Chapter 11- Chemical changes

A physical change is a change in which no new substances are formed. A physical change can be reversed.

A chemical change is a change in which a new substance is formed. A chemical change cannot be reversed.

Reactants, Conditions, Products.

Combustion is a chemical change where a substance is heated in the presence of oxygen to form new oxides of elements and relese heat or light energy.

In thermal decompostion, a single substance breaks up into two or more simpler substances when it is heated.

Sugar (heat)——> carbon + water vapour

carbon + oxygen (heat)—> carbon dioxide + heat + light

Hydro carbon + oxygen ——> carbon dioxide + oxygen + heat + light

Oxidation refers to the addition of oxygen to a substance.

Rusting and cellular respiration.

Iron + oxygen + water——> iron hydroxide

Combination is a process where 2 or more elements chemically combine to form a new substance. Combination is a term used when neither of the obove terms can be used to classify a chemical change.

Rusting is an oxidation process that occurs when iron reacts with oxygen and water.

Cellular respiration is a chemical process under oxidation where glucose is broken down in the presence of oxygen to provide cells with energy to survive and reproduce. During respiration, carbon dioxide and water are produced.

Oxygen + glucose ——> Carbon dioxide + water + energy

Photosynthesis is a chemical reaction where green plants use chlorophyll in their leaves, carbon dioxide and water to produce glucose and oxygen in the presence of light.

Carbon dioxide + water (light enegry)——> Oxygen + glucose

Electroplating is the use of electric current to plate one material with another through electrolysis.

Acid-Metal reaction, Acid-Carbonate reaction, Acid-Alkali reaction.

When dilute acids are mixed with certain metals, hydrogen gas and a salt are produced. You will observe effervescence of colourless gas due to the release of hydrogen gas.

Acid + metal ——> salt + hydrogen

Copper, gold, sliver

When dilute acids are mixed with carbonates, carbon dioxide gas, a salt and water are produced.

Acid + Carbonate ——> Carbon dioxide, water, salt

When dilute acids are mixed with alkalis, a neutralisation reaction occurs, producing a salt and water ONLY.

Acid + Alkali ——> Salt + water.

Hydrochloric acid + sodium hydroxide ——> sodium chloride + water.

Acid-Metal reaction.

Hydrochloric acid + calcium carbonate ——> calcium chloride + carbon dioxide + water.

Anatacids are common medications that contain alkaline compounds that help to neutralise excess acid produced by the stomch and relieve discomfort.

Toothpaste. Bacteria on our teeth produce acids that can cause tooth decay. Toothpaste is weakly alkaline and can help to neutralise any acid in our mouth.

Indicators are substances that show one colour in acids and another in alkalis, allowing us to identify if a subtance is neutral, acidic, or alkaline.

Litmus paper, universal indicator, natural indicator, pH meter, lit wooden splint (to test for hydrogen), limewater (to test for carbon dioxide).

If the substance is acidic, the blue litmus turns red, and the red litmus remains red. If the substance is alkaline, the blue litmus remains blue and the red litmus turns blue. If the substance is neutral, the blue and red litmus remain blue and red respectively.

The indicator changes into different colours when it is added to different substances. Each colour corresponds to a certain pH level beween 0 and 14. This allows us to identity if the substance is an acid or an alkali.

Acids and alkalis are both colourless in the first place.

The pH meters are more accurate then universal indicators. The electrode on the meter is dipped into a solution and the pH value is read from the meter.

Litmus paper is fast, and instant. But, it can only be used for acid/alkali and does not show the exact pH level of a substance, and is not accurate.

Universal indicator is fast and instant, but does not show the exact nature of substance or is not accurate

pH meter is very accurate. But it takes a lot of time or needs a lot of chemicals to be tested.

Light a wooden splint and place the lighted splint at the mouth of the test tube with the solution in it. If the lighted splint estinguishes with a ‘pop’ sound, hydrogen is present.

Bubble the gas into the testube with lime water with either a delivery tube or the dropper method. If white precipitate is formed in the limewater, carbon dioxide is present.

Decay or decompostition is the process of breaking down organic matter into simpler substances. It allows nutrients to be returned to the environment.

When certain micro-organisms start growing on food, they cause a chemical reaction that can make food go bad and give it a bad taste or smell. Consuming this can cause food poisoning.

One method of slowing down food decay is vacuum packaging where air is removed from the package prior to sealing. This reduces the amount of oxygen in the package and limits the growth of bacteria and other micro-organisms as they need oxygen to reproduce and grow.

They have benefits in nature, farming, hygiene and healthcare.

Photosynthesis, respiration and decay are all chemical processes that are very crucial for the sustainability of the ecosytsem.

When the soil is too acidic for farming and growing crops, some alkali solutions are poured into it to neutralise the pH of the soil.

Toothpaste.

Antacids.

Rusting is an oxidation reaction that occurs rapidly in the humid climates, in which a large amount of water vapour is present. When iron is exposed to the atmosphere, it reacts with the water in the water vapour and oxygen to form iron oxide, which is rust. Rust is a weak substance, and objects that are rusted can no longer be used.

Painting, oiling, galvanising (includes coating iron with protective lyer of zinc), and drying all prevent iron and steel from coming into contact with water or oxygen, preventing rusting.

Combustion of fuels in motor vehicals, factories, and power stations produces air pollutants.

Carbon monoxide, sulfur dioxide, nitrogen dioxide, ash, soot, and dust.

Burning of fuels in factories and motor vehicals in the presence of insufficient oxygen.

Carbon monoxide is a colourless, oudourless and highly toxic gas that reduces the amount of oxygen taken in by red blood cells, leading to headaches and eventually death due to suffocation. It is also very poisonous to inhale.

Burning of sulfer-containing fuels such as coal and petrol in power stations and factories. Volcanic eruptions.

Burning of fuels at high temps in motor vehicals, factories, and power stations. Reaction of nitrogen and oxygen in the air during lightening strikes and deforestation using slash-and-burn method.

Can irritate the eyes, nose and cause respiratory illnesses such as lung cancer. Can worsen symptoms of asthma and heart diseases even at low levels. Can dissolve in rainwater to form acid rain which harms human health, and damages or corrodes building. Can dissovle into the ocean and make it more acidic, thus affecting aquatic life.