Class 10 General Science NCERT Based - 1 - Free MCQ Practice Test

• During a chemical reaction atoms of one element do not change into those of another element.

• Nor do atoms disappear from the mixture or appear from elsewhere, in fact chemical reactions involve the breaking and making of bonds between atoms to produce new substances. Exothermic reactions are accompanied by release of Heat.

• When a metal is attacked by substances around it such as moisture, acids, etc., it is said to be corroded and this process is called corrosion. The black coating on silver and the green coating on copper are examples of corrosion.

• Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made up of metals, especially those of iron. When fats and oils are oxidised, they become rancid and their smell and taste change.

• Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil. Keeping food in air tight containers helps to slow down oxidation. Due to this reason, bags of chips are flushed with gas such as nitrogen to prevent the chips from getting oxidised.

• Calcium oxide is called as quicklime. Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

• Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls.

• Respiration is an exothermic process. Reactions in which heat is released along with the formation of products are called exothermic chemical reactions. As we all know, we need energy to stay alive. We get this energy from the food we eat. During digestion, food is broken down into simpler substances.

• For example, rice, potatoes and bread contain carbohydrates. These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy.

Double Displacement Reaction

1. A double displacement reaction is a type of chemical reaction in which the reactant ions exchange places to form new products.

2. Usually, a double displacement reaction results in precipitate formation.

3. The chemical bonds between the reactants may be either covalent or ionic.

4. A double displacement reaction is also called a double replacement reaction, salt metathesis reaction, or double decomposition.

The process of respiration involves the slow combustion of glucose when it combines with oxygen in our body cells. This reaction releases energy, making it an exothermic process. During this process, heat and energy are produced as by-products, which is characteristic of exothermic reactions.

• An alloy is a homogeneous mixture of two or more metals, or a metal and a nonmetal. It is prepared by first melting the primary metal, and then, dissolving the other elements in it in definite proportions.

• It is then cooled to room temperature. If one of the metals is mercury, then the alloy is known as an amalgam. The electrical conductivity and melting point of an alloy is less than that of pure metals.

• For example, brass, an alloy of copper and zinc (Cu and Zn), and bronze, an alloy of copper and tin (Cu and Sn), are not good conductors of electricity whereas copper is used for making electrical circuits. Solder, an alloy of lead and tin (Pb and Sn), has a low melting point and is used for welding electrical wires together.

• Our stomach produces hydrochloric acid. It helps in digestion of food without harming the stomach. During indigestion, the stomach produces too much acid and this causes pain and irritation.

• To get rid of this pain, people use bases called antacids. These antacids neutralise the excess acid. Magnesium hydroxide (Milk of magnesia), a mild base, is often used for this purpose.

• Our body works within the pH range of 7.0 to 7.8. Living organisms can survive only in a narrow range of pH change. When pH of rain water is less than 5.6, it is called acid rain. When acid rain flows into the rivers, it lowers the pH of the river water.

• The survival of aquatic life in such rivers becomes difficult. Tooth decay starts when pH of the mouth is lower than 5.5. Tooth enamel, made up of calcium phosphate is the hardest substance in the body.

• It does not dissolve in water, but gets corroded when the pH level in the mouth goes below 5.5. Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and thus can prevent tooth decay.

A. Human blood → 2 (7.3–7.5)

B. Milk → 3 (6.4)

C. Human saliva → 4 (6.5–7.5)

D. Human urine → 1 (5.5–7.5)

So , the correct answer is Option A