Sci Chap 11 Flashcards
How do we identify a chemical change?
Chemical reactions involve the rearrangement of atoms leading to new products formed. The atoms are not created nor destroyed, and mass is conserved.
What are the different types of chemical changes?
There are diff types of chemical changes such as combustion, thermal decomposition, oxidation, and neutralisation.
How do chemical changes occur?
Chemical changes occur when matter (substances) interact with each other and with the environment through mixing, heat, electricity, etc. These interactions can result in different types of chemical changes (reactions).
How does the knowledge of chemical reactions help us in our daily lives?
Chemical reactions can bring both benefit (e.g. cooking, respiration) and harm (e.g. rusting, decay, burning).
How can the changes caused by interactions between the substances and the environment be categorised?
- Physical changes
- Chemical changes
Define physical substances.
Physical changes do not involve the formation of new substances.
Some examples:
- Dissolving sugar in water
- Freezing, boiling, and evaporation of water
- Melting of chocolate
During a physical change, energy in the form of heat may be taken in or given out.
For instance, ice must gain heat from its surroundings in order to melt and become water. Hence, melting of a solid substance involves taking in heat.
Water must lose heat to its surroundings in order to freeze and become ice. Hence, freezing of a liquid substance involves giving out heat.
Define chemical changes.
Chemical changes occur when:
- Matter interacts with heat, light, or electricity.
- Two or more different kinds of matter interact.
They can be represented by word equations and chemical equations.
Chemical changes are also known as chemical reactions.
In a chemical change, the substances present at the start of the reactions (known as the reactants) undergo the change to form completely new substances that are found at the end of the reaction (aka products)
During a chemical change, energy in the form of heat, light, or sound (or a combination of all) may be taken in or given out. When heat is given out during a chemical change, an increase in the temperature of the surroundings will be observed. Likewise, when heat is taken in during a chemical change, a decrease in the temperature of the surroundings will be observed. Changes in temp can be measured using a thermometer.
An example of a word equation: carbon + oxygen →(heat) carbon dioxide
Examples of chemical reaction
- burning charcoal
- mixing an acid and alkali
- toasting bread
Why might effervescence be observed in a solution?
Effervescence indicates the presence of a gas, which is insoluble in water, produced by a reaction. It also indicates that a new substance has been formed.
Similarities and differences between physical and chemical changes.
SIMI: Energy may be taken in or given out during physical/chemical changes.
DIFF:
- One or more new substances are formed in chemical changes, whereas no new substances are formed in physical changes.
- Physical changes are reversible, whereas chemical changes are irreversible.
Which of the following always occurs in a chemical change?
a. New substances are formed
b. There is a change in the composition of the substance
c. Heat and light are produced
d. Particles move further or closer to one another.
- a and b 2. c and d
- a, b, and c 4. all of the above
1
Types of chemical changes w/ types of interactions.
1. Interaction of substances with heat
i) Combustion (burning)
ii) Thermal decomposition
2. Interaction of substances with oxygen
i) Combustion (burning)
ii) Rusting
iii) Cellular respiration
3. Interaction of substances with light
i) Photosynthesis
4. Interaction of substances with electric current
i) Electroplating
5. Interaction of substances- mixing
i) Reaction of acid with alkali
ii) Reaction of acid with metal
iii) Reaction of acid with carbonate
Define the two interactions that take place with heat.
-
Combustion
Combustion refers to the chemical reaction of a substance with oxygen in the presence of the heat. The combustion of fuels releases a lot of energy, which can be used for cooking, transport, and generating electrical energy. The combustion is commonly known as ‘burning’.
- The substance must be in direct contact with a flame or spark.
- The substance is known as a fuel.
Some common examples pf fuels are coal, natural gas, petrol, and diesel. In a combustion reaction, one or more new substances may be produced. They are known as products. The combustion of fuels releases a lot of energy, which can be used for cooking, transport, and generating electrical energy.
