Urvashi Yadav answered this
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Divyangana Bhaskar answered this
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Rishabh Tripathi answered this
Chemical changes take place on the molecular level. A chemical change produces a new substance. Examples of chemical changes include combustion (burning), cooking an egg, rusting of an iron pan, and mixing hydrochloric acid and sodium hydroxide to make salt and water.
Physical Changes
Physical changes are concerned with energy and states of matter. A physical change does not produce a new substance. Changes in state or phase (melting, freezing, vaporization, condensation, sublimation) are physical changes. Examples of physical changes include crushing a can, melting an ice cube, and breaking a bottle.
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Vivek Tripathi answered this
Physical changes
When the shape, size, appearance or state, of a substance is altered, but its chemical composition remains same a physical
change of matter takes place. No new substance is formed. It is usually a change, which is reversible, that is, by reversing the
process, the original substance can be obtained.
By using a piece of gold, a goldsmith can make a gold chain and can later alter it to make a gold bracelet, ring or a pair of
earings. All of them consist of the same substance, namely gold. This is an example of a physical change. A more common
everyday example is that of water, which can be converted into solid ice, liquid water, and gaseous water vapour. It can be
reconverted to its previous state by various methods. Yet, in all the three forms, the chemical composition of water is not altered.
No new substance with new chemical properties is formed. In all these, water consists of two atoms of hydrogen and one atom
of oxygen.
Thus, a Physical change is a change in which the chemical composition of the substance is not affected, i.e., no new substance is formed.
Examples of Physical Changes
(a) Dissolving of sugar in water
When crystalline sugar is dissolved in water, it seems to disappear completely, and not even a very powerful microscope can show us the molecules of sugar in water. The solution obtained is uniformly sweet. By evaporating the water the sugar can be recovered in a crystalline form.
(b) Magnetising an iron rod
An iron rod can be converted into a magnet by passing an electric current through an insulated coil wound around it. When the current flows, the iron bar gets magnetised. One end becomes North Pole and the other the South Pole. If the direction of the current is reversed, the North Pole changes to South Pole and the South Pole changes to North Pole.
(c) Glowing of an electric bulb
When an electric current is passed through the filament of a bulb, the filament starts glowing and the glow stops as soon as the current is cut off. The chemical composition of the filament does not alter in either state.
Chemical Change
A change in which the composition of a substance is altered is called as chemical change of matter. As a result, the original
properties get changed and one or more new substances are formed.
Iron is greyish white metal and conducts electricity. It is attracted by a magnet, and reacts with dilute acids to yield hydrogen.
Sulphur is a non-metal, and is yellow in colour. It dissolves in carbon disulphide. When powdered iron and sulphur are heated
together, a completely new substance, iron sulphide is formed. The properties of iron sulphide are entirely different from those of
iron and sulphur. It is black in colour, does not get attracted by a magnet and does not allow electric current to pass through it.
It reacts with dilute acids to from hydrogen sulphide gas. In short, the properties of neither iron nor sulphur are exhibited by iron
sulphide.
Conditions For Chemical Change
The criterion for a chemical change is the production of one or more new substances. Therefore, certain conditions should be
met in order to bring about a chemical change. The necessary conditions are:
1.A minimum amount of energy needed to initiate a reaction, called the activation energy, should be supplied in the form of heat,
light or electric current. In a chemical change the reactants combine to form new products. For this process it is necessary to
break the old bonds of the reactants and forms fresh bonds in order to give new products. This requires a certain amount of
energy.
2. For the occurrence of any reaction, the molecules or atoms of the reactants must collide with one another, in order to break
old bonds and form new bonds.
3. The speed with which the chemical reaction takes place is called the rate of the chemical reaction. This should be
appreciable, to bring about the change.
4. The rate of a reaction depends on following factors:
- Temperature
- Presence of light
- Presence of catalyst
- Electricity
- Pressure.
- Temperature : Certain chemical reactions do not take place at room temperature but occur readily at a higher temperature. Accordingly, heat is required to start the reaction e.g. fuels like coal and wood only start burning when heated to a certain temperature called its ignition temperature.
- Presence of Light : Some reactions take place only in light and do not take place in dark e.g. photosynthesis in green plants or reaction between H2 and Cl2 to form HCl.
- Presence of a Catalyst : A catalyst is a substance that increases the rate of chemical reaction without itself undergoing any change.Example: Hydrogenation of oils to form fats takes place faster, in presence of nickel.
- Electricity : Certain reactions take place with the help of an electric current.Example: Decomposition of acidulated water to give hydrogen and oxygen gas
- Pressure : Some chemical reactions need very high pressure to proceed. For e.g., in the commercial manufacture of ammonia from nitrogen and hydrogen by Haber's process, a pressure of over 200 atmosphere is required, in presence of a catalyst iron and a temperature of 450oC to 500oC.
Examples of Chemical Changes
Below are the examples on chemical changes -
(a) Effect of heat on lead nitrate : Lead nitrate is a white, crystalline solid. When heated, it starts decomposing with a
crackling sound, producing a reddish brown gas called nitrogen dioxide, and a colourless gas, oxygen. A yellow residue of lead
monoxide is left behind in the test tube.
(b) Action of concentrated sulphuric acid on sugar :When concentrated sulphuric acid is added to sugar, it becomes a
black mass of sugar charcoal. The acid removes all the water from the sugar, that is, hydrogen and oxygen and absorbs it
leaving a residue of spongy carbon.
(c) Action of sodium on cold water :
When a small piece (pea size) of sodium is placed in cold water, it darts about on the water with a hissing sound and produces hydrogen. The water left behind, acquires the property of turning red litmus blue. This shows the presence of a basic substance, which is sodium hydroxide
| Physical Change | Chemical Change |
|---|---|
| No new or different substance is formed. The composition of the substance, that undergoes the change, remains unchanged | Results in the formation of at least one new substance. The constituent particles of the new substance are different from the constituent particles of the original substance |
| It is temporary change and in most cases it can be reversed by the reversal of conditions | It is permanent change and cannot be reversed by mere reversal of conditions |
| No change occurs in the mass of the substances undergoing the change | Mass of the individual substances that undergoes the change, always, either increases or decreases. However, the total mass of all the reactants is equal to the total mass of all the products |
Remember :
Sometimes both changes occur together. For example, when a person eats chocolate and then digests it a physical and a
chemical change takes place. When the person chews the chocolate and breaks it into smaller pieces - it is a physical change.
No new substances have been formed yet.
Once acted upon by saliva and other digestive juices, the chocolate is broken down into other simpler substances, which can be
absorbed by the blood. This is a chemical change.
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Rishika Singh answered this
Physical Change | |
1.The chemical composition of a substance does not change. | |
2.Most changes are reversible | |
3.No new substances are formed. For example, .Ice → Water → Steam
Chemical Change 1.The chemical composition of a substance changes. 2.Most changes are irreversible. 3.New substances are formed.For example, 4.Paper → Ashes
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