Physical and Chemical Change

The changes occur in our environment is classified in two type mainly called physical change and chemical change.

Physical change

A physical change is a type of change in which the form of matter is altered but one substance is not transformed into another. It can be understand by following example:

1. The mixing of sugar into water

In the solution of sugar and water, the molecule of sugar adjusted in between the water molecule and hence water become sweet. So here no new substance is formed. Hence it is defined as physical change

2. The formation of ice from water

In the formation of ice from water, the molecule of water get cooled, and releases heat. Due to this it got condensed and form solid called ice. In this process also no new substance are formed so it is also defines as a physical change.

3. Mixture of two colour

Mixture of any two colour gives a new colour, so why it is a physical change? When we mix two colour the shade of the colour changes so that we found a new shade but in actual property of those colour does not change, hence we conclude it as a physical change.

Now as we studied about property in our last example, so we can say that when certain property of substance does not change, it is concluded to physical change. The property corresponding to physical change is termed as physical property. The property such as shape size or state etc. are come under this type of property.

Crystallization

Crystallization is defined as the process of formation of solid crystal. It leads to the formation of crystal structure. It is also a physical change.

Chemical change

A change in one or more new substance is formed with different properties is defined as chemical change. To better understand the chemical change lets discuss some example as follows:

1. Digestion of food in our body

When we eat food, the vegetable or anything we eat decomposes to their elementary form. For example caobohydrate present in sugar, so when we eat sugar it decomposes and form glucose. Similarly fats are decomposes to fatty acids and protein is decomposes to amino acid. So as wee can see that one substances formed havig very different properties that the last one so it is considered as chemical reaction.

2. Ripening of a food

The food we eat, get ripped after some time at room temperature. This is happened because the oxygen present in air oxidised or react with the food and ripe it. So this is an example of chemical reaction as the inhibit nature or property of food get change or in science language we can say that the chemical formula is changed

3. Formation of curd

Milk is converted into curd by the process of fermentation. Milk consists of globular proteins called casein. The curd forms because of the chemical reaction between the lactic acid bacteria and casein. In this reaction liquid milk is converted to semi solid curd. Lactic acid bacteria form the casein chain. Hence the new product is formed.

Chemical reaction: it is the process in which new substance with new properties are formed. Chemical reaction involves chemical changes

Chemical equation: it is defined as the method of representation of a chemical reaction with the help of symbol and formulae of the substance involved in it.

To better understand these terms, let take example, in the burning of crackers, potassium nitrate, charcoal and sulphur are ignited which produce potassium carbonate, potassium sulphate, potassium sulphide is formed with release of carbon dioxide and carbon monoxide. This is a chemical reaction we can define chemical reaction in statement the chemical equation for the above reaction is as follows:

6 KNO₃ + C₇H₄O + 2 S → K₂CO₃ + K₂SO₄ + K₂S + 4 CO₂ + 2 CO + 2 H₂O + 3 N₂

So, above we studied some common example of chemical reaction. Hence, we know that some new product with new properties substance is formed after chemical reaction.

Characteristics of chemical reaction:

1. Change in temperature: some chemical reaction is characterised by a change in temperature. For example, when quick lime react with water, then slaked lime is formed and a lot of heat energy is produced. This can be explained as chemical reaction:

Ca + H₂O à CaO

Then this slaked lime is reacted with carbon dioxide in air and form a calcium carbonate. Calcium carbonate is shinny appearance.

CaO + CO₂ à CaCO₃

Calcium carbonate is used in white washing. It is in a white powder form when water is added it release a heat so the temperature is increased without applying any external heat. So semi solid calcium hydroxide applied to the walls and then calcium hydroxide reacts with carbon dioxide and form solid calcium carbonate and shine appear on wall after white wash.

2. Change in colour: some chemical reaction is characterized by a change in colour. For example, when iron nail is placed in copper sulphate solution which is

blue in colour. Then after an hour or later the blue solution turned into green colour iron sulphate and the brown deposit is obtained on the nail which is copper. So, in this reaction we observed that the change of colour on formation of new substance.

