Grade 7 Science Pure Substances and Mixtures By

Grade 7 Science: Pure Substances and Mixtures By: Michelle and Rawan

�I can explain the components of a solution. � I can identify solutes and solvents in various kinds of solutions (liquid, solid and gas). � I can explain why water is referred to as the universal solvent. � Key terms: solute, solvent, dissolve

Recall from our last lesson: � Solution: a mixture that looks like a single pure substance; a uniform mixture of two or more pure substances also known as a homogenous mixture. � Today we will be taking a closer look at solutions.

� Solute: The smaller part of a solution; the part of a solution that dissolves in the solvent. Example: Kool-Aid powder mix � Solvent: The larger part of a solution; the part of a solution into which the solutes dissolve. Example: Water (the most common solvent. ) When the kool-aid powder mix (solute) is mixed in the water (solvent), it dissolves and becomes Kool-Aid (solution).

We are most familiar with solutions that are liquids. � Liquid solutions all have liquid solvents. � Solutes that dissolve in liquids may be solids (e. g. salt, sugar), liquids (e. g. acetic acid to from vinegar), or gases (e. g. CO 2 and nitrogen gases dissolve in our blood and are carried around our body).

The Universal Solvent is water. � Water dissolves more different substances than any other solvent. It is the solvent for many important solutions. � This is why it is called “The Universal Solvent”.

o o o Tap water usually looks and tastes like pure water but, did you know, that water is a solution that contains many solutes? These solutes include iron, aluminum, salt, fluorine, calcium, magnesium, and chlorine. As water flows in rivers and lakes and underground, it comes into contact with many types of matter. Example: Gases from the air and minerals from the rocks and soil, all dissolve in the water. Pollutants may also dissolve in the water. � Before water reaches your tap it is cleaned so it is safe for drinking (Chlorine and fluorine are sometimes added to the water. Chlorine kills bacteria and fluorine may help keep your teeth healthy).

Water in your body: � 70% of your body is water. This water dissolves many different solutes, making a variety of solutions. The solutes are salt, oxygen, sugars, and mineral components (calcium and potassium). Because they are dissolved in water they have the ability to travel around your body. � Some of the solutions produced by your body are blood plasma, sweat, urine, and tears. The solvent of all of these solutions is water.

� 70% of the Earth’s surface is water. As water moves around Earth, it dissolves many different solutes which are transported around the world as the water cycle occurs. � Solutes in water can be absorbed by living things. Plants absorb the minerals and nutrients dissolved in water through their roots. � Discuss why it is important to know this in terms of pollution.

� � � A pollutant is any pure substance or mixture that can contaminate or harm the natural environment. Polluted water is a mixture of pure water and pollutants. Pollutants can enter water from various sources such as fertilizers and pesticides from farms, waste from mining, fertilizers and pesticides from lawns, industrial waste, and sewage and household products put down the drains. It’s always important to keep our water clean so that organisms can grow and live normally in the lakes, rivers and oceans. We have to be careful not to let contaminants get into our water.

� Solutions that are solids have both solid solutes and solid solvents. � Example: The gold that is used to make jewelry often is called “ 14 karat” gold. Pure gold is 24 karat gold. This means that the jewelry gold is 14 parts gold and 10 parts of other metals(silver, copper, nickel, or palladium). Gold is the solvent and the other metals are the solutes in this case. � Remember, the solutes are the smaller part of the solution and they are dissolved into the solvent.

� Solid solutions are called “alloys” when they contain two or more metals. To make them, the metals are heated until they melt, and then mixed together and allowed to cool. Brass is an alloy of copper and tin.

� In any gas solution, the solvents and solutes are always gases. � An example of a gas solution is the air we breathe. It is 78% nitrogen gas, 21% oxygen gas and 1% argon gas. It also has small amounts of other gases like carbon dioxide. Can you tell what the solvent is of this gas solution is? How about the solutes?