PRESERVATION OF METALS (Activity 1.1)

Builders in coastal regions mainly use wooden window frames instead of steel frames for homes because:

• wood is less likely to corrode in salty air than steel

• wood is cheaper than steel everywhere

• steel is more attractive than wood

• wood is heavier than steel

Corrosion is the gradual destruction of metals by chemical reactions with their environment. Why does motorbike B, which is located in Durban, show more signs of corrosion than motorbike A from Pietermaritzburg?

• Durban is closer to the sea, so the salty air causes more corrosion.

• Pietermaritzburg has more rainfall, causing more corrosion.

• Motorbike B is older than motorbike A, causing more corrosion.

• Durban has less pollution, causing more corrosion.

Painting can reduce corrosion by:

• Providing a protective barrier that prevents moisture and air from reaching the metal surface.

• Increasing the rate of oxidation on the metal surface.

• Allowing more water to come into contact with the metal.

• Reacting chemically with the metal to produce rust faster.

Galvanising is a process used to:

• coat iron or steel with a layer of zinc to prevent rusting

• paint metal surfaces for decoration

• increase the weight of metal objects

• remove impurities from metals

Describe in your own words what you observed during the experiment.

• The rusted key once again has a copper colour.

• The key dissolved completely in the solution.

• The key turned into a silver color.

• The key broke into two pieces.

Explain in your own words why, what you observed, happened.

• An electrochemical reaction took place. With the use of an electric current one metal (the key) was covered with a thin layer of another metal (copper). The appearance of the key was improved and its use was prolonged.

• The key was simply cleaned with water, which made it look shiny and new.

• The key was heated, causing the metal to melt and form a new layer.

• The key was painted with copper-colored paint to improve its appearance.

Your mother packs you cheese sandwiches for break. You forget the sandwiches in your bag and only discover them 10 days later. Describe the smell and appearance of the sandwiches as well as the reason why they look and smell the way they do.

• The sandwiches and cheese have a black-brownish colour and smell awful. The colour and smell are as a result of the mould that started growing on it. The reason why the mould started growing on the bread and cheese is because the parcel in which the sandwich was wrapped became warm and moist in the school bag. Due to the heat and moist the mould grew even faster.

• The sandwiches and cheese remain fresh and smell pleasant because the bag was airtight and cool.

• The sandwiches and cheese turn dry and hard but do not change colour or smell because cheese preserves itself.

• The sandwiches and cheese become watery and sweet-smelling due to the sugar content in the bread.

Name at least three precautionary measures to be considered when grain is stored and also explain the importance of each.

• Grain must be kept dry because grain that becomes wet creates a favourable environment for fungi (funguses) and bacteria to grow. Grain must be kept at a constantly cool temperature since changes in temperature (increase in temperature) results in fungi (funguses) and bacteria growing even faster. Grain must be protected against insects, birds and rodents because they can damage it and spread diseases that can destroy the grain.

• Grain should be stored in open containers to allow maximum air exposure, which helps in natural drying. Grain should be mixed with water occasionally to prevent it from becoming too dry. Grain should be left uncovered to allow birds to help in cleaning it.

• Grain must be stored in direct sunlight to keep it warm and prevent any cooling. Grain should be kept moist to avoid it from drying out. Grain should be stored together with fruits to enhance its flavor.

• Grain should be stored in plastic bags without ventilation to keep out all air. Grain should be kept near chemicals to prevent pests. Grain should be stored in a humid environment to maintain its texture.

Which of the following is an example of a preserved product you might find in a supermarket?

• Canned beans

• Fresh apples

• Raw chicken breast

• Unprocessed lettuce

Which of the following lists contains three different pickled products and the chemicals commonly used for their preservation?

• Cucumber (vinegar), Olives (brine), Kimchi (salt)

• Apple (citric acid), Banana (sugar), Grape (ascorbic acid)

• Bread (yeast), Cheese (rennet), Milk (lactose)

• Potato (starch), Rice (water), Corn (oil)