Fe + CuSO4(aq) àFeSO4 + Cu

3. Change in state: some chemical reaction are characterised by a change in state. For example, the burning of magnesium in air to form magnesium oxide. In this reaction magnesium strip is burned in presence of air then gives a white powder of magnesium oxide.

Mg(s)+CO2→2MgO(s)+C

4. Evolution of gas: some chemical reaction is characterised by the evolution of a gas. For example, when baking soda is added to vinegar the gas get evolved through this process. So, this can be written as

NaHCO₃(s) + CH₃COOH(l) → CO₂(g) + H₂O(l) + Na⁺(aq) + CH₃COO^-(aq)

Here, NaHCO₃ is the chemical formula for baking soda and CH₃COOH is chemical formula for acetic acid. When we add these two CO₂ gas releases.

5. Formation of precipitate: some chemical reaction are cherecterzied by the formation of precipitate. For example, when potassium iodide solution is added to the solution of lead nitrate , then a yellow precipitate of lead iodide is precipitates formed.

2KI + Pb(NO₃)₂ à PbI₂ + 2KNO₃

Here, precipitate is solid material left settled in the bottom of solution.

Now we had done with the ways to understand the chemical reaction. Now we study about the chemical equation of iron with air and water(moisture) it called rusting. Now we study what is the cause of rusting and how can we prevent it.

Rust

Rust is the soft and porous substance which gradually falls off from the surface of an iron object, and then the iron below starts rusting. Thus the rusting of iron is a continuous process which if not prevented in time, eats up the whole iron object. The process through which rust happens is called rusting. So, rusting is the process in which iron metal are eaten up gradually by the action of air, moisture or chemical (such as acid) on their surface. The chemical equation of the process of rusting is as follows:

4Fe + 3O₂ + 2xH₂O à 2Fe₂O₃.xH₂O

The number of water molecules (x) in the rust varies, it is not fixed. For rusting air and moisture both needed, without moisuture rusting does not take place. Rusting is the unwanted reaction occur in nature it has many losses as it weakens the iron and steel objects and structures such as railing, car bodies, bridges, and ships etc; and make their life short. A lot of money has to be spent every year to prevent the corrosion of iron and to replace the damaged iron structures. There are many preventions taken to prevent the rusting. Some of them are as follows:

1. Rusting of iron can be prevented by painting: the most common method of preventing the rusting of iron is to coat its surface with paint. When a coat of paint applied to the surface of an iron object, then air and moisture cannot come in contact with the iron object and hence no rusting take place. The iron article such as window grills, railing, furniture, iron pipes, iron bridges, railway coaches, ships and bodies of car, busses, and trucks etc; are all painted to protect them from rusting.

2. Rusting of iron can be prevented by applying grease or oil: when some grease or oil: when some grease oil is applied to the surface of an iron object, then air and moisture cannot come in contact with it and hence rusting is prevented. For example, the tools and machine parts made of iron are smeared with grease or oil to prevent their rusting

3. Rusting of iron can be prevented by galvanisation: the process of depositing a thin layer of zinc metal on iron object is called galvanisation. Galvanisation is done by dipping an iron object in molten zinc metal. A thin layer zinc metal is then formed all over the iron object. This thin layer of zinc metal on the surface of iron object protects them from rusting because zinc metal does not corrode(rust)on exposure to air. The iron sheet used to make bucket, drums, dust-bin and sheds (roof)are galvanised to prevent their rusting. The iron pipes used for water supply are also galvanised to prevent rusting.

4. Rusting of iron can be prevented by alloying it to make stainless steel: when iron is alloyed with chromium and nickel, then stain steel is obtained. Stainless steel does not rust at all. Cooking utensils, knives, scissors, and surgical instrument, etc; are made of stainless steel. But stainless steel is too expensive to be used in large amounts.