The application of energy brings changes in matter. In our everyday lives, we see substances undergoing changes. Let us see few examples from our daily life, sun heat the snowy mountains, which melts and changes into water sources like rivers, lakes and ponds, fire changes the raw vegetables/meat to cooked food, the river on heating evaporates into water vapor, that condense in the upper atmosphere and change into clouds, burning of fuel, making lemonade. These all show the changes taking place in a substance. We can categorize these changes into two types: Physical change and chemical change.
Physical Change
Physical properties of a substance include appearance and observable properties. Some physical property are color, odor, taste, solubility, melting and boiling points, rigidity etc.
In physical change form of matter is altered but its chemical composition remains the same. In other words, no new substance is formed in physical change.
Example:
- Take some water in a china dish and mix some salt into it. Taste the solution. You will find it salty. Now heat the dish till all water evaporates. Taste the white residue left behind. You will find that the white residue is a common salt. This proves that no new substance is formed by dissolving salt in water and it is a physical change.
- Breaking of chalk.
- Tearing of paper.
- Evaporation or freezing of water.
- The magnetization of iron bar.
- Stretching of a rubber band.
Characteristics of a physical change
- Generally, a physical change is temporary and can be reversed back by changing the condition.
- No new substance formed hence no change in the mass of a substance undergoing physical change.
- Only physical properties of a substance changes like its size, color, state or shape.
Chemical Change
A chemical change is a permanent change in which the original substance loses its own composition and properties. During this change one or more new substance is formed with different composition and properties.
Example:
- Burning of paper produce new substances like ash, smoke, carbon dioxide and water vapour.The molecules of paper in presence of oxygen in air combine and undergo changes to form molecule of these new substances. The change here is permanent and cannot be reversed, thus burning of paper is a chemical change.
- Fermentation.
- Ripening of fruits.
- Rusting of iron.
Characteristics of chemical change
- Chemical change is permanent and irreversible.
- In a chemical change one or more new substance is formed with composition and properties different from the original substance.
- Mass of the substance that undergoes chemical change is altered however the total mass involved in a chemical change remains the same(mass is neither created nor destroyed).
Question: Is making mixed fruit smoothie using a blender a physical change or chemical change?
Answer: It is a physical change as the shape and size of the fruit pieces are changed but the chemical component still remains unchanged.
Chemical Reactions
A chemical change is also called a chemical reaction. A chemical reaction is the change of a substance into a new one that has a different chemical identity. Chemical reactions release or absorb heat or other energy or may produce a gas, odor, color or sound. If you don't see any of these indications, a physical change likely occurred. The substances that react with each other in a reaction are called reactants and the new substances that are produced by the reaction are called products.
Below are two chemical reactions. (1) The reaction of hydrogen with oxygen produces water.Hydrogen and oxygen are the two reactants and Water is the product (2) The reaction of carbon with oxygen to produce carbon dioxide. Carbon and Oxygen are two reactants and Carbon dioxide is the product.
During chemical change or chemical reaction, atoms in the molecules of the reactants rearrange themselves to form one or more products. Chemical Equations is used to represent chemical reaction symbolically.
When the chemical reaction is represented using symbols and formulae for the reactants and products involved in the reaction then it is called Chemical Equation. Example: chemical equation for carbon reacting with oxygen to give Carbon dioxide.
C + O2 —> CO2
Conditions required for Chemical Reaction to happen:
- Surface Area: The rate of a chemical reaction is slow if the surface area of the reactants is small as there will be less chance of contact between the reactants. If the surface area is big then the rate of reaction increases. For example, in lab powdered form of calcium carbonate reacts much faster with dilute hydrochloric acid than the lumps of limestone.
- Catalyst: A catalyst speeds up a reaction hence it is added in a reaction to increase the reaction rate without getting consumed in the process. For example, enzymes in our body act as catalysts as they speed up the rate of chemical reactions in a cell or outside a cell.
- Pressure: Some chemical reactions occur only after applying pressure. For example, in the manufacture of ammonia in the Haber process the rate of reaction between the nitrogen and hydrogen is increased by the use of very high pressure.
- Heat: Various reaction takes place only under a certain temperature. When heat is supplied to the reactants then they go under reaction. We use a burner or a hot plate in the laboratory to increase the speed of reactions that react slowly at room temperature. In many cases, an increase in temperature of only 10°C will approximately double the rate of a reaction.
- Light: Light is also one of the factors that stimulate the rate of reaction, also there are some reactions that undergoes only in the presence of light. The best example here is photosynthesis. A chemical reaction that starts with light being absorbed as a form of energy is called photochemical reaction.
