Water as a chemical, Hâ‚‚O, is a pure substance. But almost every glass of water you have ever seen is a mixture, because water dissolves bits of nearly everything it touches. So the answer depends on whether you mean water the molecule or the water that comes out of your tap.
Why Chemistry Classifies Water as a Pure Substance
Chemists call something a pure substance when its makeup is fixed and it has its own distinct properties.1Pearson. Classification of Matter: Pure Substances and Mixtures Pure substances come in two kinds: elements, like iron or copper, and compounds, which are made of different elements joined together.2Pearson. Classification of Matter: Pure Substances and Mixtures Water is a compound, built from the elements hydrogen and oxygen.3Pearson. Classification of Matter: Pure Substances and Mixtures
The formula Hâ‚‚O is its fixed recipe. Each water molecule has two hydrogen atoms bonded to one oxygen atom, and the small 2 shows there are two hydrogens for every oxygen.4Water Security. What is the formula of water? That ratio never varies. If it changed, the substance would no longer be water.5Water Security. What is the formula of water?
A mixture is different. Its ingredients can be combined in any ratio, so its makeup varies.6Pearson. Classification of Matter: Pure Substances and Mixtures Salt water can be a little salty or very salty, but Hâ‚‚O is always the same two-to-one recipe.
It helps to compare water with a mixture of its own ingredients. Hydrogen gas and oxygen gas stirred together form a mixture that is not water at all. In water, the atoms are chemically bonded into new molecules with new properties. Water is a liquid that puts out fire, while its ingredients are gases, one of which burns. And separating water back into hydrogen and oxygen takes a chemical change, not filtering.
Being pure does not mean being made of one kind of atom. Water contains two elements and is still pure because they are always joined the same fixed way, so every sample has the same makeup and properties, such as freezing and boiling temperatures. A mixture lacks that sameness.
Be careful with the word “pure” on labels. There it often means clean or safe. In chemistry it means only one substance is present, so spring water sold as pure is still a mixture in the chemistry sense.
Water Molecules Are Not All Identical
Even perfectly pure water breaks apart slightly, all the time, at room temperature.7PubMed Central. Probing the self-ionization of liquid water with ab initio deep potential molecular dynamics One molecule hands a hydrogen nucleus to a neighbour, leaving one charged hydroxide piece and one charged hydronium piece.8Wikipedia. Self-ionization of water These pieces keep rejoining into ordinary water, so the substance stays water. At any moment only a very small fraction of molecules are split, so nearly everything in a glass is ordinary Hâ‚‚O.
Some molecules differ in weight, too. Some hydrogen atoms in nature are a heavier version called deuterium, about 1 hydrogen in 6,400.9Wikipedia. Heavy water As a result, in ordinary water about 1 molecule in 3,200 contains one heavy hydrogen.10Wikipedia. Heavy water Molecules with two heavy hydrogens, called heavy water, are only about 1 in 41 million.11Wikipedia. Heavy water Heavier oxygen atoms occur naturally in water as well.12Wikipedia. Heavy water
None of this turns water into a mixture. The charged pieces form from water itself and turn back into water. Deuterium is still hydrogen, only a heavier atom, so a molecule with one heavy hydrogen still has two hydrogens per oxygen. Heavy water behaves almost like ordinary water in chemistry, though it is a little denser. It is a variant of the same compound, not a contaminant. Water molecules also cling loosely into short-lived clusters, which don’t change what water is.13PubMed Central. A hierarchical clustering method of hydrogen bond networks in liquid water undergoing shear flow
Tap Water, Mineral Water and Seawater Are Mixtures
As water moves through soil and rock, it dissolves small amounts of natural minerals.14USGS. Hardness of Water Groundwater gains minerals the longer it travels, which is why deep, old water can be heavily mineralized.15National Ground Water Association. Dissolved Mineral Sources and Significance Eventually it reaches a balance and stops dissolving more.16National Ground Water Association. Dissolved Mineral Sources and Significance The minerals in a supply depend on the rock the water passed through, so a town on limestone gets different water from a town on granite. Because the source differs from place to place, the ingredients and amounts differ too, which is the defining trait of a mixture.
Tap water often contains calcium, magnesium, sodium, and bicarbonate.17mypurewater.com. Why Does My Water Taste Different Depending on Where I Go? It may also contain disinfectants from treatment and traces picked up from pipes.18mypurewater.com. Why Does My Water Taste Different Depending on Where I Go? These extras explain why water tastes and smells different from town to town.19mypurewater.com. Why Does My Water Taste Different Depending on Where I Go? In the US, legal limits apply to more than 90 contaminants in public drinking water.20US EPA. Drinking Water Regulations
Chemists describe a mixture that looks the same throughout, like clear salt water, as homogeneous. One with visible parts, like muddy river water, is heterogeneous. Most tap water is an evenly blended mixture, which is why it looks pure.
“Dissolved” has a specific meaning. When salt dissolves, its particles spread out among the water molecules until they are too small to see. The salt has not turned into water or vanished. Evaporate the water and the salt is left behind. Each keeps its own identity, which is why the result is a mixture and not a new compound.
