Class 7 Science Chapter 5 Question Answers - Chemical And Chemical Changes

Short Types Questions

A chemical substance is a material that has constant composition and distinct properties.

Q.2. What are chemical symbols?

Chemical symbols are shorthand notations used to represent elements in the periodic table.

Q.3. Define chemical equation.

A chemical equation is a symbolic representation of a chemical reaction, showing the reactants and products.

Q.4. What is the difference between physical and chemical changes?

Physical changes are changes in which the state or appearance of a substance changes, but its chemical composition remains the same. Chemical changes, on the other hand, involve a change in the composition of a substance, resulting in the formation of new substances.

Q.5. What is crystallization?

Crystallization is the process of forming a solid crystal from a solution or a melt, in which the atoms or molecules arrange themselves in a highly ordered, repeating pattern.

Long Types Questions

Chemical substances are the materials that have a definite composition and distinct properties. Chemical symbols are the shorthand notation used to represent chemical elements. The importance of chemical symbols lies in their ability to represent chemical substances in a concise and standard manner. For example, the chemical symbol for hydrogen is H and for oxygen is O. The combination of these two elements results in the formation of water, which is represented by the chemical formula H₂O. This formula indicates the presence of two hydrogen atoms and one oxygen atom in the water molecule.

Q.2. Differentiate between physical changes and chemical changes. Provide examples to support your answer.

Physical changes are the changes that do not alter the chemical composition of a substance. Examples of physical changes include changes in state, such as melting, boiling, and freezing. Chemical changes, on the other hand, involve the formation of new substances with different chemical properties. Examples of chemical changes include rusting of iron, burning of wood, and digestion of food. In a chemical change, the original substance undergoes a chemical reaction to form a new substance with different properties.

Q.3. What is crystallization? Explain the process of crystallization with an example.

Crystallization is the process of forming crystals from a solution or a melt. It is a physical process that involves the formation of a solid crystal from a liquid or a gas. The process of crystallization involves the slow cooling of a hot solution or the evaporation of a solvent from a solution. As the solvent evaporates or cools, the concentration of the solute increases, and it starts to form crystals. An example of crystallization is the formation of sugar crystals from a hot sugar solution. When the sugar solution cools down, the sugar molecules start to solidify and form crystals.

Q.4. Define chemical formulae and chemical equations. Explain the importance of balancing chemical equations with an example.

Chemical formulae are the symbolic representation of the chemical composition of a substance. Chemical equations are the symbolic representation of a chemical reaction. The importance of balancing chemical equations lies in the conservation of mass law, which states that the total mass of the reactants is equal to the total mass of the products in a chemical reaction. Balancing chemical equations ensures that this law is followed. For example, the chemical equation for the reaction between hydrogen and oxygen to form water is:

2H₂ + O₂ → 2H₂O. This equation is balanced, as there are equal numbers of atoms of each element on both sides of the equation.

Q.5. Explain the concept of chemical changes with an example. Describe the different types of chemical changes that can occur in a substance.

Chemical changes involve the formation of new substances with different chemical properties. An example of a chemical change is the reaction between iron and sulfur to form iron sulfide. This reaction involves the exchange of atoms between the reactants to form a new substance with different chemical properties. There are different types of chemical changes that can occur in a substance, including combustion, oxidation, reduction, and precipitation. Combustion is a chemical change that involves the reaction of a substance with oxygen to produce heat and light. Oxidation is the process of adding oxygen to a substance, while reduction is the process of removing oxygen from a substance. Precipitation is the process of forming a solid from a solution due to a chemical reaction.