Complete combustion occurs when there is an abundant supply of oxygen. This will ALWAYS produce carbon dioxide and water at the products.
e.g. Burning of coal in power stations to generate electricity: carbon + oxygen → (heat) carbon dioxide
Heat and light is typically given out during combustion e.g. sparklers release heat and light when burned.
- Thermal decomposition
Thermal decomposition is a process in which a substance is broken down into two or more simpler substances by the effect of heat. It is an irreversible chemical reaction.
examples:
- Glucose breaks down into carbon when heated for too long.
word equation: glucose → (heat) carbon + water vapour
Thermal decomposition of limestone
- Limestone is mixture which is mainly up of calcium carbonate. When heated strongly, it decomposes to form calcium oxide and carbon dioxide.
word equation: calcium carbonate → (heat) calcium oxide + carbon dioxide
Define the 3 interactions that take place with oxygen. (oxidation)
Oxidation refers to a process in which a substance reacts with oxygen.
- Combustion
Combustion reactions are also oxidation reactions as the substance reacts with oxygen.
- Rusting
When iron or an object made of/contains iron is exposed to moist air (e.g. water vapour) for some time, a red-brown substance slowly forms on the surface of iron/object. This substance is known as rust. The rust is a new substance, hence making this change a chemical change.
Conditions for rusting to take place:
- presence of oxygen (in air)
- presence of water
Rusting is a chemical change which occurs when iron or iron containing substances such as steel react with oxygen in air and water, or water vapour in the air, to form rust.
word equation: iron + oxygen + water → rust
It is important to prevent rusting of iron. some methods:
→ Painting (coating an obj w/ a layer of paint) (barrier method)
→ Oiling (coating obj w/ a layer of grease) (barrier method)
→ Galvanizing (electroplating: zinc plating- coating the object w/ a layer of zinc) (barrier method)
→ Drying
The barrier methods prevent iron or steel objects from coming into contact with water and/or oxygen.
CELLULAR RESPIRATION
Cellular respiration is the process by which living cells of plants and animals take in oxygen with water and /or oxygen to release the energy stored in glucose. This process takes place to provide cells with energy to survive and reproduce. (an oxidation reaction that takes place in the cells of living things)
word equation: glucose + oxygen → carbon dioxide + water
food molecules break down in the presence of oxygen.
Define the interaction that takes place with light.
Photosynthesis
Photosynthesis is the process in which green plants make food in the presence of light.
Photosynthesis is a chemical change as carbon dioxide and water react in the presence of sunlight and chlorophyll to form glucose and oxygen.
word equation: carbon dioxide + water → (sunlight and chlorophyll) glucose +oxygen
Define the interaction that takes place with electric current.
ELECTROPLATING
Coins and trophies are sometimes covered by a thin layer of metal to make them more attractive or to prevent the metal from rusting or corrosion. This is done through electroplating.
Electroplating is the process in which a substance is coated with a metal with the passage of an electric current. different metals can be used by changing the type of solution
and type of metal used during electroplating.
How it works: An electric current is passed through the solution (e.g. copper(II) sulfate solution). The metal plate (e.g. copper) becomes smaller as the coin becomes coated with the metal (e.g. reddish brown copper).
Which of the following cannot bring about a chemical change?
- heating mater
- conversion of state
- exposing matter to light
- passing an electric current through matter.
2
Which of the following is not a chemical change?
a. iron forming rust
b. cooking an egg
c. forest fire
4. dissolving salt in water
4
Is charging a rechargeable battery a chemical or physical change? explain briefly.
Chemical change. This is because the process involves an oxidation reaction, which is a chemical reaction.
Explain the movement of atoms in a chemical reaction.
-> In a chemical reaction, the total number of atoms in the products is the same as the total number of atoms in the reactants.
-> The total mass of the reactants is the same as the total mass of the products in a single chemical reaction. We say that mass is conserved during a chemical reaction.
-> The atoms of the reactants rearrange to form new products.
This means that the atoms in a chemical reaction are neither created nor destroyed. e.g. sulfur reacts with oxygen to form sulfur dioxide.
