Physical change is a temporary change that occurs in the physical state of matter. It does not form any new substances. These changes are reversible and require little energy absorption. An example of physical change is the melting of ice into water. It is a temporary change that can reverse with time. The water can turn back into ice or evaporate as gas. Therefore, physical change only affects the physical properties of the substance. The mass of the substance remains unchanged.

Chemical Change

Chemical changes alter the composition of substances permanently. These are irreversible changes that occur inside the atoms and molecules. Chemical changes in substances involve the release and absorption of energy. An example of chemical change is when paper or thermocol burns. The paper or thermocol turns to ashes and cannot return to its original form. The ash cannot turn back to the actual substance. During the process of chemical change, the mass of the concerned substance changes as the molecular structure inside changes.

Process of Chemical Change

Chemical changes are also known as chemical reactions. These follow a complicated process. Chemical changes occur when two or more substances combine with each other to form a new substance. This process either releases or absorbs energy. Energy increases kinetic force, which causes the atoms to move faster. Subsequently, the bond between molecules breaks. Thus, the chemical composition of substances changes. This change is irreversible.

 

 

Rusting of iron is a chemical change that weakens the Iron. Rusting slowly degrades the Iron, which is an essential requirement for construction of buildings. Therefore, rusting will also affect the construction if preventive measures are not taken. The process of rusting involves Iron (Fe), Oxygen (O2) and Water (H2O). These components together form rust or iron oxide( Fe2O3). Rusting requires all three factors, or it will not take place. If the moisture quantity is excessive, the Iron will rust sooner. This reaction happens because moisture is an essential component in the rusting process.

Prevention of Rusting

Since rusting is so harmful to Iron and all things made out of Iron, it must be prevented. Some methods that help in preventing Iron from rusting are the following: 

1. Apply a covering: The main reason rusting happens is that Iron comes in contact with oxygen and moisture or water vapour. Applying a covering or a shield will prevent the rusting as it will block the oxygen and moisture in the air from coming in contact with Iron. This method will also keep the iron sturdy and prevent it from deteriorating.
2. Paint: Painting the metal is another effective way to prevent rusting. The paint coat can prevent moisture and oxygen from coming in contact with the Iron. Without water vapour, rusting will not take place. This way, painting helps in slowing down the process of rusting.
3. Applying oil: Applying oil is yet another effective method of preventing rusting. Oil lubricates the iron surface and enables the parts to move with less friction. Consequently, the iron parts move faster. Water and oil do not mix. Therefore, oil prevents water from entering the iron surface. This way, Iron doesn’t rust. 
4. Bluing: Bluing is another good way to prevent rusting. Bluing is the controlled oxidation of ferrous metals having similar properties to rust. The process requires a high temperature and a salt solution. In the end, it produces a magnetite or black iron oxide layer. This layer transforms the metal into blue from black and makes it thicker. Therefore, in this way, bluing prevents rusting from taking place. Although it is an effective method, it only works for a short time and does not produce long-term results. Bluing also prevents corrosion in metals.
5. Galvanising: Galvanising involves zinc that forms a barrier on the iron surface. Zinc is an active metal that forms a preventive coating on iron. It prevents oxygen and moisture from coming in contact with the metal. Zinc does not undergo corrosion. Hence it prevents rust. In places like the roofs of houses, the Zinc Hydroxide reacts with carbon dioxide and forms a layer of Zinc Carbonate. Zinc Carbonate is impermeable and creates a shield against oxygen and moisture. This way, galvanising helps in preventing the process of rusting and protecting iron.

Crystallisation is a natural process of obtaining large crystals of pure substances from solutions. The method of evaporation plays an essential role in Crystallisation. Evaporation occurs when the liquid water converts to gas moisture or water vapour. In Crystallisation, the solution evaporates and leaves behind the crystals. Crystallisation is a purification technique where the molecules of the substances arrange themselves in a compact state. It also reduces the energy of the entire system. Both Crystallisation and Evaporation are examples of physical change because they don’t produce any new substances, but enhance the original form. The reactions are easily reversible.

1. A candle undergoes both physical and chemical changes. Explain

Physical changes are temporary and are reversible. On the other hand, in chemical change, the substance changes completely. When a candle burns, it undergoes both physical and chemical changes. With the burning of the candle, the wax starts to melt. The melting of the wax is a physical change as the candle transforms physically. The burning of the candle also brings out emissions of carbon dioxide. This emission of carbon dioxide is a chemical change.

2. How is making curd a chemical change?

A chemical change is a permanent change. Once an item transforms with chemical change, it cannot transfer back to its original state. The shape, size, volume and depth of the product changes completely. Curd is made from milk. Milk is a liquid. When people churn out milk, it changes completely and becomes curd. Curd is a solid product that has a sour taste. On the other hand, milk does not have any strong flavour. Once milk changes to curd, it cannot change back to milk again.

3. Why is rusting faster in coastal areas than in deserts?

Rusting of iron requires moisture and oxygen. Without these two components rusting of iron will not be possible. The climate in deserts is quite hot and dry. Deserts barely have any water vapour in the air. Therefore, although oxygen is present in the atmosphere of deserts, moisture is not present. Hence, rusting is not possible. Coastal areas have water bodies. These water bodies provide an abundance of moisture in the air. Both oxygen and moisture are present in the air. Therefore, the rusting process becomes faster. 

4. What is crystallisation? How are crystals of copper sulphate prepared?

Crystallisation is a purification process that includes forming crystals from pure solutions. The structure of the atoms changes and forms crystals through the process of evaporation. To prepare crystals of copper sulphate, take water in a beaker. Then add a few drops of diluted sulphuric acid to it. Heat the water and stir it. Then add the copper sulphate powder to the solution till the solution becomes saturated. Lastly, pour the solution into a china dish. Let it cool; the crystals will form on their own.

Physical and chemical reactions are important in daily life. Therefore, students must learn the chapter properly. This will help them understand the practical applications of physical and chemical changes and the logic behind the frequent changes in nature. MSVGo is an online learning platform that provides all the study materials of CBSE and ICSE subjects. It also offers experienced faculty members to teach the students at their own pace. So, if you are looking for a platform to guide you through this chapter, join MSVGo. Download the MSVGo app to access exclusive content related to this chapter now.