Chemical Changes and Reactions
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Questions and Answers

What happens to the temperature of the system when heat is evolved in a chemical reaction?

The temperature of the system rises.

Describe the change in temperature when barium hydroxide reacts with ammonium chloride.

The temperature of the system decreases.

What type of reaction occurs when quick lime reacts with water?

An exothermic reaction.

What state change occurs when hydrogen chloride gas is passed over ammonia gas?

<p>Solid ammonium chloride is obtained.</p> Signup and view all the answers

What is the pH of a neutral substance like water?

<p>The pH is 7.</p> Signup and view all the answers

How does the addition of sodium hydroxide affect the pH of hydrochloric acid until all HCl reacts?

<p>The pH of the solution becomes 7.</p> Signup and view all the answers

What kind of chemical reaction involves the breaking of chemical bonds without the formation of new ones?

<p>A physical change.</p> Signup and view all the answers

What happens to the pH of an acidic solution when a base is added?

<p>The pH increases.</p> Signup and view all the answers

Give an example of a reaction where the temperature of the system increases.

<p>CaO(s) + H2O (1) → Ca(OH)2 (aq) + heat</p> Signup and view all the answers

What is the outcome when Ba(OH)2 reacts with 2NH4Cl?

<p>BaCl2 and 2NH4OH are formed.</p> Signup and view all the answers

Study Notes

Chemical Changes and Reactions

  • There are two types of changes: physical and chemical changes
  • Physical changes: no new substance is formed, and the chemical composition remains the same
  • Chemical changes: a permanent change in which the original substance gives rise to one or more new substances with different properties

Types of Chemical Changes

  • Direct combination
  • Decomposition
  • Displacement
  • Double decomposition

Conditions Necessary for a Chemical Change

  • Close contact between reactants
  • Reactants in an aqueous or molten state
  • Heat
  • Light
  • Electricity
  • Pressure
  • Catalyst

Characteristics of Chemical Reactions

  • Change in colour
  • Formation of a precipitate
  • Evolution of a gas
  • Change in temperature
  • Change of state
  • Change in pH

Energy Changes in a Chemical Change

  • Exothermic reactions: heat is evolved, and the temperature of the system rises
  • Endothermic reactions: heat is absorbed, and the temperature of the system decreases

Examples of Chemical Changes

  • Rusting of iron on exposure to moist air
  • Burning of wood to give gases and ash
  • Curdling of milk when left at room temperature during summer
  • Ripening of fruits
  • Cooking of food
  • Iron dipped in a solution of copper sulphate
  • Magnesium ribbon heated to produce magnesium oxide
  • Sodium metal reacting with distilled water to produce sodium hydroxide solution and liberating hydrogen gas

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Description

Study of physical and chemical changes, including the formation of new substances and their chemical composition

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