When copper(II) carbonate is heated, a black solid (copper(II) oxide) and a colourless gas are produced. What type of reaction is this? Write the word equation for this reaction.
thermal decomposition.
copper(II) carbonate → (heat) copper(II) oxide + carbon dioxide.
What condition(s) must be fulfilled for an acid to react with alkalis, carbonates, and metals?
Acids must be in an aqueous state in order to react with alkalis, carbonates, and metals.
State the physical properties of acids and provide examples and applications.
Acids have a sour taste. They change the colours of diff indicators to diff colours:
- Acids will turn blue litmus paper red
- Acids will turn green universal indicator solution red/orange/yellow.
Acidic solutions can conduct electricity. Some examples of acids would be:
~ Dilute hydrochloric acid
~ Dilute nitric acid
~ Vinegar
~ Lemon juice
- Antacids (for gastric pain)
State the physical properties of alkalis and provide examples and applications.
Alkalis have a bitter taste and feel soapy when touched.
Alkalis will change the colour of different indicators diff colours::
- Alkalis will turn red litmus paper blue
- Alkalis will turn green Universal Indicator Solution blue/violet/purple
Alkali solutions can conduct electricity. Some examples/applications:
~ Ammonia solution, used to make fertilisers
~ Sodium hydroxide, used to make soaps and detergents
~ Sodium hydroxide
~ Potassium hydroxide
What is the neutral colours of universal indicator solution?
Green.
Define indicators with examples.
An indicator is a substance that changes colour depending on whether the solution being tested is acidic or alkaline. They come in the form of solutions or indicator papers. Indicators usually come from natural sources such as coloured plants like red cabbage, purple, grasp, curry powder etc.
The two main types of indicators are litmus paper and universal indicator solution.
LITMUS
- A litmus paper is used to test whether a solution is acidic or alkaline. However, it cannot provide the pH of the solution.
There are two colours of litmus paper: red and blue. They are dipped into a solution to observe for any colour change.
When the blue litmus paper turns red and the red litmus paper remains red, the solution is acidic.
When the blue litmus paper remains blue and the red litmus paper turns blue, the solution is alkaline.
When the blue litmus paper remains blue and the **red litmus paper remains red*, the solution is Neutral. This solution has no effect don’t the litmus papers.
UIS
The UIS indicates the strength of acids and alkalis through a colour change. The UIS also tells us a rough (not precise) pH value of an acid/alkali.
The pH scale goes from 1-14.
red -> pink
orange -> orange-yellow -> yellow -> yellow-green
green
dark green -> green-blue -> pale blue -> blue
dark blue -> violet -> purple
The pH levels can be categorised into:
Strong acids: pH 0-2
Weak acids: pH 3-6
Weak alkalis: pH 8-11
Strong alkalis: pH 12-14
Green represents the neutral pH, 7. It is neither a weak acid nor a strong acid. When the UIS turns green, we say that the solution has a natural pH or a pH of 7.
What is pH?
The pH value os a solution is a measure of how acidic or alkaline a solution is.
Many ppl suffer frm ‘acid ingestion’ caused by too much hydrochloric acid in the stomach. The problem can be cured by using an ‘antacid’, which is weakly alkaline. What is the best pH for an antacid?
a. 3
b. 6
c. 9
d. 14
C
What are the 3 chemical properties of interactions of substances through mixing?
1: Acids react with reactive metals to produce a salt and hydrogen gas.
general word equation (GWQ)
acid + metal → salt + hydrogen gas
This is also known as neutralisation.
2: Acids react with carbonate to form salt, water, and carbon dioxide.
GWQ
acid + carbonate → salt + water + carbon dioxide
3. Acids react with alkalis to form sal and water.
GWQ
acid + alkali → salt + water
If Hydrochloric acid were to react with a metal, what salt would be produced?
metal chloride
If sulfuric acid were to react with a metal, what salt would be produced?
metal sulfate
If Nitric acid were to react with a metal, what salt would be produced?
metal nitrate
Applications of the reaction between acid and reactive metal.
