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Grade 8 Chemistry

Short notes to remember:

• A mixture consists of two or more components which are not chemically combined.

• A mixture has variable composition, that is, the proportion of the different components can vary.

• A mixture can be separated into its components by simple separation techniques, for example, hand picking, magnetic attraction, filtration, decantation and evaporation.

• A solution consists of a mixture containing a solute dissolved in a solvent.

• A solute is the dissolved component in a solution.

• A solvent is the liquid in which the solute is dissolved.

• A suspension consists of a mixture of a liquid with undissolved fine solid particles.

• Magnetic materials can be separated from non-magnetic materials by using a magnet.

• Decantation is a fast method for separating a mixture of a heavier insoluble solid from a liquid. Decantation involves the pouring out of a liquid lying on top of a heavier solid.

• Filtration involves the separation of a finely divided insoluble solid mixed from a liquid. Filtration is possible when the size of the particles is larger than the pores in the filter.

• The liquid which passes through the filter paper during filtration is called a filtrate.

• The solid which remains on the filter paper during filtration is called the residue.

• Evaporation as a separation technique is used to separate a soluble solid (solute) from a solution. During evaporation, heat is used to evaporate the solvent.

• An alloy is a mixture of one metal and one or more metal or non-metal element.

• Alloys have improved properties of their constituent metals.

• Some examples of alloys include bronze (copper and tin), brass (copper and zinc), steel (iron and carbon) and stainless steel (iron, carbon and chromium and nickel).

• Bronze has a shiny surface; it is resistant to corrosion. It is used to make statues and medals.

• Brass is harder than pure copper. It is used to make kitchen utensils and musical instruments.

• Steel is hard and strong. It is used to make car bodies and bridges.

• Stainless steel is shiny and does not rust. It is used to make cutlery and surgical instruments.

Short notes to remember:

• An element is a substance that cannot be broken down into simpler substances.

• A symbol is a shorthand way of representing an element.

• The valency of an element is its combining power.

• An atom is the smallest indivisible part of an element that can exist.

• A molecule is made up of non-metal atoms that are chemically joined together.

• A molecule is the smallest particle of an element or a compound which can exist independently, e.g. O2, H2, H2O, NH3.

• A radical is a group of atoms having a formula and a valency. Examples of radicals include hydroxide, carbonate, sulfate, ammonium and nitrate.

• The formula of a compound shows the ratio of the different number atoms of each element in a compound.

• The elements and/or radicals present in a compound can be deduced from the chemical name of the compound. Some compounds have common names fromwhich the elements and/or radicals cannot be deduced.

• Chemical reactions are chemical changes which produces new substances.

• Chemical reactions can be represented by word equations.

• The starting substances in a chemical reaction are called reactants.

• The substances formed in a chemical reaction are called products.

• In a word equation, reactants are placed on the left side of the equation and products are placed on the right side of the equation.

Short notes to remember:

• Acids are compounds containing hydrogen and other elements or radicals. They can be represented by formulae.

• Acids present in the laboratory are sulfuric acid (H₂SO₄), nitric acid (HNO₃) and hydrochloric acid (HCl) and they are corrosive when concentrated. They are usually kept in glass containers.

• Acids present in food have a sour taste for example citric acid in lime, ethanoic acid in vinegar, malic acid in apples, lactic acid in milk or carbonic acid in soft drinks. These acids are not corrosive.

• Acids have the following properties; water soluble, sour taste, corrosive when concentrated, reacts with reactive metals to liberate hydrogen, reacts with carbonates to liberate carbon dioxide.

• Copper, silver and gold do not react with acids.

• Hydrogen gas burns in oxygen with a 'pop' sound.

• A base is a metal oxide or a metal hydroxide. When a base dissolves in water, an alkali is formed.

• Strong alkalis are sodium hydroxide (NaOH), potassium hydroxide (KOH) and weak alkalis are calcium hydroxide (Ca(OH)₂) and aqueous ammonia (NH₃(aq)).

• Strong alkalis are caustic and must be handled with care.

• A pH value is a unit used to measure acidity or alkalinity of a solution. It tells us if a substance is acidic or basic. It differentiates between a strong and weak acid or an alkali.

• A pH scale ranges from 0 to 14. Strong acids have low pH near 0 that is 0, 1 and 2. Weak acids have higher pH just below 7 namely 4, 5 and 6. Strong bases have high pH near 14 for example 13 and 14. Weak bases have lower pH just above 7 that is 8 or 9. Neutral solution has a fixed pH value of 7.

• Indicators are chemical substances that can differentiate between an acid and an alkali by showing different colours. Example of indicators are litmus papers, methyl orange and phenolphthalein. Litmus paper is red in acid and blue in alkali. Methyl orange is red in acid and yellow in alkali. Phenolphthalein is colourless in acid and pink in alkali.

• Indicators can also exist in nature for example red cabbage juice or pigments present in Hydrangea flowers.

• Neutralisation reaction is a chemical reaction where an acid reacts with an alkali or vice versa to form salt and water. During the reaction heat is also liberated. Neutralisation reactions are commonly applied to daily life experiences for example liming of the soil, neutralising bee sting with baking soda or wasp sting with vinegar.

• When dilute acids react with metals, heat is also liberated. A thermometer is used to measure the change in temperature.