there.

A change in which only the physical properties of the substance gets changed, while its chemical composition remains unchanged, is called a physical change. Crystallisation of substance and Demagnetisation of magnets comes under this change.
A change in which chemical composition and chemical properties of the reacting substances undergo a change is called a chemical change.
Curdling of milk, Digestion of food, Rusting of iron etc. comes under chemical change.

Physical change would not be in the components of a mixture by Filtering.
Filtering just separates solids from liquids. It does not involve any phase change.

In the given example, water only changes its state by the change in temperature, but its properties remain same hence it is an example of physical change not a chemical change.

Yes, melting of butter is a physical change because the butter can be solidified again (i.e. reversible change).

Boiling water is a physical change because the gaseous water produced is chemically identical to the liquid water i.e both of them have the same molecular structure of the water.

When an electric bulb glows a physical change is observed. As the light produced by the electric bulb is due to the heating effect of the conductor. Even though there is an occurrence of temperature change since no new substances are produced it is considered to be a physical change.

When two solutions are mixed and no observable changes like evolution of gas or precipitate formation takes place then the reaction is reversible and the reaction is a physical reaction.

Melting point is defined as the temperature at which solid is converted to liquid.

When two colourless solutions react to form a cloudy precipitate which settles at bottom of a solution then the reaction is said to be precipitation reaction.

No change in temperature $-$ neither exothermic  or endothermic reaction.

Sugar dissolves in water is a physical change because in this change no new substance is formed and the process is reversible in which by vaporization followed by condensation and crystallization water and sugar can be separated.

In general, the metallic substance expands on heating and contract on cooling.

If the substance is expanded on melting, the melting point increases when the pressure is increased, because in increased pressure substance try to be in a state where volume decreases.

When water between rocks is freezing its volume increases due to which rock can break into pieces.

Since ionic compound has the much higher boiling point than a molecular compound. This is due to stronger intermolecular attraction.

A filter is a substance added to make a material less expensive or for changing its physical properties.