Changes occur around us all the time. Milk turns sour, seed germinate, the weather changes and fruit ripen.
Changes can be of different types: desirable and undesirable changes, slow and fast changes, temporary and permanent changes, reversible and Irreversible changes and physical and chemical changes.
All these changes involve one or more substances. Each substance has a unique composition.
Physical changes: When a substance undergoes a physical change, it does not give rise to a different kind of substance or there is no new substance is formed. A physical change is a reversible change.
For example; heating of water melting of butter, cutting of paper etc.
Chemical changes: A Chemical change occurs when two or more chemical substances react to form new product that have an entirely different set of properties. A chemical change is mostly Irreversible. For example; cooking food, ripening of fruit, lighting a matchstick, aging of leaves and digestion of food etc.
Chemical reaction:
A chemical change occurs when chemicals react with each other. Thus, a chemical change is also called a chemical reaction.
Some of the properties of a chemical change or chemical reaction are:
Change in energy
Change in color
Evolution of gas
Formation of precipitate
Change in energy: Almost all chemical reactions involved energy change. Some chemical reactions produce heat while some take away heat.
A change or reaction in which heat is released is called an exothermic reaction.
For example; In a reaction between carbon and oxygen, carbon dioxide is formed with the generation of heat.
Carbon + Oxygen ……. Carbon dioxide + Heat
A change or reaction in which heat is absorbed from the surrounding is called an endothermic reaction.
For example in a reaction between carbon and sulphur, carbon disulphide is formed with the absorption of heat.
Carbon + Sulphur + Heat …….. Carbon disulphide
Change in colour: The colour of the product may differ from that of the reactants.
For example; copper sulphate is blue in colour, but after reacting with iron, the product formed ( iron sulphate) is of green colour.
Evolution of gas: Some chemical changes also produce gases. Generally the gases produced can be carbon dioxide, hydrogen, ammonia etc. The presence of carbon dioxide can be confirmed as it turns lime water milky. Activity: Reaction between vinegar and baking soda:
Take vinegar (acetic acid) in a glass beaker and add a pinch of baking soda (sodium hydrogen carbonate) to it. We will observe gas bubbles coming out of the beaker. This is due to the release of carbon dioxide. This can be confirmed by making the gas produced pass through freshly prepared lime water. The lime water turns milky when carbon dioxide is passed through it due to the formation of calcium carbonate.
Carbon dioxide (CO2) + Lime water [Ca(OH)2 ] -> Calcium Carbonate (CaCO3) + Water (H2 O) Formation of precipitate: In some chemical changes, the product formed separates from the solution and form a different mass of layer. This insoluble solid is called a precipitate.
For example; in a reaction between copper sulphate and hydrogen sulphide, copper sulphide and sulfuric acid are formed. The copper sulphide formed is given out as a precipitate.
To understand chemical reactions, let us learn about elements, molecules and compounds.
Elements: Substances that consists of only one type of atoms and cannot be broken down into simple substances either physically or chemically are known as elements.
Elements can be classified into two broad groups- metals and nonmetals.
For example: Iron, copper, oxygen and carbon.
Compounds: Substances in which two or more elements are combined together chemically in definite proportions are known as compounds.
For example; two atoms of hydrogen and one atom of oxygen combine in a definite proportion to form a molecule of water.
Mixtures: Substances in which two or more substances are mixed together physically in no definite proportion are known as mixtures.
For example; sugar solution.
Chemical symbols:
The representation of an element using the short form or abbreviation of its name is known as its chemical symbol. The name is based on English name or Latin name. For example; the name Copper and its symbol 'Cu' came from the Latin word cuprum.
All the names and symbols of the elements are approved by the International Union of Pure and Applied Chemistry (IUPAC).
Chemical formula:
The symbolic representation of the composition of a chemical compound is known as its chemical formula. For example; the chemical formula of sodium chloride is NaCl.
Valency:
The combining capacity of the atom of an element with the atom of other elements is known as its valency.
Valencies of Some common elements are given below Chemical equations:
A Chemical change resulting in the formation of one or more new substances is called a chemical reaction. The representation of a chemical reaction using symbols and formulae of the elements or compounds involved in the reaction is known as chemical equation.
The chemical substances that participate in a chemical reaction are known as reactants.
The new substances formed due to a chemical reaction are known as products.
For example; Consider the reaction of magnesium with oxygen. Magnesium reacts with oxygen to form magnesium oxide.
$\text{Magnesium} + \text{oxygen} \rightarrow \text {Magnesium oxide}$
$2\text{Mg} + O_2 \rightarrow 2MgO$
Types of chemical reactions:
Chemical reactions are classified in the following types:
Combination reaction
Decomposition reaction
Single displacement reaction
Double displacement reaction
Synthesis or Combination reaction:
In this type of reaction, two or more substances combine to form one new substance. For example; hydrogen and oxygen react to form water.
$2H_2 + O_2 \rightarrow 2H_2O$
Decomposition reaction:
In this type of chemical reaction, a compound breaks down into two or more substances. For example; silver chloride when exposed in sunlight breaks into silver and chlorine gas.
$2Ag + CL_2 \rightarrow 2AgCl$
Single displacement reaction:
In this, atoms of one element displays the atoms of another element from a compound.
For example; when iron is dipped in copper sulphate solution then iron displaced copper from copper sulphate solution and copper and iron sulphate solution is formed.
$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$
Double displacement reaction:
In this, Atom of two compounds switch places to form two new compounds.
For example; when hydrochloric acid and sodium hydroxide react then two new compounds sodium chloride and water are formed.
$HCl + NaOH \rightarrow NaCl + H_20$
Crystallization:
Some pure substances which are solid at room temperature from crystals. The process of formation of crystals is known as crystallization.
The process of crystallization is an example of a physical change. By obtaining crystals, we are able to purify certain substances. Pure sugar, urea, copper sulphate, alum and table salt form crystals.
The salt obtained by evaporation consists of many impurities such as sand, magnesium chloride etc. The impure salt can be purified by the crystallization process.
For crystallization a large amount of salt is dissolved in water and allowed to boil to obtain a saturated solution.
Filter the saturated solution to remove impurities.
Now, let the solution cool slowly to form Salt crystals Also Read