Acid etching, where decorative patterns can be carved onto metal objects.
Magnesium reacts with hydrochloric acid.
Magnesium reacts with sulfuric acid.
Applications of the reaction between acid and carbonate.
Bread-making.
Calcium carbonate reacts with hydrochloric acid.
Magnesium carbonate reacts with nitric acid.
Applications of the reaction between acid and alkali (neutralisation)
Using toothpaste to brush our teeth.
- Toothpaste contains an alkali that reacts with acid formed when bacteria act on food particles in our teeth.
Hair conditioner contains weak acids to neutralise the alkali from the shampoo.
Liming of soil.
Most plants grow best in neutral or slightly acidic soil (pH 6-7). Plants will not grow in soil that is too acidic, which can happen when too much fertiliser is added or during acid rain. Hence, chemicals such as calcium hydroxide (slaked lime) are added to the soil to adjust its pH.
Sodium hydroxide reacts with hydrochloric acid.
Calcium hydroxide reacts with nitric acid.
How do we test whether hydrogen gas has been produced after a chemical reaction?
Place a lighted splint (match) near the mouth of the test tube containing the gas. The lighted splint will extinguish with a ‘pop’ sound in the presence of hydrogen gas.
How do we test whether carbon dioxide gas has been produced after a chemical reaction?
fill a test tube with limewater. Place a stopper on the mouth of the tube with the solution and connect a delivery tube from the test tube with limewater to the tube containing the solution. The carbon dioxide gad forms a white precipitate in limewater.
What are the benefits of chemical changes?
- Chemical changes help us cook food.
- The decomposition of dead organisms breaks down unwanted matter and releases nutrients back to the earth.
- Chemical changes brought about by household detergents and fertilisers help in cleaning and increasing corp from a variety of raw materials in production industries.
- Chemical changes allow us to manufacture various useful products such as plastics, iron, and glass from a variety of raw materials in production industries.
- Obtaining metals and alloys from the earth by chemical reactions and used as structure support in buildings, bridges, and vehicles.
- Chemical reactions are used in the production of medicine to treat medical conditions, and insecticides to kill mosquitoes that cause dengue fever.
- Fuel combustion is a type of chemical reaction in engines that release energy lights in engines that release energy to move vehicles.
- Ligth sticks ‘light up’ after it is bent. The bending of a light stick results in a chemical reaction within the stick, releasing light energy.
- Using baking soda to remove stains.
Baking soda contains a chemical known as sodium hydrogen carbonate. NaHCO3-. Baking soda reacts chemically with some substances. This property can be used to clean coffee or tea-stained cups by using a baking soda paste.
What are the harms of chemical changes?
1. Combustion of coal and fossil fuels for energy releases soot and sulfur dioxide into the atmosphere. Soot and sulfur are air pollutants. Sulfur dioxide, when breathed in, leads to the irritation of the eyes and lungs. Sulfur dioxide also reacts with oxygen in the atmosphere and dissolves in rainwater to form acid rain.
acid rain can cause the following negative effects:
- corrodes limestone buildings and metal structures
- acidifies lakes and streams, destroying and gaming aquatic life
- leaches nutrients from the soil, destroying vegetation.
2. rusting. When iron rusts, structures such as bridges become corroded and unsafe. These structures need to be repaired and replaced. Rusting causes the loss of finite natural resources.
3. decomposition of food leaders to the wastage of food supply and resources as the decomposed doof is not safe for consumption anymore.
WQ: Barium carbonate + hydrochloric acid → __________ + ___________ + __________
→ barium chloride + carbon dioxide + water
Some examples of reactive metals.
sodium, zinc, copper, magnesium, calcium
Some examples of metal carbonates.
sodium carbonate, zinc carbonate, magnesium carbonate, calcium carbonate.
Some examples of alkalis.
sodium hydroxide, potassium hydroxide, lithium hydroxide, aqueous ammonia.
