Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define chemical change and tell it apart from physical change.
• Identify signs that a chemical reaction has happened.
• Define what 'rate of reaction' means.
• List the factors that can change the speed of a reaction.

Lesson summary

Chemical reactions are happening all around us, all the time. From the food we cook (think of how an egg changes when you fry it) to the rusting of corrugated iron roofs in our homes, to the burning of fuel in cars and power stations that provide us with electricity – chemical changes are fundamental to our lives. Understanding these changes is crucial, not just for science, but also for solving problems in our communities and our country. For example, understanding how to prevent corrosion can save money on infrastructure maintenance. Knowing about combustion can help reduce air pollution.

Lesson notes

2.1 Chemical vs. Physical Change A physical change alters the form or appearance of a substance, but it doesn't change what that substance is. Think about melting ice. It goes from solid ice to liquid water, but it's still H₂O. Other examples include tearing paper, dissolving sugar in water (the sugar molecules are still sugar), and boiling water. The substance’s chemical identity remains unchanged. A chemical change, on the other hand, results in the formation of a new substance(s) with different properties. This involves the breaking and/or forming of chemical bonds. Burning wood is a great example. The wood (mostly cellulose) reacts with oxygen in the air to produce ash, carbon dioxide, water, and heat. The wood is gone; it has transformed into entirely new substances. Rusting is another example where iron reacts with oxygen to form iron oxide, a new substance.

Key Differences Summarized: | Feature | Physical Change | Chemical Change | |-----------------|-----------------------------------------|------------------------------------------| | Substance Identity | Remains the same | Changes; new substance(s) are formed | | Bonds | No bonds are broken or formed | Bonds are broken and/or formed | | Reversibility | Often easily reversible | Usually difficult to reverse | | Examples | Melting, boiling, tearing, dissolving | Burning, rusting, cooking, digestion | 2.2 Indicators of a Chemical Reaction How do we know if a chemical reaction has taken place?

Here are some common clues: Colour Change: A significant change in colour often indicates a new substance is forming. For example, when you mix iodine solution with starch solution, the mixture turns a dark blue-black colour.

Gas Production: Bubbles forming (other than from boiling!) can signify a gas is being produced. For example, adding vinegar (acetic acid) to baking soda (sodium bicarbonate) produces carbon dioxide gas, which you see as bubbles.

Precipitate Formation: A precipitate is an insoluble solid that forms when two solutions are mixed. Imagine you mix solutions of lead nitrate and potassium iodide. A bright yellow solid, lead iodide, precipitates out of the solution.

Heat Change (Exothermic/Endothermic): Chemical reactions can either release heat (exothermic) or absorb heat (endothermic). If a reaction releases heat, the surrounding area will feel warmer (e.g., burning wood). If a reaction absorbs heat, the surrounding area will feel cooler (e.g., some instant cold packs).

Light Emission: Some chemical reactions produce light (e.g., burning magnesium).

Example: Think about cooking an egg.

Colour Change: The clear egg white turns opaque white.

Texture Change: The runny liquid egg solidifies. These changes suggest a chemical reaction is taking place – the proteins in the egg are denaturing and forming new bonds. 2.3 Rate of Reaction The rate of reaction tells us how fast a chemical reaction is proceeding. A fast reaction occurs quickly (e.g., an explosion), while a slow reaction takes a long time (e.g., rusting). At this introductory level, we'll focus on describing the rate qualitatively, meaning without exact numerical values.

Qualitative Measurement: We can describe the rate of reaction by observing how quickly reactants disappear or how quickly products appear.

Example 1: If you drop an Alka-Seltzer tablet into water, and the fizzing (production of gas) stops very quickly, we can say the reaction rate was fast. If the fizzing continues slowly for a long time, the reaction rate was slow.

Example 2: Imagine placing a piece of iron outside to rust. After a week, you only see a tiny bit of rust. After a year, you see a lot of rust. This tells us the rusting process is slow. 2.4 Factors Affecting Rate of Reaction Several factors can influence how fast a chemical reaction occurs: Temperature: Generally, increasing the temperature increases the rate of reaction. Higher temperature means the particles have more kinetic energy and are more likely to collide with enough energy to react. Think about cooking food – it cooks much faster at higher temperatures.

Concentration: Increasing the concentration of reactants usually increases the rate of reaction. A higher concentration means there are more reactant particles in a given volume, leading to more frequent collisions.

Imagine a classroom: the more people, the more likely they are to bump into each other.

Surface Area: Increasing the surface area of a solid reactant increases the rate of reaction. A larger surface area means more of the reactant is exposed to the other reactants, leading to more collisions.

Think about starting a fire: small, thin pieces of wood catch fire faster than a large log because they have a higher surface area exposed to the oxygen in the air.

Catalyst: A catalyst is a substance that speeds up a chemical reaction without being used up itself. Catalysts provide an alternative reaction pathway with a lower activation energy.