Seawater is a strong example. It is about 96% water and 4% dissolved substances by weight.21EBSCOhost. Seawater Composition Table salt makes up about 85% of what is dissolved.22EBSCOhost. Seawater Composition
Rain is a mixture too, since it picks up gases like carbon dioxide while falling. Mineral water labels list dissolved minerals, and even “purified” bottled water often has minerals added back for taste.23PubMed Central. Photochemistry of the Cloud Aqueous Phase: A Review
Signs of the Mixture in Your Home
Water hardness is the amount of dissolved calcium and magnesium in water.24USGS. Hardness of Water In hard water, soap reacts with calcium to form soap scum instead of lather.25Wikipedia. Hard water26USGS. Hardness of Water So you need more soap or detergent to clean hands, hair, or laundry.27USGS. Hardness of Water
You can often spot hardness without a test. Soap scum on sinks and shower walls, soap that seems to rinse off poorly, spotty glasses after the dishwasher, and stiff-feeling laundry all point that way. They are clues, not measurements, since the amount of soap used can also cause them. A test kit or your water supplier’s report gives an actual figure.
The white crust on kettles and taps is minerals left behind when water evaporates or heats. Pure Hâ‚‚O would leave nothing behind. Vinegar dissolves the scale, a sign it is a mineral deposit.
For a simple test, let a spoonful of tap water and one of distilled water evaporate on a dark plate. Tap water usually leaves a faint ring of minerals, while distilled water leaves little or nothing.
Dissolved things also change freezing and boiling. Adding something dissolved lowers the temperature at which water freezes.28Physics Classroom. Chemistry Tutorial – Solutions – Colligative Properties – Freezing Point Depression It also raises the temperature at which water boils, as salt does.29Chemistry LibreTexts. Colligative Properties These effects depend on how many dissolved particles there are, not what they are.30Chemistry LibreTexts. Colligative Properties Pure water freezes at 0 °C and boils at 100 °C at sea level, but mixtures don’t.
That is why salt on icy roads works: salt water freezes below 0 °C. A pinch of salt in pasta water barely changes its boiling point, though. The change grows with the amount dissolved, so a lightly mineralized glass of tap water freezes and boils very close to pure water’s temperatures. A kitchen thermometer won’t show the difference, but a very salty solution shows it clearly.
Getting Water Close to Pure
Distillation boils water and condenses the steam, leaving minerals, metals, and most germs behind.31Morui Water. How Does Reverse Osmosis and Distillation Work to Purify Water? A Detailed Explanation32ServAPure. Distilled vs. Deionized Water: Which Purification Method Is Right for You Some substances that boil at lower temperatures than water can travel with the steam, so distillation alone doesn’t guarantee perfect purity.
Reverse osmosis pushes water through a fine membrane that holds back dissolved solids.33Morui Water. How Does Reverse Osmosis and Distillation Work to Purify Water? A Detailed Explanation34AXEONSupply.com. Reverse Osmosis vs. Deionization: Water Purification Guide Normally water would flow toward the saltier side of a membrane; here pressure forces it the other way. The pores let water molecules through but block most larger dissolved particles. The blocked material is washed away in a stream of waste water, so these systems use more water than they deliver.
Deionization swaps out charged mineral particles using special resin beads.35ServAPure. Distilled vs. Deionized Water: Which Purification Method Is Right for You36AXEONSupply.com. Reverse Osmosis vs. Deionization: Water Purification Guide It swaps them for hydrogen and hydroxide pieces that join into water. It does little against uncharged things such as many germs and some organic substances. The methods tackle different impurities and are often used together.37AXEONSupply.com. Reverse Osmosis vs. Deionization: Water Purification Guide
Distilled water from a shop is much closer to pure Hâ‚‚O than tap water, but not perfectly pure; once opened, it absorbs gases from the air. It is used in irons, car batteries, and some medical devices to avoid mineral build-up. Home filters are different. A jug filter mainly improves taste and removes some contaminants, leaving most dissolved minerals in, so the water stays a mixture. Under-sink reverse osmosis systems remove far more.
Perfectly pure water conducts electricity very poorly, because there are almost no dissolved charged particles to carry a current. So labs measure purity by electrical resistance, and absolutely pure water reaches about 18.2 megohm-cm at 25 °C.38Asianometry. The PUREST Water in the World But very pure water slowly dissolves metal from pipes and containers, such as nickel from stainless steel.39SilcoTek. Preventing Metal Ion Leaching in DI Water That makes it hard to keep pure once it’s made.
Is Drinking Pure Water a Good Idea
Regularly drinking water with very low minerals can disturb the body’s balance of minerals and water.40Environmental Toxicology and Medicine / National Institute of Public Health Czech Republic. Health Risks from Drinking Demineralised Water It can increase urine output and with it the loss of important body salts.41Environmental Toxicology and Medicine / National Institute of Public Health Czech Republic. Health Risks from Drinking Demineralised Water Water with almost no minerals also adds nothing toward your daily intake.
Many people assume food makes up for any missing minerals, but your diet does not automatically do so.42PubMed Central. Health Effects of Demineralization Drinking Water How well food covers the shortfall depends on the whole diet, and some diets supply less than others. Early signs of low body salts include tiredness, weakness, and headache.43PubMed Central. Health Effects of Demineralization Drinking Water
An occasional glass of distilled water won’t harm a healthy adult. The concern is making it your only drinking water for months. People on home reverse osmosis can ask about systems that add minerals back. Ordinary treated tap water meets safety limits and supplies some minerals.
Babies are most at risk. Infants whose drinks were made with distilled or low-mineral bottled water have developed a dangerous blood chemistry problem called metabolic acidosis.44PubMed Central. Health Effects of Demineralization Drinking Water For infant drinks, use tap water or water recommended for infant formula, not distilled water, unless your doctor advises otherwise.