Chemical Changes and Reactions Chapter Notes | IGCSE Cambridge Science for Year 6 - Class 6 PDF Download

Making compounds

Word Equations:

• In a chemical reaction, substances (reactants) react together to form new substances (products).
• A word equation is a model that represents a chemical reaction using the names of reactants and products.
• Example: magnesium + oxygen → magnesium oxide
• Reactants are written on the left, products on the right, and an arrow shows the direction of the reaction.

Particle Model:

• A particle diagram is another way to represent chemical reactions.
• It shows atoms as circles, and how they are arranged in reactants and products.
• The same number of each type of atom must be present on both sides of the diagram.
• Particle diagrams provide more information about the arrangement of atoms than word equations.

Particle Diagram

Making Compounds:

• Magnesium and oxygen are elements that react to form the compound magnesium oxide.
• Compounds contain atoms of different elements strongly held together.
• Properties of compounds differ from their constituent elements.
• Compounds can only be separated into elements through chemical reactions.

Identifying Chemical Reactions:

• Temperature changes, reactants being used up, products being formed, color changes, and precipitation can indicate a chemical reaction.
• Signs include disappearance of reactants, gas bubbles, color changes, and formation of a solid precipitate.

Testing for Hydrogen:

• Some reactions produce hydrogen gas.
• To test for hydrogen, hold a lighted splint near the gas – it will ignite with a 'pop' sound if hydrogen is present.
• Hydrogen is flammable and burns quickly.

Testing for Hydrogen

Testing for Oxygen:

• Reactions can produce oxygen gas.
• To test, hold a glowing splint just inside the gas – it will relight if oxygen is present.
• Oxygen allows the hot wood to start burning again.

Testing for Carbon Dioxide:

• Carbon dioxide gas can be produced in reactions.
• Pass the gas through limewater (calcium hydroxide solution) – it will turn milky white if carbon dioxide is present.

Limewater turns Milky

Identifying Risks:

• Experiments may involve hazards (substances or procedures that can cause harm).
• Hazard labels indicate potential risks.
• Identify hazards and plan to control risks (reduce chances of harm) before starting an experiment.

Forming Precipitates

Soluble and Insoluble Substances:

• Some substances are soluble in water, meaning they can dissolve to form a clear solution.
• Example: Sodium chloride (table salt) is soluble in water and forms a clear solution.
• Solutions can be colorless (like sodium chloride solution) or colored (like blue copper sulfate solution).
• Other substances are insoluble in water, meaning they do not dissolve and cannot form a solution.
• If an insoluble substance is mixed with water, it remains visible, making the mixture cloudy.
• Example: Calcium carbonate is an insoluble white solid that makes water cloudy when mixed.

Precipitates:

• Substances dissolved in water can still participate in reactions (like acids).
• When soluble reactants react and form one or more insoluble products, an insoluble product formed is called a precipitate.
• A precipitate cannot dissolve in the reaction mixture, causing it to turn cloudy.
• A reaction in which a precipitate forms is called a precipitation reaction.

Modelling Precipitation Reactions:

• Word equations and particle diagrams can be used to model precipitation reactions.
• Example word equation: copper sulfate + sodium silicate → copper silicate + sodium sulfate

• Particle diagrams show the arrangement of particles but have limitations:
  1. They may not show all atoms present in the reaction mixture.
  2. They do not show the forces holding particles together or indicate solubility/insolubility.
• While these models provide some information, they have limitations in representing all aspects of precipitation reactions.

Neutralisation Reactions

Measuring and Estimating pH:

• pH of a solution can be measured using a pH meter, which consists of a pH probe connected to a meter.
• Indicators are substances that change color depending on the pH of a solution.
• Universal Indicator is a mixture of indicators that changes color across the pH scale, from red to purple.
• Universal Indicator is available as a solution or test paper.
• To estimate pH using Universal Indicator, add a few drops of the solution or a small drop on test paper.
• Match the resulting color to a pH color chart to estimate the pH value.

What is Neutralization?

• When an acid is added to an alkali (base), a chemical reaction called neutralization occurs.
• In a neutralization reaction, water and another substance (which may dissolve in water) are formed.
• During neutralization, the pH of the solution changes.
• As an alkali is added to an acidic solution, the pH increases toward 7 (neutral).
• When the pH reaches 7, a neutral solution is formed.
• If more alkali is added after reaching pH 7, the solution becomes alkaline, and the pH increases above 7.
• Neutralization involves the gradual change of an acidic solution to a basic solution (or vice versa) as the neutralizing agent is added.